A HANDBOOK OF COLLOID-CHEMISTRY OSTWALD V*" (Frontispiece) A HANDBOOK OF COLLOID-CHEMISTRY THE RECOGNITION OF COLLOIDS, THE THEORY OF COLLOIDS, AND THEIR GENERAL PHYSICO-CHEMI- CAL PROPERTIES BY DR. WOLFGANG OSTWALD PRIVATDOZENT IN THE UNIVERSITY OF LEIPZIG FIRST ENGLISH EDITION TRANSLATED FROM THE THIRD GERMAN EDITION BY DR. MARTIN H. FISCHER PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CINCINNATI WITH THE ASSISTANCE OF RALPH E. OESPER, PH. D. AND LOUIS HERMAN, M. D, INSTRUCTOR IN CHEMISTRY, NEW YORK STAFF PHYSICIAN, MOUNT SINAI UNIVERSITY HOSPITAL, NEW YORK PHILADELPHIA P. BLAKISTON'S SON & CO. 1012 WALNUT STREET 9 COPYRIGHT, 1915, BY P. BLAKISTON'S SON & Co, TH.TC MAP UK PKKSS Y O H K. J'A TRANSLATOR'S PREFACE The day is past when the importance of colloid-chemistry to the worker in the abstract or applied branches of science needs emphasis. The endeavor of the "pure" chemist to reduce all substances to crystalloid form and from the knowledge of their behavior to resynthesize the phenomena of nature has been a good one, but the limitations of such a point of view have grown daily more apparent. It happens that nature has chosen the colloid form in which to show her face. Crystalloid behavior is the exception, colloid behavior the rule, in the cosmos. Whether we deal with the regions above the earth, as the color of sky, the formation of fogs, the precipitation of rain and snow, or with the earth itself in its muddied streams, its minerals and its soils, or with the molten materials that lie under the earth, the problems of colloid-chemistry are more to the fore than have ever been the crystalloid ones. To the abstract thinker in science colloid-chemistry there- fore, because of its universality, represents the larger field. But the practical worker knows, too, that in a better knowledge of the properties of those very materials which the orthodox chemist has too often cast aside in his jellies, pastes arid glues, is found the explanation of so much that interests him. Is it any wonder then that colloid-chemistry appeals to the agriculturalist, the metallurgist, the dealer in precious stones, the tanner of skins, the manufacturer of wood pulps and paper, the dyer, the his- tologist, the steel worker, the weaver of textiles, the smelter, the manufacturer of paints? Not only the inorganic world but the organic also has chosen the colloid realm in which to manifest itself. Living matter, whether of plants or animals, and under normal or pathological conditions, is chemistry in a colloid matrix; whence colloid- chemistry comes to concern every botanist and zoologist, the 3375S8 / vi physiologist, the pathologist and the practical man in medicine and surgery. Under the circumstances, does this volume, known the world over as the authoritative and classical text, need an introduction to any of our people who think in the day's work? It can only seem somewhat strange that three large German editions and seven years were required before its first issue in the tongue of Thomas Graham and the brilliant modern group of English- speaking colloid-chemists. Wolfgang Ostwald's writings repre- sent in colloid-chemistry what those of Charles Gerhardt represent in organic, Justus Liebig in agricultural, and Wilhelm Ostwald in physical chemistry. MARTIN H. FISCHER. EICHBERG LABORATORY OF PHYSIOLOGY, UNIVERSITY OF CINCINNATI. TABLE OF CONTENTS PRACTICAL INTRODUCTION PAGE i. Identification of Colloid Systems by Elementary Methods: 1. General Considerations i 2. The Colloid State is Independent of Chemical Composition. ... 2 I. ELEMENTARY GENERAL COLLOID ANALYSIS 3. Chemically Homogeneous and Heterogeneous Liquids 3 4. True Solutions, Mechanical Suspensions and Colloid Solutions. . . 4 5. The Properties of Mechanical Suspensions 4 6. The Instability of Mechanical Suspensions 5 7. Differentiation of a True from a Colloid Solution . 6 8. The Tyndall Phenomenon 7 9. The Distinction of True from Colloid Solutions on the Basis of Their Mechanical Properties 9 10. Dialysis Experiments 10 u. Transition Systems 12 II. ELEMENTARY SPECIAL COLLOID ANALYSIS 12. Suspensoids and Emulsoids 12 13. Viscosity 13 14. Coagulation 13 15. Influence of Concentration . 14 1 6. The Electric Properties of Colloids 14 17. The Mutual Precipitation of Colloids 16 18. Electrophoresis 16 19. Summary 16 PART I GENERAL COLLOID-CHEMISTRY CHAPTER I THE GENERAL CONSTITUTION OF COLLOID SYSTEMS 2. The Colloids as Heterogeneous Systems: 1. The Concept of Heterogeneity 21 2. Physical and Chemical Heterogeneity 22 3. Colloids as Disperse Heterogeneous Systems: i. The Phases are in Contact with Each Other under Conditions vii Vlll TABLE OF CONTENTS PAGE Which Permit the Development of Much Surface between Them. 23 2. The Phases are so Distributed within the System That Externally the Whole Appears Homogeneous 23 4. The Disperse Phase and the Dispersion Means 25 5. Specific Surface in Dispersoids; Degree of Dispersion 26 6. Classification of the Dispersoids According to Their Degree of Dispersion . 29 1. Classification of Zsigmondy 29 2. Classification of Dispersoids According to Their Degree of Dis- persion 31 3. Defects of this Principle of Classification. 34 4. Polydispersoids 35 5. Dispersoids Varying with Changes in Concentration 35 6. Temperature-variable Dispersoids 36 7. Complex Dispersoids 36 8. Transition Phenomena 39 7. General Colloid-chemical Nomenclature 40 CHAPTER II RELATIONS BETWEEN THE PHYSICAL STATE AND THE GENERAL PROPERTIES OF COLLOID SYSTEMS 8. Classification of Dispersoids According to the States of Their Phases: 1. The Physical State of the Disperse Phases as a Principle of Classi- fication 42 2. Classification of the Dispersoids According to the Physical State of Their Phases 43 9. Transition Phenomena. Complex Dispersoids 44 1. General Considerations. Influence of Temperature and Degree of Dispersion 44 2. Influence of Concentration upon State in Complex Dispersoids . . 45 A. COMPLEX SYSTEMS HAVING THE COMPOSITION LIQUID + LIQUID (a) Influence of Concentration upon the State of the Dispersoid as a Whole 47 (b) Influence of Concentration on the State of the Disperse Phase 48 B. COMPLEX DISPERSOIDS HAVING THE COMPOSITION LIQUID + SOLID (a) Influence of Concentration on the State of the Dispersoid as a Whole 48 (&) Influence of Concentration on the State of the Disperse Phase. 49 10. Colloid Systems as Suspensoids and Emulsoids 49 1. General Considerations 49 2. The Empirical Establishment of Two Classes of Colloids 50 3. The Theoretical Characterization of the Two Classes of Colloids . 51 4. The Frequency of Occurrence of Complex Emulsoids 53 5. Relation of These Two Colloid Classes to Molecular Dispersoids. . 54 6. Suspensoids and Emulsoids 54 n. Transition Phenomena between Suspensoids and Emulsoids 55 12. The Crystalline (Vectorial) Constitution of the Disperse Phase 56 1. The Concept of Crystallinity 56 2. Direct Proof of Crystallinity in Colloids 58 TABLE OF CONTENTS IX PAGE 3. Indirect Proof for the Crystallinity of Colloid Phases 58 4. Dependence of Crystallinity upon Size of Particles 61 5. Crystallinity of Emulsoids 64 CHAPTER III GENERAL ENERGETICS OF THE DISPERSOIDS 13. Surface Energies 66 1. Forms of Energy Characteristic of Dispersoids 66 2. Surface Energy of the First Order 66 3. Surface Energy of the Second Order 67 4. The Relation of Surface Energy of the Second Order to Other Forms of Energy 71 14. Dependence of Surface Energies upon Specific Surface 72 1. General Considerations 72 2. Surface Energy of the First Order and Specific Surface 74 3. Surface Energy of the Second Order and Specific Surface 74 4. Dependence of Surface Tensions upon Specific Surface 76 15. Reciprocal Effects of the Two Surf ace Energies 77 1. General Considerations 77 2. Discontinuous Increase in Surface 78 3. Theory of Dispersion 80 4. Consequences of the Energetic Theory of Dispersion 82 5. Discontinuous Diminutions in Surface 84 6. Theory of Condensation 88 16. Influence of the Specific Surface upon the Relations between Surface Energies and Other Forms of Energy 91 1. Specific Surface and Volume Energy; Capillary Pressure 91 2. Specific Surface and Changes of State 91 3. Specific Surface and Electrical Energy '.''. 92 4. Specific Surface and Chemical Energy 93 5. Specific Surface and Radiant Energy 97 CHAPTER IV DISTRIBUTION OF THE COLLOID STATE AND THE CONCEPT OF COLLOID CHEMISTRY 17. The Fundamental Independence of the Colloid State of the Chemical Nature of the Phases 99 1. Statistical and Experimental Development of the Idea of the Universality of the Colloid State 99 2. Universality of the Colloid State as a Necessary Consequence of Characterizing Colloid Solutions as Disperse Systems 101 18. Isocolloids 102 19. Multiplicity of the Colloid State of One and the Same Substance. Ex- ample: Colloid Ice 106 i. Isocolloids of H 2 O 107 TABLE OF CONTENTS PAGE 2. Chemically Heterogeneous H 2 O Colloids 109 The Concept of Colloid-chemistry .in PART II SPECIAL COLLOID-CHEMISTRY CHAPTER V MECHANICAL PROPERTIES OF COLLOID SYSTEMS I. RELATIONS OF VOLUME AND MASS IN COLLOIDS 21. Volume and Density Relations in Colloids 115 1. Volume Relations of Colloid Systems 115 2. Density and Space Relations in Colloid Systems 120 3. The Concentration Function of Density in Colloid Systems. ... 124 4. Thermal Coefficient of Expansion in Colloids 126 22. Vapor Tension, Boiling Point and Freezing Point of Colloid Solutions. . 128 1. General Remarks 128 2. Measurements of Vapor Pressure of Colloid Solutions 129 3. Elevation of Boiling Point of Colloid Solutions 130 4. Depression of Freezing Point of Colloid Solutions 131 23. Mass-relations in Colloids 132 1. Concentration of Colloid Systems 132 2. Experimental Work on Saturation in Colloid Solutions 134 3. Theoretical Considerations Bearing on the Saturation of Colloids . 136 4. Supersaturation in Colloid Systems 138 24. Molecular Weight of Substances in the Colloid State as Measured by Changes in the Constants of the Dispersing Medium 140 1. General Remarks 140 2. Examples of the "Molecular Weights" of Substances in the Colloid State as Determined by Changes in the Constants of the Dispersing Medium 142 II. INTERNAL FRICTION AND SURFACE TENSION OF COLLOIDS 25. Internal Friction of Colloid Systems 145 1. General Remarks 145 2. Internal Friction of Suspensoids 146 3. Effects of External Conditions upon the Viscosity of Suspensoids. 150 4. Mechanical Theory of the Viscosity Relations in Suspensoids . . . 152 5. Viscosity of Emulsoids. Literature 153 6. Viscosity Changes in Emulsoids with Time 154 7. Effect of Mechanical Treatment on Viscosity of Emulsoids . . . .158 8. Influence of "Inoculation" on Viscosity of Emulsoids 158 9. Influence of Thermal History on Viscosity of Emulsoids 159 10. Influence of Concentration on Viscosity of Emulsoids 161 11. Influence of Temperature on Viscosity of Emulsoids 164 12. Influence of Added Substances on Viscosity of Emulsoids 165 TABLE OF CONTENTS XI PAGE 13. Effect of Added Substances on Viscosity of Emulsoids; Behavior of Protein Solutions 169 14. Influence of Added Substances on Viscosity of Emulsoids. Effects of Non-electrolytes and Mixture of Dispersing Media 173 15. Viscosity and Electrical Charge of Disperse Phase 174 16. Viscosity and Degree of Dispersion; Viscosity of Coarse and Com- plex Dispersions 175 17. Viscosity and Type of Disperse Phase 179 26. Surface Tension of Colloid Solutions 180 1. General Remarks 180 2. Experimental Facts 181 CHAPTER VI MECHANICAL PROPERTIES OF COLLOID SYSTEMS III. MOVEMENT IN COLLOID SYSTEMS AND ITS RESULTS 27. Browian Movement 186 1. General Remarks 186 2. The Independence of Brownian Movement of External Sources of Energy 189 3. More Exact Determination and Measurement of Brownian Move- ment 192 4. Uniformity of Brownian Movement 195 5. Influence of the Specific Surface of the Particles 196 6. Influence of the Concentration of the Dispersoid 196 7. Influence of the Viscosity of the Dispersion Means 197 8. Influence of Temperature 198 9. Influence of Added Substances 199 10. Influence of Electrical Charge 201 11. Influence of Gravity on the Distribution of Vibrating Particles . . 201 12. Validity of Stokes' Law for Highly Dispersed Particles 204 13. Kinetic Theory of Brownian Movement 205 14. Determination of the "Molecular Weight" of Dispersed Particles from Their Brownian Movement . 209 28. Diffusibility of Colloids 210 1. General Remarks 210 2. Experimental Study of Diffusion of Colloids 211 3. Experimental Facts Regarding Diffusion of Colloids. 213 4. Influence of Degree of Dispersion on Diffusion Velocity 215 5. Theory of Colloid Diffusion 217 6. Effect of Added Substances on Colloid Diffusion. Spurious Diffu- sion of Colloids 219 29. Dialysis of Colloid Systems 222 1. General Remarks 222 2. Methods of Dialysis 222 3. Experimental Facts Regarding the Dialysis of Colloids 224 4. Special Observations Regarding the Dialysis of Colloids 227 30. Osmosis of Colloid Systems 231 i. General Remarks and Literature 231 xl TABLE OF CONTENTS PAGE 2. Methods of Measuring the Osmotic Pressure of Colloids 233 3. Instability of Osmotic Pressure of Colloids 235 4. Influence of Concentration on Osmotic Pressure of Colloids. . . . 238 5. Influence of Temperature on Osmotic Pressure of Colloids .... 242 6. Influence of Added Substances on Osmotic Pressure of Colloids. . 244 7. On the Theory of Osmotic Pressure of Colloids 253 8. Determination of the "Molecular Weight" of Colloid Systems by Osmotic Means 258 9. On the Moleculo-kinetic Theory of Osmosis in Colloid Systems . . 261 ADDENDUM OTHER TYPES OF MOVEMENT IN DISPERSOIDS 262 31. Filtration and Ultrafiltration of Colloid Systems 263 1. Filtration of Colloid Systems 263 2. Ultrafiltration of Colloid Systems 264 AUTHOR INDEX 267 SUBJECT INDEX ' 273 PRACTICAL INTRODUCTION i. Identification of Colloid Systems by Elementary Methods (The Elements of Qualitative Colloid-chemical Analysis) i. General Considerations. The teachings of colloid-chem- istry are by no means so familiar to all who encounter colloid substances in their scientific or practical work that the questions: "How can we recognize a colloid?" or "When is a body said to be a colloid?" are no longer raised. These questions have often been put to me, not only by such men of science as physicists, physical chemists, physicians and mineralogists, but by technicians who for years perhaps have worked exclusively in such practical colloid problems as the manufacture of rubber. Even the organic and inorganic chemists frequently encounter phenomena, par- ticularly when they work with highly polymerized and highly complex substances that remind them of what they know of the properties of colloids, and which make them ask how they can de- termine quickly and simply whether colloid-chemical principles will help them in the solution of their problem or no. As a matter of fact I am of the opinion that such questions have not been asked frequently enough, say in organic chemistry, where examination of the colloid behavior of one and the same organic substance in different solvents would throw much light on the properties observed. 1 The youth of colloid-chemistry itself justifies such questions, and their discussion is by no means either useless or superfluous. An answer to the question: "How do we know when we 1 We need but call to mind the modern problem of the relation to each other in solutions of various kinds of color, chemical constitution, molecular state and char- acter of solvent as studied by A. Hantzsch and his pupils. It seems to me that a colloid-chemical (dialytic or ultramicroscopic) examination of such variously colored solutions would bring light especially in those cases in which molecular weight deter- minations have been exhausted without result. The failure of Beer's law governing the proportionality between thickness of layer and light absorption when applied to colloids and to solutions of dyes, of oxime salts, organic ammonium salts (see chapter on optical properties in this volume) as well as other facts seem to me to indicate that suitable colloid-chemical investigations in this field will bring to light as surprising facts as did those of J. Amann (Koll.-Zeitschr., 6, 235, 7, 67 (1910) on the colloid and molecular solubility of iodine in various solvents. 2 'COLLOID-CHEMISTRY are dealing with a colloid?" would consist in a presentation of the elementary properties and the experimentally observed be- havior of colloid substances. Such an analysis would constitute the elements of a qualitative colloid-chemical analysis. A possible method of procedure in attempting to discover the colloid nature of any substance is indicated in the following: 2. The Colloid State is Independent of Chemical Composition. At first sight one might hope to obtain information about the question under consideration by constructing a comprehensive table of all the colloid substances or groups of substances known. As a matter of fact such attempts 1 have been made even recently, but never with the full approval of competent workers in the field. It was soon noticed 'that we cannot speak of colloid substances in the same way as we (still) do of "liquid-crystalline" or "radio-active" substances. We have been compelled to rec- ognize that colloid properties are in no way connected with substances of definite chemical composition to the end that only certain elements or certain compounds, for example, appear as colloids. We can speak of " colloids " only as we speak of "crys- tals," "amorphous" substances, "soluble and insoluble" sub- stances, or better still of "gaseous, liquid, and solid" substances. All substances can appear as colloids under appropriate conditions. This peculiarity of colloid-chemistry, through which it thus pre- sents itself not as a study of colloid substances but as a study of the colloid state, will be discussed in detail later. But it is of great importance for even an elementary characterization of col- loid substances to know that depending upon experimental con- ditions one and the same chemical compound can appear either as a colloid or as a non-colloid. Generally speaking, the knowledge of the chemical constitu- tion of a substance furnishes no trustworthy indication as to whether or not we are dealing with a colloid. Only one law has thus far been deduced governing the relation between chemical constitution and colloid state: The more complex chemically the compound, the greater the probability that it is in a colloid state. Thus most, of the native proteins appear in a colloid state; and the chemical composition of the original colloid, namely gelatine, is so complex that we are still largely ignorant concerning it. 1 See, for example, Koll.-Zeitschr., 2, 53 (1907). PRACTICAL INTRODUCTION 3 Solid and liquid and even gaseous bodies may appear in the colloid state. 1 The liquid colloids are the most numerous and the most important, and thus far have been most studied. Whenever we deal with the class properties of the colloids we therefore usually refer to these. I. ELEMENTARY GENERAL COLLOID ANALYSIS 3. Chemically Homogeneous and Heterogeneous Liquids. If we wish to enquire into the possible colloid nature of a given liquid, it is well to decide first whether it is chemically homoge- neous or chemically heterogeneous. In an ideal case a chemically homogeneous liquid has the following properties: i. It is susceptible of hylo tropic change, that is, it can be evaporated or frozen without changing its com- position at any time during the manipulation. 2. The hylo tropic transformations take place within narrow limits of temperature and pressure; there is only one boiling temperature and one congelation temperature; we speak of melting and boiling points. Among further properties of an ideal liquid is to be mentioned the fact that the temperature coefficient of its molar surface energy equals 2.I2. 2 As is well known, there are a great number of substances, or- ganic liquids, more particularly, which fulfill these requirements in part only. They are the mixtures of isomeric, metameriCj and polymeric substances; to which we may add the so-called asso- ciated liquids. Even though all these liquids show the same elementary analysis in every state of aggregation, yet they can be separated by fractional distillation, for example, into parts having different boiling points; or, notwithstanding the analytic- ally identical composition of the liquid undergoing distillation and the distillate, it is noted that the former is not completely evaporated at any definite temperature. These facts are illus- trated by the behavior of polymerized liquids such as styrol- metastyrol. Again, as in associated liquids, the molar surface energy is found to be less than the normal. The following rule may be stated regarding the relation of 1 See Chapter III of this volume. 8 See the textbook of Wilh. Ostwald, Grundr. d. allgem. Chemie, 4 Aufl., 1909 for a discussion of the general concepts of physical chemistry employed here. 4 COLLOID-CHEMISTRY these properties to the possibility of the appearance of a colloid state in liquids of constant composition: The more a liquid approaches the ideal of chemical homogeneity, the less probable that it is in the colloid state. Therefore, if from general physico- chemical examination we know a liquid not to be " normal" with regard to exactness of .boiling point, molar surface energy, etc., it is possible that we are dealing with a "physical mixture/' and therefore with a molecular or colloid solution. Colloid liquids showing the same analytical composition with every hylotropic transformation are by no means rare. Thus far these have been little studied from a colloid-chemical point of view. Details regarding the peculiarities of these so-called isodispersoids, more especially the isocottoids, 1 will be given later. Nearly all the colloid solutions investigated thus far belong to the class of the chemically heterogeneous liquids discussed below. Since the fundamental properties of colloid liquids depend, not upon chemical composition but upon other physical conditions which are especially to be encountered in chemically heterogeneous liquids, we shall also discuss in the succeeding paragraphs the general data by means of which we recognize the colloid character of chemically homogeneous liquids. 4. True Solutions, Mechanical Suspensions and Colloid Solu- tions. Chemically heterogeneous liquids can be separated by changes in temperature and pressure (distillation, freezing, etc.) into at least two components of different chemical composition. When we have thus determined in our unknown that we are deal- ing with a chemically heterogeneous liquid it may appear in any one of three states : (a) The unknown may be an ordinary or "true" (molecular- disperse) solution of one or more substances. (b) It may be a coarse "mechanical suspension" of one or more substances which form true solutions to a limited degree only, if at all. (c) It may be a colloid solution. 5. The Properties of Mechanical Suspensions. In a qualita- tive analysis for the determination of the "degree of dispersion" 2 1 Examples of such liquid isocolloids are oils, petroleum, paraffin, styrol-metastyrol, liquid sulphur at temperatures above 170, highly polymerized liquids, etc. * See page 29 for a discussion of the concept "degree of dispersion;" the three classes of systems mentioned above are distinguished from each other by their different degrees of dispersion. PRACTICAL INTRODUCTION 5 in a heterogeneous liquid the second of the above possibilities can be disposed of most easily. Typical " mechanical sus- pensions" of substances but slightly soluble in liquids, as sus- pensions of quartz, kaolin, or oil in water, are turbid in trans- mitted light, and their individual particles can be recognized under the microscope (though sometimes only with high magni- fications and special optical means) . If no microscope is available, filtration is the next simplest method by which a suspension can be recognized. Ordinary filter paper holds back particles having a diameter greater than about 5/,t; a hardened filter (Schleicher and Schiill, No. 602 e.h.} 9 those about 2/z in diameter. Clay cylinders and the so-called Pukall filters which are frequently employed in bacteriology will even hold back particles about 0.4 to o.2/x in diameter. 1 The size of the particles in question can therefore be roughly measured by the employment of such differently permeable filters. When applied to emulsions, that is, suspensions of droplets in a liquid, . filtration is successful only when the suspended droplets are not materially deformed during filtration. As the investigations of E! Hatschek 2 on the filtration of emulsions show, this difficulty does not appear if the droplets are moderately viscous, as are the droplets of castor oil or olive oil; or when their surfaces are in a condition which gives the droplets themselves sufficient stiffness. Such stiffness may result from the formation of thin elastic membranes about the droplets, of the nature of the well-known saponin or peptone films, 3 or it may be due and this seems most important to the small size of the droplets with its accompanying increase in surface energy. As E. Hatschek has shown, it is often possible to separate emulsions into their components by means of appro- priate filters. 6. The Instability of Mechanical Suspensions. Another characteristic of coarse suspensions of solid and liquid particles is their instability, that is their tendency to separate " spon- taneously" into their components. If we can exclude the sta- bilizing effects of additions of viscous substances such as gelatine, tragacanth, etc., as well as the peculiar " protective action" of 1 For details regarding permeability and size of pores in various filters see H. Bechhold, Zeitschr. 1. physik. Chem., 64, 342 (1908). 2 See the chapter on Adsorption Phenomena in Part III. 3 E. Hatschek, Koll.-Zeitschr., 6, 254 (1910); 7, 81 (1910). 6 COLLOID-CHEMISTRY small amounts of soap, saponin, albumose, etc., separation occurs in typical coarse suspensions in accordance with the difference in the densities of their components. 1 Considerable acceleration in separation can be effected by moderately centrifuging the mix- ture. A hand-centrifuge such as is employed in the study of blood does very well. The suspended component then separates out, in accordance with the difference in density, either in the form of a precipitate or of a supernatant layer. After such a separation has been accomplished either spontaneously or with the assistance of a centrifuge, the original system can in most instances be restored by shaking the components together again. When the suspended particles are in a very finely divided condition, indefinite or negative results are obtained by these procedures. Under such circumstances two possibilities still remain: the liquid in question is either a "true" or a "colloid" solution. 7. Differentiation of a True from a Colloid Solution. It is generally harder to distinguish a true from a colloid solution than to distinguish a coarse suspension from either, yet this problem is precisely the one that arises most frequently. We must there- fore discuss the methods involved in detail. A. Optical Diferences. Absolutely clear liquids are formed as a rule by substances in molecular or true solution. If a liquid (which is not chemically homogeneous, and which is not a coarse suspension) is seen to be turbid, we may suspect that it is a colloid solution. The existence of a slight turbidity may be recognized on inspection of a rather thick layer of the liquid in a thin-walled glass vessel against an opaquely black background (black paper, or better, black velvet). If the liquid is colorless but turbid, the background shining through it assumes a grayish-white appearance. In the case of colored liquids (in the examination of which it may be necessary to employ a particular dilution or thickness of layer) an optical effect appears which is similar to that observed on mixing water-colors with small quantities of opaque white (the colors become milky). Different varieties of 1 In this experiment it is well to use very long tubes and relatively "dilute" systems. The temperature must be kept constant to prevent ^mixing of the layers by convection currents. Closed basement rooms may be used if a suitable thermo- stat is not available. PRACTICAL INTRODUCTION 7 nephelometers have been constructed for the more exact deter- mination of the degree of turbidity. 1 8. The Tyndall Phenomenon. A far more delicate method of demonstrating the presence of a very fine turbidity lies in the use of the so-called Tyndall phenomenon. It is well known that when, for example, the air of a room is intensely illuminated, say by sunlight,^from one side only, dust particles are rendered visible which cannot be seen when illumination is equal on all^sides. This is the prototype of the so-called Tyndall phenomenon, the theory of which will be discussed later. Extraordinarily fine turbidities FIG. i. Tyndall phenomenon. can be rendered visible by such means; in fact this holds true to such an extent that special, measures become necessary if we would obtain, for example, an absolutely " optically empty" distilled water; ordinary distilled water regularly shows individual dust particles. Tyndall experiments can be best carried out where sunlight and a darkened room are available. The phenomenon becomes beautifully evident if we but let a sharply defined ray of light, entering a darkened room through a^hole bored in the shutter 1 For simpler forms of such apparatus see H. von Oettingen, Zeitschr. f. physik. Chem.,33, i (1900); J. Frie-dlander, ibid, 38, 430 (1901). 8 * COLLOID-CHEMISTRY of a window, pass through the liquid in question contained in a thin-walled test tube. Very good results are also obtained if a projecting lantern is used from which the light rays are concen- trated as much as possible by means of a condenser and dia- phragm (see Fig. i). A powerful incandescent lamp 1 enclosed in a box that is impervious to light and provided with a small opening, and if possible with a condenser, is generally satisfac- tory also. The thinnest and clearest glass vessels must be used. Special advantages are offered by vessels with parallel walls which reflect light least. When we work with hot or very cold liquids we use cotton-stoppered, double- walled tubes from which the air has been exhausted (Dewar tubes). When cold liquids are used in these, the tubes may be immersed in alcohol to avoid the condensation of water on their outer walls. It should now be noted that it is not the presence of many more or less evident particles which may be recognized either macroscopically or microscopically that distinguishes a colloid from a molecular-disperse (true) solution. It is rather the in- tensity of the unbroken light-cone passing through the solution which betrays the state of the liquid. It is safe to say that liquids which show no definite Tyndall light-cone or show it only in high concentrations are molecular-disperse solutions. Practically all colloid solutions give a positive Tyndall effect. The Tyndall phenomenon is not to be confounded with fluorescence. When a ray of light enters many solutions, such as those of certain dye-stuffs and alkaloids, the path of the beam betrays itself in brilliant colors even though these solutions may not be in the colloid state. The fluorescence can be dis- tinguished from the Tyndall effect by looking at the light-cone with a Nicol prism. If we look at the Tyndall cone of a colloid solution through a Nicol prism we find that it disappears when the prism is rotated, to light up again at a certain angle. Fluorescent light remains visible at all angles. Emphasis should be laid on the fact that the Tyndall effect is of particular value in the recognition of isocolloids. It should further be mentioned that the microscopic ex- amination of a Tyndall cone with the highest available magnifi- 1 Not only electrically lighted but gas-lighted projection apparatus as used in photographic enlarging, when combined with a condenser is suited for this purpose. PRACTICAL INTRODUCTION cations at times permits us to see the individual particles which in their totality give rise to the light-cone. This is called ultra- microscopy. Since for ultramicroscopy special apparatus and powerful sources of light are necessary which are by no means generally available, and since the technique of ultramicroscopy is by no means simple, we cannot further discuss the subject in this elementary outline of colloid analysis. 9. The Distinction of True from Colloid Solutions on the Basis of Their Mechanical Properties. B. Mechanical Differences. Diffu- sion and dialysis experiments provide us with two further simple methods for distinguishing molecular-disperse (true) from colloid solutions. These might be called the " classical" methods for the qualitative analysis of solutions, for it was by them that Thomas Gra- ham in 1 86 1 first distinguished be- tween the "states" of different solu- tions and thus introduced the concept "colloid." (a) Diffusion Experiments.- Per- haps the simplest and most convenient experimental method of estimating the diffusion velocity of a dissolved substance depends upon the fact that moder- ately concentrated jellies of gelatine, agar, etc., offer only slight or no resis- tance to the diffusion of substances through them, as determined by com- parison with the diffusion of these same substances through the pure solvent. For such tests we prepare a 5 per cent. gelatine, or better a 2 per cent, agar solution, fill some test tubes about halfway with the hot solution, and allow it to con- geal. It is well to use gelatinizing substances that have been thoroughly washed and purified. The solution under exami- FIG. 2. Diffusion experi- ments with gelatine gels at end of 24 hours. (a) (Colloid) congo red; (b) (molecularly dispersed) safranin. 10 COLLOID-CHEMISTRY nation is then poured upon these gelatine or agar layers and the tubes are left standing, variations in temperature being avoided as far as possible. A true solution in water, either of a dye such as a safranin, or of a colored salt such as copper sulphate is taken as a control. If the solution undergoing analysis is colored a picture similar to that shown in Fig. 2 may be seen after a day or two. While non-colloids, that is molecular-disperse or true solutions, gradually spread down into the jelly, colloid solutions do this only very slightly or not at all. In other words, substances in the colloid state practically do not diffuse at all. At the best they diffuse with extreme slowness when compared with the behavior of substances in molecular solution. If it is feared that a liquid 'of high specific gravity may by mechanical means force itself into the jelly, a small tube half filled with gelatine or agar may be placed mouth downward into the solution contained in a second larger vessel. The tube is removed after a few days and carefully washed when it also will show the phenomena that have been described. If the liquid under examination is light colored or colorless the test tube con- taining the gelatine or agar is dipped for an instant into hot water so that the jelly slips out. This is then divided into several slices of equal size, and the individual slices are examined analyt- ically for their content of the substance in question. This method is not generally applicable to the analysis of isocolloids, nor when marked chemical or colloid-chemical reac- tions take place between the jelly and the liquid under examina- tion. Under such circumstances it is necessary to resort to other methods. 10. Dialysis Experiments. (b) Dialysis, a process closely re- lated to diffusion, depends upon the fact that animal, plant, and artificial membranes hold back substances in colloid solution while they allow substances in molecular solution to pass through them whenever such a membrane separates the liquid under examination from the pure dispersion means (the solvent) . Parch- ment bags, so-called diffusion sacs made in one piece (see Figs. 3 and 4), pig and fish bladders, and artificially prepared colloid membranes form the most convenient as well as the most fre- quently employed of these. The last-named are made by stick- ing a large, well-cleansed test tube into collodion dissolved in PRACTICAL INTRODUCTION II ether and alcohol, permitting the collodion layer formed to harden slightly by evaporation, repeating the process if necessary, and then hardening the whole by washing in water. The collo- dion bag is then carefully drawn off the tube. 1 When only small amounts of liquid are to be analyzed colloidally, diffusion sacs (Schleicher and Schiill) arranged as shown in Fig. 3 are par- ticularly useful. For this purpose a small Erlenmeyer flask is used, into the neck of which the diffusion sac fits snugly; the flask is first filled with the pure solvent while the liquid under examina- tion is poured into the sac which is then closed with a cork stopper. In this way, aided by the slight swelling of the sac which usually occurs, evaporation and the entrance of dust into the liquid are FIGS. 3 and 4. Simple arrangement for dialytic analysis. largely prevented. It is evident that if the solutions under ex- amination are alcoholic or ethereal in character, collodion sacs cannot be used. When dealing with such volatile liquids it is advisable to employ glass-stoppered vessels in which the dialyzer is placed or suspended as shown in Fig. 4. The dialyzer dis- tinguishes colloid from crystalloid solutions in that it does not allow the former to pass through the membrane into the outer liquid. Oc- 1 Details of various methods of preparation may be found in A. Cotton and H. Mouton: Les Ultramicroscopes, 117, Paris, 1906; L. Bigelow, Journ. Am. Chem. Soc., 29, 1576 (1907); J. Duclaux, Journ. Chem. Phys., 7, 430 (1909); W. Biltzand A. von Vegesack, Zeitschr. f. physik. Chem., 63, 369 (1909). 1 2 COLLOID-CHEMISTRY casionally we find that a colloid "phase" will pass with a molec- ularly dissolved phase into the outer liquid. But this happens only at first. After the outer liquid has been renewed once or twice, no more of the colloid phase comes through. Sometimes a dissolved substance will penetrate a collodion sac when it is held back by the less porous parchment paper. In such cases we are evidently dealing with a " highly disperse" (finely divided) colloid, or to put it in another way, with a substance occupying a position midway between the colloid and molecular-disperse state. So-called ultrafilters are used for more exact determinations of the degree of subdivision, but they cannot be discussed here be- cause they are rather complex (see later) . 11. Transition Systems.- It will nearly always be possible to determine by one or more of the methods described whether a substance in solution is in the colloid or in the molecular-dis- perse state. At the same time it must be admitted that we encounter cases in which one and the same liquid yields different results with different methods. Thus a pure congo red shows only a faint Tyndall cone,^ yet it scarcely diffuses through parchment paper. Protein solutions behave in a similar way in certain concentrations, etc. For a complete analysis it is therefore not only advisable but necessary to employ several methods. But even then it may occasionally be doubtful whether we are dealing with a colloid or with a molecular-disperse solution. These cases constitute the extremely interesting transitional types between the two kinds of solution. Their state can be completely analyzed only by application to them of the more refined methods of colloid and physical chemistry ultramicroscopy, ultrafiltration, molecular weight determination, etc. H. ELEMENTARY SPECIAL COLLOID ANALYSIS 12. Suspensoids and Emulsoids. When one undertakes detailed work with substances in the colloid state one soon dis- covers that the individual illustrations arrange themselves in two classes of systems which differ markedly from each other, in spite of the fact that all are possessed of the same general prop- erties that we have already discussed. These two groups of colloid solutions are the suspension colloids (suspensoids) and PRACTICAL INTRODUCTION 13 the emulsion colloids (emulsoids), or as they are also called, the lyophobic (hydrophobic) and lyophilic (hydrophilic) colloids. The theoretical basis for such nomenclature will be discussed later. In passing, it should be noted that the two terminologies are not entirely synonymous, though for practical purposes they may be so regarded. When by the general methods previously discussed we have discovered that we are dealing with a colloid solution we need next to determine whether it is a suspensoid or an emulsoid. Of the many means of doing this we describe the following because they are particularly characteristic and simplest in character. 13. Viscosity. The viscosity of a suspension colloid, par- ticularly in low concentration, is imperceptibly greater than that of the pure dispersion means (the pure solvent). In contra- distinction, the viscosity of an emulsion colloid even in low con- centration is much greater than that of its dispersion means; in fact at higher concentrations this becomes so great that the colloid solution assumes an oily or even a gelatinous consistency. Further, the viscosity of an emulsion colloid generally increases rapidly with decrease in temperature which is not the case with a suspension colloid. The simplest way of estimating experimentally the viscosity of a colloid solution and its varia- tions with temperature and concentration is to measure the time of outflow of a constant volume of liquid from a standard volumetric (10 cc.) pipette. Roughly, the viscosity is inversely proportional to the time of outflow. 14. Coagulation. It is characteristic of colloid solutions that the substance in colloid solution may be easily precipitated or coagulated' ' through various agencies (see Figs . 5 and 6) . Electro- lytes such as neutral salts are particularly effective. The suspension colloids are easily coagulated when minute quantities of salts, especially those having a polyvalent ion, are added to them, while the emulsion colloids are precipitated only after the addition of much larger quantities of salt. This is particularly true of hydrosols, that is of colloids having water as the dispersion means. If, for example, aluminium sulphate (ordinary alum serves the same purpose) is selected as the coagulant, it is found that almost all suspension colloids are precipitated by this as soon as it is present in a i per cent, concentration. Much COLLOID-CHEMISTRY higher concentrations are necessary to precipitate the typical emulsion colloids. In fact the coagulation of many emulsion colloids is not brought about until the neutral salts have been added to the point of saturation. In making these qualitative analyses one must not use salts of the heavy metals, for they frequently produce entirely abnormal coagulation effects. 15. Influence of Concentration. One will occasionally en- counter instances in which neither viscosity nor coagulation deter- minations will serve to distinguish clearly a suspension colloid from an emulsion colloid. It is then advisable to compare with each other rather dilute solutions of suspension colloids and FIG. 5. Non-coagulated. FIG. 6. Coagulated through addition of 2 per cent, sulphuric acid. Coagulation of an aqueous suspension of lamp-black. (After E. E. Free.) rather concentrated solutions of emulsion colloids. We encounter here also a series of interesting transitional types which can be accurately analyzed only through quantitative study. The suspensoid or emulsoid state is not a constant or integral property of a chemical substance, it is the result of a series of physico- chemical variables which bring about a particular state in a chemical substance. 16. The Electric Properties of Colloids. Colloid solutions have a characteristic electric behavior which explains many of PRACTICAL INTRODUCTION 15 their peculiar properties. Most substances in colloid solution assume an electric charge toward their dispersion means, though the magnitude of this charge varies greatly. We are able to distinguish between negatively and positively charged sub- stances in colloid solutions. The simplest method of determining with which of these we are dealing in a given case is to make use of their difference in behavior (as noted by F. Fichter and N. Sahlbohm) when they are subjected to capillary analysis by means FIG. 7. Ascent of positively and negatively charged colloids. (According to N. Sahlbom.) The dispersion means ascends the paper in all these experiments, but as shown in the left half of the photograph, the positively charged colloids (metallic oxides) are precipitated at once at the margin of immersion. The negatively charged colloids, on the other hand, (gold, silver, arsenic sulphide, antimony sulphide, Berlin blue, selenium) ascend with the dispersion means, being separated from its upper margin by a diffusely stained area. of filter paper. If the lower end of a strip of filter paper is im- mersed in a colloid solution one of two things may happen de- pending upon the character of the electric charge of the colloid; if the colloid carries a negative charge it wanders up the strip of paper along with its dispersion means. The colloid may rise to a height of 20 centimetres or more depending upon the kind of paper used and the special properties of the colloid. If the 1 6 COLLOID-CHEMISTRY colloid carries a positive charge the dispersion means continues to rise to the normal height, but the colloid phase does not. It rises to a point but slightly above the level of the liquid in which the filter paper is immersed, becomes highly concentrated here and finally coagulates. Positively charged colloids may there- fore be separated from their dispersion means through the cap- illary action of strips of filter paper. The behavior is illustrated in the accompanying Fig. 7, taken from N. Sahlbohm. 17. The Mutual Precipitation of Colloids. Another means of determining quickly the character of the charge of a substance in colloid solution depends upon the fact that oppositely charged colloids precipitate each other. If two typical test solutions are kept in stock (for example, a positive colloid such as ferric hy- droxide, and a negative colloid such as sulphur or arsenious sul- phide) the charge of an unknown colloid may frequently be determined by ascertaining with which of the two solutions it yields a precipitate. The charge of the unknown colloid is then the opposite of that of the precipitating colloid the charge of which is known. This method is not, however, universally applicable (see the section on coagulation of colloids). 18. Electrophoresis. The character of the charge of the colloid phase may be determined by noting the direction in which it moves when subjected to the action of an electric current (migration in an electric field) . To do this the colloid is poured into a U tube closed with corks and provided with platinum electrodes which dip into the solution. When a stronger current is not available, that from a few storage cells will frequently suffice to produce a movement of the colloid toward one or the other pole if only sufficient time be allowed. If the colloid wanders toward the anode it is negatively charged; if it wanders toward the cathode it is positively charged. Disturbing secondary effects often enter into the behavior of a colloid when subjected to the electric current, and so it is advisable to employ along with it the methods of colloid analysis already described. If the colloid under examination is colorless it may be necessary to call in the aid of simple analytical methods. 19. Summary. The following is an outline of the methods of qualitative colloid-chemical analysis discussed above. PRACTICAL INTRODUCTION A. ELEMENTARY GENERAL COLLOID ANALYSIS I. Chemically Homogeneous Liquids. (a) Always hylotropically transformable, definite boiling and freezing points, normal molecular surface energy, etc. (&) Physical mixtures of substances having the same chem- ical composition, but different "body properties" as different boiling points, molecular surface energies, etc.; mixtures of isomeric and polymeric substances, highly associated liquids, etc. II. Chemically Heterogeneous Liquids. (c) Microscopically heterogeneous, components separable by ordinary filtration, may be easily sedimented espe- cially if centrifuged, precipitate spontaneously, can usually be easily resuspended on shaking. (d) Optically homogeneous, at least in low concentrations, diffuse well (as into gelatine or agar-agar) and pass through membranes (as when subjected to dialysis), etc. (e) Often turbid macroscopically, Tyndall test positive (apply Nicol prism to differentiate from fluorescence), non-diffusible or only slightly so, will not pass through a membrane, etc. Normal liquids. Isodispersoids, possibly Isocolloids. 15 Coarse dispersions (suspensions and emulsions) . Molecujar-dis- perse solutions. Colloid solutions. B. ELEMENTARY SPECIAL COLLOID ANALYSIS (/) Viscosity not perceptibly greater than that of dis- persion means; easily coagulable by electrolytes, espe- cially by salts with polyvalent ions (i per cent. alum). (g) Viscosity substantially greater than that of pure dis- persion means even in low concentrations. Viscosity increases with lowering of temperature; difficultly coagu- lable by salts which at times need to be added to saturation point to accomplish flocculation. (K) Electric behavior: 1. Colloids can be separated from their dispersion means by capillary analysis (filter-paper experiment); are precipitated on adding colloid solutions of sul- phur or arsenious sulphide; wander toward cathode. 2. Can be separated but slightly, if at all, from their dis- persion means (filter-paper experiment); are precipi- tated by colloid hydroxide solutions; wander toward anode. 15 For details see 18, page 102. Suspension col- loids (lyophobic colloids). Emulsion colloids (lyophilic col- loids). Positive colloids. Negative anodes. PARTI GENERAL COLLOID-CHEMISTRY (THEORY OF THE COLLOID STATE) CHAPTER I THE GENERAL CONSTITUTION OF COLLOID SYSTEMS 2. The Colloids as Heterogeneous Systems i. The Concept of Heterogeneity. The typical colloids, more particularly the colloid solutions, belong to the group of systems designated as poly phasic or heterogeneous by physical chemists. This is the broadest as well as the best established generalization thus far derived from the study of colloid-chemistry. By a phase is meant any homogeneous part of a system differ- ent from other parts of the system and separated from these by abrupt transitions. Thus we distinguish a gaseous and a liquid phase in a closed vessel which is half filled with water ; we obtain two liquid phases when we mix water with carbon disulphide; we get a solid and a liquid phase when we shake up quartz dust in water. Sudden changes, more particularly in physical proper- ties, are encountered as a rule as we pass from one phase to another in these systems, or, to put it more simply, we recognize that there exist boundaries or surfaces between them. These changes may be due either to the fact that the different phases possess totally different properties, or there may be simply a quantitative difference in the properties possessed by each. We may therefore speak of different kinds and of different grades of heterogeneity. Thus two phases may be heterogeneous optically but homogeneous electrically or thermally. It has further been found that, as a rule, more of the physical and physico-chemical properties of the different constituents of a system change at the planes of contact when a solid phase and a liquid phase coexist than when two (non-miscible) liquid phases coexist. Failure to state the exact properties to which they referred in arguing for the heterogeneity of colloid systems has undoubtedly been the cause of much misunderstanding in the discussion of this question. It must also be remembered that the general constitution of boundary planes must tend to vary the more, the greater the 21 22 GENERAL COLLOID-CHEMISTRY differences in the nature and the larger the number of the prop- erties which change abruptly at them. In this sense the surface where a liquid comes in contact with another liquid may be said to be less well denned than that where a liquid comes in contact with a solid. The importance of making such a distinction has repeatedly shown itself in the literature of colloid-chemistry as we shall see later. 2. Physical and Chemical Heterogeneity. The relation be- tween chemical (analytical) and physical heterogeneity is of especial importance for the exact characterization of the colloids as 'heterogeneous systems. It has been found that two spatially different parts of a system having the same chemical composition (chemically homogeneous therefore), may physically show typical boundaries between them, in other words appear as different phases. In illustration of this fact we need but call to mind the allotropic modifications of the elements which may coexist for a long time, as in the case of sulphur. But compounds of the same composi- tion, particularly isomers and polymers, may also form hetero- geneous systems. Thus the solid particles of metastyrol may be suspended in liquid styrol; rubber may be " dissolved" in iso- prene, etc. It is even possible to have hylotropic phases (different states) of one and the same chemical substance coexist, though usually but temporarily. Thus water and steam, or water and ice may coexist. It follows from all this that the spatial or other heterogeneity of the colloids is not connected with a chemical heterogeneity if by the latter is meant a difference in analytical composition. The existence of physical boundary planes in them can alone be regarded as characteristic of them. As a matter of fact we can distinguish between the different phases having the same chemical composi- tion in the illustrations instanced above not only by differences in their chemical reactivity, but by optical differences, by differ- ences in states of aggregation, etc., in other words by their different "body properties." But whenever we speak of (physically) heterogeneous systems we mean spatial combinations of coexisting phases. There are several types of these. Thus we have systems in which all the phases are liquid, others in which they are all solid, still others in which solid, liquid and gaseous phases are mixed. Heterogeneous sys- GENERAL CONSTITUTION OF COLLOID SYSTEMS 23 terns composed of gases only cannot long exist, for gases are miscible with each other in all proportions (see p. 43). 3. Colloids as Disperse Heterogeneous Systems Ordinarily the number of phases and their state of aggregation in a heterogeneous system can be determined by mere macroscopic examination. We need but to call to mind the above-mentioned examples of heterogeneous systems. The following two pecu- liarities serve to distinguish the colloid solutions from these macroheterogeneous systems. 1. The Phases are in Contact with Each Other under Condi- tions which Permit of the Development of much Surface between Them. First, the amount of the absolute surface with which the various phases are in contact with each other is very great. Second, and more important and characteristic is the fact that the magni- tude of the specific surface, 1 that is the quotient of the surface divided by the volume is extraordinarily great in colloid systems. This property may also be expressed by saying that there is a great concentration of surface in the unit volume. At first sight one might believe that the small size of the individual particles characterizes this class of heterogeneous systems. In fact we frequently speak of the great dispersion or subdivision of the phases. Nevertheless, it should be noted that it is only because a great number of such small particles exist in a relatively small volume that the peculiarities of such systems are produced. From this it should also be clear that it is not the absolute amount of surface which is of such importance as the relative or specific surface. 2. The Phases are so Distributed within the System that Externally the Whole Appears Homogeneous. Unless an ex- ceedingly small amount is studied, every fraction of a colloid solution has the same average composition. This second pecu- liarity is evidently closely connected with that given above, for a spatially uniform distribution of the phases is made possible only through the existence of a great specific surface. To express it in more exact terms, the uniformity of this distribution increases with the degree of subdivision of the phases. 1 Wo. Ostwald, Pfluger's Arch. f. d. ges. Physiol., 94, 251 (1903). See also J. M. van Bemmelen, Die Absorption, Ges. Abhandl., 23, Dresden, 1910. 24 GENERAL COLLOID-CHEMISTRY It should be emphasized that the first-mentioned property is more important and more characteristic of these bodies than the uniform distribution of the phases in space. It must be admitted that the conception of "colloid" or, more correctly speaking, of "colloid state" was developed in connection with the study of bodies belonging to the spatially homogeneous systems, that is to say, in connection with the study of colloid solutions. But there are systems which also belong to the field of colloid-chemistry in which the phases are not uniformly distributed within each other. To the former systems belong, for example, bodies which are in the course of swelling or bodies which have attained their maxi- mum of swelling in the presence of an excess of the solution in which they are swelling. On the other hand, colloid solutions in the process of coagulating are systems which are losing their condition of spatial homogeneity. It is evident that processes of swelling tend toward the production of a spatially homogeneous system in the sense in which this term was used above. On the other hand, the process of coagulation arises in a homogeneous system. It is because these bodies have in their past been in, or tend in their future to assume, a spatially homogeneous condition that they belong to the class of systems here under discussion. It follows further from these considerations that the colloid solutions occupy an intermediate position in colloid-chemistry, and should always be meant when colloids in general are under discussion. Thomas Graham very expediently introduced the special name of sol for these spatially homogeneous systems. In contradistinction, systems in the course of passing to or from a condition of spatial homogeneity he calls gels. Heterogeneous systems with the two peculiarities mentioned are known as disperse heterogeneous systems, as suggested by Wolfgang Ostwald 1 or simply as "dispersoids," an abbreviation proposed by P . P . von Weimarn . 2 The term ' ' microheterogeneous ' ' systems, put forward by G. Bredig, 3 is synonymous with these. The broadest generalization resulting from the investigation of the colloids seems therefore to be the establishment of the fact that they are a part of the general class of the dispersoids, in *Wo. Ostwald, Koll.-Zeitschr., I, 291, 331 (1907). 2 P. P. von Weimarn, Koll.-Zeitschr., 3, 26 (1908). von Weimarn uses this term in a somewhat modified sense. 3 G. Bredig, Zeitschr. f. Elektrochem., 12, 589 (1906). GENERAL CONSTITUTION OF COLLOID SYSTEMS 25 other words the colloid state of a substance is a special case of the dispersoid state. 4. The Disperse Phase and the Dispersion Means Detailed examination of a simple dispersoid, a suspension let us say, at once lets us recognize the fact that the two phases are geometrically or structurally different from each other. Most frequently one phase is composed of a great number of individual particles which are separated from each other. Because these particles have for the most part the same properties, we are accustomed to consider them as a whole and so to designate them as one phase. The individual particles of such a phase may approximate the spherical in form, but they may also be crystal- line; further, they may be mobile or immobile. The second phase is usually continuous and lies between the particles, droplets or bubbles of the first phase. The first phase is therefore generally suspended in the second. As we must frequently distinguish between the two phases they have received special names. The finely subdivided discontinuous phase is called the disperse phase; the other continuous or " closed" phase is known as the dispersion means. 1 English writers are in the habit of calling the disperse phase the "internal phase," the dispersion means the " external phase." French investigators distinguish between the "micelles, granules colloidaux" and the "milieu exterieur." This normal geometrical* constitution may in some instances give place to a more complex one depending particularly upon the behavior of the disperse phase. The disperse phase may also be continuous and may then extend through the dispersion means in the form of a reticulum or network. Such systems are formed in the first stages of many processes of coagulation. Evidently when the disperse phase and the dispersion means bear such a relationship to each other the distinction between them disappears; one can at best only designate the phase present in excess the dispersion means. Relations become more complex when heterogeneous systems with more than two phases are considered. In the more im- portant and typical cases several disperse phases may exist in 1 Wo. Ostwald, Koll.-Zeitschr., i, 291, 331 (1907). 26 GENERAL COLLOID-CHEMISTRY the form of spatially discrete particles in a common dispersion means. All reactions between colloids, like those of the immune bodies, take place in a common dispersion means. On the other hand, systems in which one of the disperse phases is continuous while another consists of individual particles are represented by membranes of colloid origin through which a colloid solution is being filtered. 5. Specific Surface in Dispersoids; Degree of Dispersion It has been stated that the chief characteristic of the dis- persoids is the great development of surface by their phases or the great value of their specific surface. It should now be noted that the following three kinds of specific surface may be dis- tinguished in a typical diphasic dispersoid: The absolute surface of the entire disperse phase The total volume of the disperse phase The absolute surface of the dispersion means The total volume of the dispersion means The absolute surface of an individual particle of the disperse phase The volume of an individual particle of the disperse phase The first-named specific surface is undoubtedly the most characteristic of such a system. All three specific surfaces may, but two of them must, change in value whenever changes occur in a dispersoid, which are accompanied by changes of surface. Thus if a suspension of quartz particles is diluted, only the two first- named specific surfaces change in value. In colloid systems, how- ever, the third specific surface or the size of the disperse particles often also varies with the changes in concentration, as will be shown later. As a rule all three specific surfaces are diminished in processes of coagulation. Let us now turn to a specific illustration of how great are the values of the absolute and the specific surfaces in a dispersoid, and how quickly these values increase with the progressive sub- division of one of the phases. If we assume that the given volume undergoing subdivision has and maintains cubical dimensions then Table i illustrates the increase in the total and specific volumes if the division takes place decimally. GENERAL CONSTITUTION OF COLLOID SYSTEMS TABLE i. INCREASE IN THE SURFACE OF A CUBE WITH PROGRESSIVE DECIMAL SUBDIVISION Length of one edge Number of cubes Total surface Specific surface i cm. I 6 square cm. 6 i mm. = X io~ cm. IO 3 60 square cm. 6.I0 1 o . i mm. = X io~ cm. I0 6 600 square cm. 6.io 2 o.oi mm. = X io~ cm. I0 9 6000 square cm. 6.io 3 i .0 /* = X io~ cm. I0 12 6 square m. 6.io 4 O.I /i = X io~ cm. I0 15 60 square m. 6.io 6 O.OI /i = X io~ cm. I0 18 600 square m. 6.io 8 1 . /i/i = X io~ cm. IO 21 6000 square cm. 6.io 7 O.I /i/i = X io~ cm. 10" 6 hectares 6.io 8 O.OI (JLfJi X io~ 9 cm. 10" 60 hectares 6.io 9 O.OOI fjtfj, = X io~ 10 cm. I0 30 6 square km. 6.I0 1 ' Particles somewhat less than IQ/JJU in diameter may be dis- tinguished optically by means of the ultramicroscope of H. Siedentopf and R. Zsigmondy. A cube of metallic gold subdivided up to the limit of ultramicroscopic visibility would therefore have a total surface of over 600 square meters and a specific surface of 6. io 6 . Even at this point we begin to enter the sphere of molecular dimensions. Lobry de Bruyn and Wolff 1 for instance, calculated an approximate diameter of 5/i/x for the starch molecule. If a cubic centimeter of dry starch could be subdivided into its molecules, that is if it could be " dissolved" in the ordinary sense of the word, the starch would present a total surface of several thousand square meters toward the solvent. When we deal with the molecular dimensions of gases and of substances in crystal- loid solution, assuming for their average molecular diameter the value of i.io" 8 , we obtain values of several hectares for i cubic centimeter of dissolved substance. Thus in 100 cc. of a io per cent, sugar solution there would be an "internal surface" of about 50 hectares when the smallest possible surface, the surface of a sphere, is assigned to the sugar molecule. Finally, if it is assumed that ions and electrons are also separated through surfaces from their dispersion means (and an electrical heterogeneity and the existence of electrical surfaces must be postulated in these) the absolute and especially the specific surfaces attain enormous values. 1 Lobry de Bruyn and Wolff, Rec. Trav. chim. Pays, Bas., 23, 155 (1904). 28 GENERAL COLLOID-CHEMISTRY It should further be noted that the increase in the surface of a cube with progressive subdivision may be expressed by the formula: in which SO is the total surface in square centimeters, a, the length of one edge in centimeters, and m*, the number of cubes. The original volume was taken as equal to i cc. (H. Mayer). 1 o Since the unit of specific surface = > then if a is taken as iccm equal to i, the calculated values obtained for surface represent at the same time the specific surface or the degree of dispersion. 2 The concept of specific surface may conveniently be replaced by the somewhat clearer one of "degree of dispersion" Thus we may say that the degree of dispersion increases greatly with progressive subdivision of a given phase, etc. As is well known, the surfaces of solid and liquid bodies of even ordinary dimensions already exhibit a whole series of peculiar phenomena, the intensity of which increases in direct proportion with the absolute and specific surfaces of the bodies. As examples might be mentioned the condensation of gases on solid surfaces, the manifold effects of surface tension in liquids, the fact that the majority of electrical phenomena appear at surfaces, etc. It should be remembered, however, that in such behavior the absolute surface is less responsible for these phenomena than the specific surface. Thus a few milligrams of platinum black have an effect upon an explosive gas mixture which is not equaled by that of several square meters of sheet platinum, for while they may have approximately equal absolute surfaces the former has an enormously greater specific surface. We are driven to conclude that all the phenomena observable at ordinary surfaces increase enormously in intensity and that they may even change qualitatively when we come to deal with dispersoids with their immense internal surfaces. There are also certain forms of energy that play an insignificant r61e in macroheterogeneous systems, but which play an enormous one in dispersoids. These are discussed in detail later. 1 H. Mayer, Kolloidchem. Beihefte, i, 62 (1909). 2 See also Wilh. Ostwald, Grundr. d. allg. Chemie, 4 Aufl. 531, Leipzig, 1909. GENERAL CONSTITUTION OF COLLOID SYSTEMS 2 9 6. Classification of the Dispersoids According to Their Degree of Dispersion i. Classification of Zsigmondy. It is evident that either the degree of dispersion or the number of phases in a system may be used for classifying the dispersoids. The mere number of phases is relatively unimportant as a means of classification, for the majority of the dispersoids and of the colloids in particular are either diphasic or triphasic. Classification on the basis of the degree of subdivision permits of finer distinctions. R. Zsigmondy 1 has developed a classification on this basis. According to him the field of colloid-chemistry occupies a middle position among the dispersoids thus far known. Particles about o.iju in diameter, that is, particles with a specific surface of about 6.io 5 (see Fig. 8), are stated by R. Zsigmondy to represent the lower limit of dispersion. The size of such particles is about that of the particles in emulsions and suspensions which no longer undergo separation. The value o.i/z about represents the limit of microscopic visibility. According to Zsigmondy the field of colloid-chemistry begins with particles of this size and extends up to particles about IMM in size, that is, to such as have a specific surface or degree of dispersion of about 6.io 7 , assuming that the particles are cubiform. The value i//ju is somewhat less than the diameter of the smallest particles hitherto observed by ultramicro- scopic means (about 6/*ju). On this basis of classification the colloids represent dispersions of a magnitude varying between 6.io 5 and 6.io 7 . H. Siedentopf 2 and R. Zsigmondy 3 have proposed a nomen- clature for the individual particles of typical dispersoids which is based upon their degree of dispersion. Particles visible under the microscope are termed "microns," while those which can be seen only by the application of ultramicroscopic methods are called "submicrons" or "ultramicrons." The disperse phase of colloid solutions would therefore be made up of submicrons (ultramicrons). It can be shown in several ways that particles exist whose size we know to be beyond that of ultramicro- scopic visibility. They must therefore be less than I/AJU in 1 R. Zsigmondy, Zur Erkenntnis der Kolloide, 22, Jena, 1905. 2 H. Siedentopf, Berl. klin. Woch., Nr. 32, (1904). 3 R. Zsigmondy, Zur Erkenntnis der Kolloide, 87, Jena, 1905. GENERAL COLLOID-CHEMISTRY diameter. These particles to which molecules and the products of their dissociation belong, are called " amicrons" The accompanying Fig. 8 (based chiefly on the data of R. Zsigmondy) is designed to illustrate approximately the rela- Precipil-al-ed parhcie of gold, about- 75 w Starch Chloroform Hydrogen snolecule molecule molecule -StJfj abouf0.dv{j abouKUuv FIG. 8. Comparison of particles of different size. The large circle corresponds to the diameter of a human red blood corpuscle (about 7.5 /*); the large pentagon to that of a rice starch granule of medium size (about 7.0 n). The particles enclosed in the frame are, in comparison with the rest of the figure, enlarged 333 times. The figure has been constructed from data and tables given in R. Zsigmondy (Zur Erkenntnis der Kolloide, Jena, 1905). The values for the mastic suspension are taken from /. Perrin's studies [Kolloidchem. Beihefte i, 221 (1910)]. tive sizes of the particles in typical dispersoids which have been the object of study. According to this diagram human blood corpuscles, starch granules, kaolin, and mastix particles would be microns, gold particles would be submicrons, while the finest gold GENERAL CONSTITUTION OF COLLOID SYSTEMS 31 particles, starch molecules, etc., which cannot be made out ultra- microscopically would be amicrons. It seems of interest to give here the estimated diameters of a number of molecules. The smallest molecule seems to be that of hydrogen gas, 0.067 to o.i 59^^; water vapor has a molecular diameter approximating o.i 13^^; carbon dioxide one of about 0.285/zju, 1 etc. Different methods of calculation yield different values, yet all approach the magnitude O.I/ZM or i.io~ 8 cm. The molecular diameters of hydrated ions have recently been measured in several ways. 2 The molecular diameter of NaCl was found to be 0.26^; that of sugar, o.7//ju, etc. 2. Classification of Dispersoids According to Their Degree of Dispersion. It follows from Zsigmondy's classification that dispersoids having a very small or a very high degree of dispersion do not belong to the systems to be specially considered in this book. Such dispersoids should have special names. Dispersoids with a degree of dispersion of less than 6.io 5 , that is microscopic sus- pensions, emulsions, and foams, might be called "true or coarse dispersions" while dispersoids with a degree of dispersion higher than 6.io 7 might be termed "molecular dispersoids." Roughly, the molecular dispersoids correspond with Thomas Graham's " crystal- loids." As this term is based upon a property which need scarcely determine the degree of dispersion it is not as free from objec- tion as that which I suggest. Since molecules may dissociate into smaller particles, into ions, we obtain systems which may be designated as "ionically disperse" or as "ionic dispersoids," as suggested by The Svedberg. 3 It should be remembered, how- ever, that ions are by no means always the products of dissociated molecules, and especially is this not true if such appear in colloid solutions. Such ions need not therefore have a higher degree of dispersion than the colloid particles themselves. This will be discussed later in the section on the electrochemistry of the colloids. It has further been found that the specific surface of the 1 These figures are taken from a table on p. 64 of the excellent publication of W. Mecklenburg, Die exper. Grundlegung der Atomistik, Jena, 1910. The various methods of calculation may also be found there. 2 See, for example, the summaries of G. H. Washburn, Jahrb. d. Radioaktivitat, 5, 493 (1908); 6, 69 (1900). 8 The Svedberg, Stud. z. Lehre v. d. koll. Losungen. Nov. Act. R. Soc. Scient. Upsaliensis, Ser. IV, II, i (1907). 32 GENERAL COLLOID-CHEMISTRY colloids may vary within the limits calculated by Zsigmondy, that is to say, between 6.io 5 and 6.io 7 . We may therefore expect to find that colloid solutions contain particles of different sizes. Experimental study has confirmed this expectation. Not only have different colloids very different degrees of dispersion, but one and the same substance may exist in different degrees of subdivision in a given dispersion means. As an example may be cited a series of carefully studied aqueous gold dispersoids in- vestigated by R. Zsigmondy. 1 TABLE 2. AQUEOUS GOLD DTSPERSOJDS or DIFFERENT DEGREES OF DISPERSION Designation of the solution* Color of the dispersoid Au 3 - a Rose ................... | About 6 . o Au 92 Bright red .............. j About 10 .o Au 97 Bright red .............. i 15.3 Au 9 2 b Bright red .............. j 17.0 Au 9 i a Violet red .............. About 23 .o Au 8 s a Violet red .............. 32.0 Au 2 Purple red .............. ! 38 . o Gold suspension a .......... Violet red .............. 45 .o Gold suspension b .......... Bright red .............. 95 .o Gold suspension c .......... Bluish ................. 130 . o * The designations are those of Zsigmondy (I.e.}. Zsigmondy and other investigators have prepared gold dis- persoids in which the size of the particles could not be determined. They must therefore have been smaller than 6^. This variability in the degree of dispersion within the limits characteristic of colloid solutions has been recognized in the literature of colloid-chemistry by distinguishing between sub- stances having a "strong or a weak colloidality," and different "degrees of colloidality." Substances have also been designated as systems "slightly, intermediately, highly, or completely colloid," or "coarsely disperse, finely disperse," etc. The term highly colloid is synonymous with highly disperse, etc. It is also at times advisable to distinguish between super molecularly-dis per sed phases (as in the case of ions) and submolecularly-dispersed phases. 1 R. Zsigmondy, Zur Erkenntnis d. Kolloide, 104, Jena, 1905. GENERAL CONSTITUTION OF COLLOID SYSTEMS 33 The following outline gives graphically a classification of the dispersoids according to their degree of dispersion. True or coarse dispersions (suspensions, emulsions, etc.). Size of the particles of the disperse phase greater than o.i/t. Specific surface < 6.io 5 . DISPERSOIDS Colloid solutions. Size of the particles of the disperse phase between 0.1/4 and 1/j.fj.. Specific surface between 6.io 5 and 6.io 7 . Colloidality decreases Molecular and supermo- lecular dispersoids (solu- toids). 1 Size of the particles of the disperse phase about i/u/i or less. Specific surface > 6.io 7 . Degree of Dispersion increases P. P. von Weimarn 2 has repeatedly emphasized that the so-called " super saturated solutions" (in which we are justified in assuming the existence of larger molecular aggregates) occupy a position between the colloid and the molecular-disperse systems. But there seems to be no reason for believing as von Weimarn does that supersaturated solutions always represent transitions between colloid and molecular-disperse systems; or for believing that such transition types must appear every time we pass from a high degree of dispersion to a lower one or vice versa. The con- cept of supersaturation embodies in itself no information regarding degree of dispersion which alone is the criterion for the type of classification here under consideration. Supersaturation consti- 1 This name was proposed by P. P. von Weimarn, Koll.-Zeitschr., 7, 155 (1910). It should be noted that von Weimarn wishes the terms "colloid," "colloid solution," etc., avoided and replaced by the more general terms " dispersoid," "dispersed solu- tion," etc., while the term "colloid -chemistry" is to be replaced by "dispersoid chemistry." In spite of the fact that I was the first to propose the extension of the study of the colloids to that of the disperse systems and first suggested a suitable nomenclature, yet, for obvious reasons I do not deem it advisable to eliminate the use of the term "colloid." Even the fact that the word "colloid" originally had a different meaning, namely, a more special one than it now has, does not justify the proposed measure. The word "molecule," for example, has not disappeared from science even though its exact meaning has changed frequently and consider- ably. A dispersoid chemistry, in other words, a chemistry dealing with disperse systems of all degrees of dispersion, does of course exist. Nevertheless, persistence in the use of the term colloid for at least that portion of this more general science with which this work deals seems to be justified on historical and other grounds. See my preface to the second edition of this work. 2 See P. P. von Weimarn, Koll.-Zeitschr., 6, 179 (1910); and for greater details Kolloidchem. Beihefte, i, 331 (1910). 34 . GENERAL COLLOID-CHEMISTRY tutes a possible but not the sole means of preparing submolecular dispersoids. Such may be prepared by " direct methods of dispersion." 3. Defects of this Principle of Classification. The following should be noted regarding the classification of the dispersoids according to their degree of dispersion. The degree of dispersion is manifestly a continuously variable quantity, and so it is self-evident that it may have any possible value between the extremes which characterize individual classes of dispersoids. As a matter of fact, transitional values between those which characterize the field of colloid solutions and those which characterize the molecular dispersoids, or between those of the former and those of the -coarse dispersions are not only con- ceivable but have been demonstrated experimentally. The exist- ence of transitional values may be deduced from Table 2, for at its top are gold dispersoids with particles approaching molecular values in size, while at its bottom are suspensions which can be resolved under the microscope. An analogous series of dispersoids in which the degree of dispersion varied between points lying beyond either side of the field embraced by the colloid solutions was, among others, prepared by H. Picton and S. E. Linder 1 at an early period in the history of colloid-chemistry. Dispers- oids of arsenious trisulphide in water were used. The size of the particles in these could not be determined directly, but that their degree of dispersion varied greatly was clearly demonstrated by their very different degrees of diffusibility. In the face of these facts it must be admitted that the classi- fication of the dispersoids according to their degrees of dispersion is an arbitrary one. But while this is so, there is undoubtedly a practical justification for the distinctions proposed. The dispersion values given were chosen because with changes in them abrupt changes occur in other properties of the dispersoid also. Thus the particles of a dispersoid with a diameter of o.i/i are not only no longer visible under the microscope, but at this de- gree of dispersion they also lose their diffusibility, they no longer settle out spontaneously, they do not pass through a dialyzing membrane, they no longer produce changes in the freezing and boiling points of their dispersion means, etc. On the other hand, 1 H. Picton and S. E. Linder, J. Chem. Soc., 61, 148 (1892); ibid., 67, 63 (1895). GENERAL CONSTITUTION OF COLLOID SYSTEMS 35 all these properties rise and fall in value, greatly and suddenly, when molecular dimensions are approached. These ^continuous changes of other properties therefore form the true basis for the classification of the dispersoids on the basis of their degree of dispersion. But that a quantitative characterization of the dispersoids according to their degree of dispersion is important is evidenced by the fact that dispersion in itself must be regarded as the chief characteristic of the substances with which this book deals. 4. Polydispersoids. It has frequently been found in deter- mining the degree of dispersion in dispersoids, such as colloid solutions, that the individual particles of the disperse phase are of different sizes, in other words the degree of dispersion of the disperse phase must be described as multiple. Accord- ing to L. Michaelis, 1 examples of such systems are found in the aqueous solutions of certain dyes, such as fuchsin, methyl violet, etc. In these there is a molecular-disperse phase in addi- tion to a phase' observable under the microscope or ultra-micro- scope. Many protein solutions probably behave in an analogous way, as may be inferred from their ultrafiltration behavior (see later) ; but even the individual, ultramicroscopically observable particles in dispersoids (as in those of gold) are frequently of different sizes. It follows therefore that in practice we can only speak of an average dispersion value. The importance of the simultaneous existence of particles of y different sizes in one and the same dispersion means in many questions of colloid- chemistry, for example in that of their stability, will be discussed in detail later. These systems in which the disperse phase is composed of particles having different degrees of dispersion may be called polydisperse systems or poly dispersoids. 5. Dispersoids Varying with Changes in Concentration. In a number of molecular as well as colloid dispersoids the re- markable fact has been observed that the degree of dispersion varies progressively with changes in concentration. In all the cases thus far studied it decreases with increasing concentration. Cane sugar, for example, in dilute solution has all the typical 1 L. Michaelis, Deutsche medizin. Wochenschr, Nr. 24 (1904); Virchow's Arch., 179, 195 30 GENERAL COLLOID-CHEMISTRY attributes of a molecular dispersoid. But when cane sugar solu- tions of higher concentrations are investigated by applying the Tyndall test to -them it is found that they show an intense light- cone thus proving themselves submolecularly disperse. Entirely analogous observations have been made on solutions of various salts such as aluminium sulphate, and on those of certain dyes, proteins, etc. No doubt careful investigation will demonstrate the wide-spread nature of this remarkable fact. It should be added that such a progressive decrease in the degree of dispersion by simply changing the quantitative relations of the dispersoid to the dispersion means may be demonstrated by yet other than purety optical methods. Further details will be given in discussing the individual physico-chemical properties of the colloids especially in the chapter on their internal changes of state. Analogous phenomena are encountered in studying the properties of mo- lecularly and supermolecularly dispersed systems, being then de- scribed as "polymerizations, condensations," etc. We will call these systems, among which many colloid disper- soids appear, "concentration-variable systems" 6. Temperature -variable Dispersoids. Just as the degree of subdivision of a dispersoid may vary with changes in concentra- tion, it may also vary with changes in temperature. As far as we know now, raising the concentration of a dispersoid produces the same type of change as lowering its temperature. A disperse system therefore tends to become less disperse when the tem- perature is lowered. Such anomalous behavior in "true" solu- tions has generally been explained by saying that the substances "polymerize" or "condense." 1 An analogous behavior, resulting in diminutions of degree of dispersion, is found in even greater degree in colloid systems. Here we can only point out the fact; it is dealt with in detail in the chapters on internal changes in state, more particularly in that on gelation. Dispersoids showing this property are called "temperature- variable dispersoids" 7. Complex Dispersoids. There exists another class of com- plex systems which is interesting for both the theory and the practice of colloid-chemistry. It is characterized by the fact that each component of such systems, both disperse phase and dispersion 1 For examples and literature see H. Schade, Koll.-Zeitschr., 7, 26 (1910). GENERAL CONSTITUTION OF COLLOID SYSTEMS 37 means, is in itself a dispersoid. Evidently the degree of dispersion in these individual dispersoids must always be higher than that of the compound dispersoid. And in fact, the best-known examples of such systems are those in which the individual dispersoids have a molecular degree of dispersion, while the compound dispersoid is colloid or molecular-disperse in character. The best examples of such "complex dispersoids" are found among the emulsions, that is among those systems in which both phases are liquid. Pretty instances are formed by the so-called critical mixtures of liquids and their analogues. As is well known, it is possible at suitable concentrations and at suitable temperatures to make a dispersoid of two liquids which have a limited molecular solubility in each other. As an example may be cited the production of an emulsion of phenol in water. Since all liquids are mutually soluble to some extent at least, this type constitutes the bulk of the dispersion systems having a liquid-liquid composition. Such complex dispersoids are characterized by the fact that changes in the concentration or in the temperature of the macrodisperse system are accompanied by changes in the composition of the microdispersoids. Thus when droplets of phenol are dis- persed in water, both phases contain phenol as well as water. If the concentration of the emulsion is changed through the addition of one of its components, for example water, the composition of both microdisperse phases is also changed. As more water is added the phenol phase becomes progressively richer in water until a limit is reached (until the phenol is saturated with water), etc. Variations in temperature produce analogous effects. Another peculiarity of these complex dispersoids which should be emphasized is that in addition to the fact that the composition of the individual phases changes with variations in concentration or temperature, their degree of dispersion does also, and apparently whenever a change is produced in the total concentration. Thus the droplets in mixtures of liquids of limited mutual solubility become progressively smaller as the mixtures approach the so-called critical concentration, and disappear altogether at the " critical point;" in other words, the droplets become molecular-disperse. But at constant temperature the critical concentration is always less than the concentration of the fluids in a coarsely disperse state. Here again there exists exactly the same variation in 3 GENERAL COLLOID-CHEMISTRY degree of dispersion with change in concentration that was previously described. 1 What was said above regarding simple dispersoids holds for the influence of temperature on composition and degree of dispersion in complex dispersoids also. The complex dispersoids are con- centration-variable and temperature-variable systems. Dispersoids with a liquid dispersion medium and a solid dispersion phase may also form complex systems, but up to the present time these have been studied but little. It is self-evident that a solid particle floating in a liquid may either take up part of it into itself, or attach a layer of it to itself. Such behavior may be observed macroscopically when solid gelatine is pulverized and thrown into cold water. Each particle then " swells," that is, it absorbs water, but if the temperature is low enough it does not lose entirely the properties of a solid. But the properties of a solid, such as constancy of form and elasticity, become less marked as the solid particles take up more water or as the temperature rises. This very important behavior, which therefore consists in an approximation of the previously solid state to that of a liquid, will be discussed in detail later (see page 44). Let it further be pointed out that complex dispersoids may be expected to appear more frequently in systems composed of two liquid phases than in those composed of a liquid and a solid phase. This depends upon the fact that a greater mutual molecular miscibility may be assumed to exist in the case of two liquids than in the case of a liquid and a solid phase, and second, upon the fact that the "solubility" of two liquids is usually mutual. Both phases will therefore be disperse in a liquid-liquid dispersoid. On the other hand, while we may be able to speak of the "solu- bility" of a solid phase in a liquid dispersion means, we will only infrequently be able to speak of the "solubility" of the dispersion means in the solid disperse phase. It seems of interest to mention in this connection that many reasons have recently been found for assuming the existence of similar phenomena in molecular and ionic dispersoids. Such complexes are called "solvates," or if they occur in aqueous 1 It should be noted that concentration is always regarded as the quotient of the amounts of the ;. In the range above the critical point this fraction dispersion means is reversed, in that the dispersion means becomes the disperse phase and vice versa. GENERAL CONSTITUTION OF COLLOID SYSTEMS 39 solutions, " hydrates." In these compound disperse phases there also occur variations in composition with changes in concen- tration or in temperature entirely similar to those discussed above. 8. Transition Phenomena. The transition phenomena ob- served in passing from the members of one class of dispersoids to those of another having a different degree of dispersion are par- ticularly interesting. Our knowledge of the properties of dis- persoid systems is at present distributed in such a way that we may say we know a great deal about typical molecular disper- soids, somewhat less about typical colloids, and still less about typical coarse dispersions. But the atypical representatives of all three classes, that is, the transition forms between coarse dis- persions and colloids on the one hand, and between colloids and molecular dispersoids on the other, have been almost entirely neglected. There is an historical reason for this state of affairs. As is well known, the founder of colloid-chemistry, Thomas Graham, was so impressed by the differences between typical colloids and typical molecular dispersoids that he declared the two to represent "different worlds of matter." He endeavored in consequence to contrast them as much as possible. The ma- jority of his successors followed him in this, and only recently has the effort been made to cease discovering rare and sharp distinctions between colloids and molecular dispersoids. As a matter of fact no such sharp distinctions exist. But the realiza- tion of this fact was important in that it yielded a new point of view, on the basis of which it became possible to formulate the concept of the dispersoid, and with it to obtain a rational systematization of these bodies 1 (see later). It must be em- phasized, however, that even today comparatively few investi- gations are carried out with the conscious purpose of studying these transition phenomena, more especially the changes which the individual physical and physico-chemical properties exhibit with progressive variations in the degree of dispersion. In some forthcoming chapters of this book (Part V, on the History of Colloid- chemistry) we shall call attention to many of these transition phenomena. 1 See Wo. Ostwald, Koll.-Zeitschr. i, 291, 331 (1907). 4 797) 1009, etc. (1907)] first classified colloids according to the type of the dis- perse phase is not correct. The statements of Quincke which no doubt led to this historical error, do not refer to colloid systems but to coarsely dispersed ones. Quincke holds all colloid solutions (including those of arsenious trisulphides) to be mixtures of two fluid phases (I.e., 1009, 1034, etc.). See in this connection G. Bredig [Ann. d. Physik., 4, n, 221 (1903)]. 2 B. J. Richter, see Wilh. Ostwald, Koll.-Zeitschr., 4, 5 (1909). 3 M. Faraday, Philos. Mag. (4) 14, 401, 512 (1859). 4 J. Berzelius, Lehrb. d. Chemie, 2 Aufl., 2, 244 (1823). GENERAL PROPERTIES OF COLLOID SYSTEMS 53 differ from each other in the matter of the state of their disperse phases and in the properties which result from this fundamental difference. We shall of course not disregard the necessity of proving this assumption later. 1 4. The Frequency of Occurrence of Complex Emulsoids. Let us anticipate a particularly important generalization which follows from characterizing the "lyophilic" colloids as systems having the composition Liquid + Liquid. The behavior of such colloids demonstrates clearly the liquid character of the disperse phase in them and emphasizes, first, that the "lyophilic" colloids are complex dispersoids, that is to say, their individual phases are in themselves dispersoids of a higher degree of dis- persion, and, second, that the composition of these individual dispersoids as well as their degree of dispersion varies greatly with concentration, temperature, etc. Further, in consequence of the complex character of these systems the state of the disperse phase may pass progressively from liquid through semisolid to solid and back again. The possibilities of variation, in the state of a complex dispersoid with degree of dispersion, temperature, concentration, etc., as discussed on p. 45, appear very clearly in these colloids and undoubtedly constitute one of the chief reasons why their behavior is so much more varied than that of colloids having the composition Liquid + Solid. 2 The value of this conception will also show itself in discussing the individual phenomena characteristic of these systems, when it will be- come evident that the different theoretical views held regarding the properties of these two classes of colloids may not only be corre- 1 An exact classification and description of both classes of colloids according to the type of the phases was indicated in the first edition of this book. Since then I have found only further and very excellent support for this view. I hope in the near future to publish a monograph on the physical theory of colloids of the composition Liquid + Liquid (see the following footnote). 2 Even in the first edition of this book (pp. in, 328, 356, 374, etc.) I empha- sized that lyophilic colloids are not "only" systems of the composition Liquid + Liquid, but also dispersoids of a "higher order," or, in the words used above, complex dispersoids. This has not been taken into account by those writers who have objected to my characterizations by pointing out that colloid mercury which consists of two fluid phases has no lyophilic properties. They have erroneously ascribed to me the view that all Liquid + Liquid systems have such properties, while actually I merelv 11-1.1 *.*. i *i* ii*ii.* r ims. But it may view tnat all .Liquid -f- Liquid systems nave sucn properties, wnile a< held the narrower opinion that lyophilic colloids belong to these systei be pointed out again that for the reasons given on p. 47, complex systems may be expected in systems of the type Liquid + Liquid with greater certainty than in those of the type Liquid + Solid. A complex composition is therefore more general and commoner in a Liquid + Liquid dispersoid. If colloids of the composition Liquid + Liquid were unknown it would be necessary to seek them and they would no doubt be easily found. 54 GENERAL COLLOID-CHEMISTRY lated but be simplified and explained if the differences in the types of the disperse phases and the consequences thereof are kept in mind. 5. Relation of These Two Colloid Classes to Molecular Dis- persoids. Many investigators have pointed out that greater similarities exist between "lyophilic" colloids and typical mo- lecular dispersoids than between the latter and colloids of the type Liquid + Solid. Without going into details which must be reserved for later, we may emphasize that such relations may be expected on the mere basis of our characterization of the "lyophilic" colloids as Liquid + Liquid systems. Physical chemists have recently become increasingly certain that the highly disperse phases of molecular and supermolecular solutions must be conceived of as combined with a number, sometimes a very considerable number (100 and more), of the molecules of the solvent as sohates. Even though, as emphasized before, we cannot speak of the state of aggregation of a molecule, such a union of solvent with molecule cannot be conceived of physically other than as a highly disperse liquid. As a matter of fact, re- cent workers on the theory of solution speak of " droplets." 1 This widespread impression of the existence of a closer relation- ship between "true solutions" and "lyophilic" colloids than between the former and "lyophobic" systems therefore corre- sponds with the conceptions above presented. 6. Suspensoids and Emulsoids. In view of the relations exist- ing between these two classes of colloids and the corresponding coarse dispersions it seems expedient to give the former special names. R. Hober 2 introduced the name suspension colloids for the colloids of the type Liquid + Solid. An analogous term for the second class would be: emulsion colloids. The abbrevia- tions suspensoids and emulsoids have been suggested by P. P. von Weimarn. 3 These will be employed in the succeeding pages of this book. If one wishes to characterize a colloid in greater detail one may speak of "poly suspensoids" (systems composed of solid phases having different degrees of dispersion), of "com- plex emulsoids," etc. No objection can, of course, be raised against expressions like "lyophilic emulsoids." Only it should be re- membered that the terms "suspensoids" and "emulsoids" in 1 See K. Drucker, Zeitschr. f. physik. Chem., 67, 634 (1909). 2 R. Hober, Physik. Chem. d. Zelle., 2 Aufl., 208, Leipzig, 1906. 3 P. P. Von Weimarn, Koll.-Zeitschr., 3, 26 (1908). GENERAL PROPERTIES OF COLLOID SYSTEMS 55 contradistinction to "lyophilic" and "lyophobic colloids" have the advantage of expressing more definite and hence more fruitful views regarding the properties of the dispersoids. A view which connects the state of the disperse phase with the general concep- tion of the dispersoid seems incomparably more concrete, more useful, experimentally, and more suggestive than, for example, the conception of "lyophilia." 11. Transition Phenomena between Suspensoids and Emulsoids As already mentioned, it is possible for a phase to pass smoothly from a solid to a liquid state and vice versa. Often such progressive changes may occur during the process of coagulation in one and the same system, as in a complex dispersoid. Thus an originally liquid disperse phase may be precipitated in an almost solid condition by appropriate means of coagulation. Such a transition from emulsoid to suspensoid demonstrates par- ticularly well the properties which result from a change in the state of the disperse phase. According to J. Friedlander, 1 for example, two kinds of systems may be prepared from alcohol, rosin and water, both of which are turbid, thus proving them disperse heterogeneous systems. The first of these is made by pouring a few drops of an alcoholic solution of rosin into an excess of water when the rosin, which is practically insoluble in water, separates out as a solid disperse phase while the alcohol, in greater part at least, is dissolved in the water. The second is made by adding a few drops of water to a concentrated alcoholic solution of rosin. In this case the first drops of water probably dissolve in the rosin-alcohol, but further amounts can dissolve only in the alcohol or, perhaps, succeed in withdrawing this from the solution so that small droplets of water-alcohol (liquid) appear in the liquid, alcoholic solution of rosin and make it turbid. A disperse hetero- geneous system with a solid disperse phase as well as one with a liquid disperse phase may therefore be prepared from the same three components by appropriate changes in their concentration. Friedlander found the behavior of the two systems to be entirely different. "Such a turbid mixture (a concentrated alcoholic solution of rosin to which a little water has been added) behaves very differently from the ordinary rosin suspension in that it is 1 J. Freidlander, Zeitschr. f. physik. Chem., 38, 430 (1901). 56 GENERAL COLLOID-CHEMISTRY not coagulated by an increase in temperature or on the addition of electrolytes. When the temperature is lowered the rosin phase becomes solid but is not coagulated, for a rise in temperature restores the system to its previous condition. Although pre- viously irreversible, the system is now completely reversible." 1 Friedlander further found the internal friction of the second kind of system to be greater than that of the first. In this respect the second system closely resembles typical emulsions such as those of isobutyric acid in water. A detailed study, qualitative and quantitative, of these systems would evidently be of great interest for the classification and characterization of. disperse systems on the basis of the state of the disperse phases entering into their composition. Transitions from suspensoids to emulsoids and vice versa exist also among the colloids proper. Nearly all protein solutions, for example, are emulsoid in character; they are viscous, flocculated only by large quantities of electrolytes, etc. Yet 0. Hammarsten, 2 found that a neutral solution of salt-free serum globulin is coagulated by minute quantities of salt (o.i to 0.3 per cent. Nad); and according to W. Erb 3 the same is true of a plant protein, vitellin. According to H. Freund- lich and W. Neumann, 4 many dyes show an emulsoid character in aqueous solutions and a suspensoid character in alcoholic solutions. Solutions of these substances in mixtures of the two dispersion means must evidently exhibit transitions between suspensoids and emulsoids similar to those which Fridlander discovered. Systematic investigations in this field would also be of importance for the theory of the colloid state. Finally, we will here point out that one and the same substance may appear either in the suspensoid or in the emulsoid state in one and the same dispersion means depending only upon the conditions under which it is prepared. 12. The Crystalline (Vectorial) Constitution of the Disperse Phase i. The Concept of Crystallinity. As is well known, most solid substances as well as a limited number of liquids are character- 1 J. Freidlander, I.e., 432, 433. 2 O. Hammarsten, Pfluger's Arch., 18, 38; see also Zeitschr. f. physiol. Chem., 395 3 W. Erb, Zeitschr. f. Biol., 41, i (1901). 4 H. Freundlich and W. Neumann, Koll.-Zeitschr., 3, 80 (1908). GENERAL PROPERTIES OF COLLOID SYSTEMS 57 ized by the fact that when their viscosity is sufficiently great their optical, elastic, dielectric, etc., properties are dependent upon the arrangement of their molecules in space. Besides the vectorial nature of these and other properties of such systems, which we usually designate as crystalline, these systems assume a definite external shape when their internal friction is sufficiently great. In the most characteristic cases this external shape is made up of a series of plane surfaces. A detailed discussion of the distribution of the crystalline state in nature, or of the question of whether so-called amorphous solids are only under-cooled liquids 1 is out of place here. Let it be noted, however, that some investigators like M. L. Frankenheim 2 and P. P. von Weimarn are so convinced of the wide distribution of crystal- linity or vectoriality that they have declared the crystalline state "the only internal state of matter." P. P. von Weimarn, es- pecially, believes that the crystalline (vectorial) state is char- acteristic of all solid, liquid and even gaseous substances, and that generally speaking no amorphous substances exist in nature. 3 But evidently there has been confused here the possibility of demonstrating crystalline (vectorial) properties in all manner of substances in every state with the actual existence of vectoriality in these as postulated by P. P. von Weimarn. While all gases may be transformed into liquids and most of these into crystalline solids, only a relatively small number of liquids (and of these only certain ones which exhibit special chemical properties such as "molecular chain formation," etc.) are possessed of experimentally demonstrable crystalline properties when in the liquid state; and up to the present time no evidence at all is at hand to indicate the existence of a crystalline structure in gases. From this it follows that the "intensity of the vec- torial chaining together of the molecules" (P. P. von Weimarn) is so slight in all gaseous and most liquid systems that it is of no importance. The assumption of vectoriality in these systems is in consequence superfluous, for it leads to no fruitful deductions. 1 See especially the recent and extensive discussion with references to the litera- ture by C. Doelter, Koll.-Zeitschr., 7, 29, 86 (1910). 2 P. P. Von Weimarn. See his numerous discussions in Koll.-Zeitschr., 2, and subsequent volumes, especially 6, 32 (1910). 3 The earlier literature is extensively discussed and in part cited verbatim by O. Lehmann, Molekularphysik, I, 716, Leipzig, 1888. 58 GENERAL COLLOID-CHEMISTRY It must further be emphasized that the concept of crystal- linity or vectoriality is as ambiguous a one as is that of hetero- geneity (see the next paragraph) . For a system may be vectorial or crystalline in certain of its properties while it is isotropic in others. All solid crystals, for example, are vectorial in shape, but crystal- line liquids have, generally speaking, only an optical vectoriality. On the other hand, all solid crystals of the regular system, for instance, are not vectorial in their refraction coefficients. Other types of crystals exhibit different degrees of optical vectoriality. A characterization of systems according to their vectoriality is therefore somewhat arbitrary, since it is always necessary to state which of the properties are vectorial. The failure of investigators to consider that different kinds of crystalline systems and different kinds and degrees of vectoriality must be distinguished accord- ing to the kind and the number of the properties of the vectorial state has undoubtedly contributed its share toward confusing the problem of the relations between crystalline and amorphous, solid and liquid states of substances. 2. Direct Proof of Crystallinity in Colloids. The most fre- quently applied and simplest practical method of recognizing crystalline properties is the optical. As indicated in their definition, it must be impossible to prove by any direct methods such as the microscopic, that colloids possess a crystalline struc- ture. Ultramicroscopic methods in place of microscopic can only be of limited use, for they give no direct "image" of the object. A whole series of optical facts have, nevertheless, been accumulated in favor of a crystalline constitution of the disperse phase of metallic sols. These will be discussed in detail when we consider the optical properties of colloid sys- tems. Upon such and similar grounds, investigators like R. Zsigmondy, H. Siedentopf, A. Cotton and H. Mouton have been led to believe in the possibility if not in the prob- ability of the crystalline nature of metallic sols at least. 3. Indirect Proof for the Crystallinity of Colloid Phases. The Crystallinity Theory of P. P. von Weimarn. Since we have no direct evidence besides the ultramicroscopic upon which to base conclusions regarding the vectorial state of colloid disperse phases we are compelled to resort to indirect means based upon theoretical considerations and exterpolations. Most of these conclusions GENERAL PROPERTIES OF COLLOID SYSTEMS 59 are based upon the assumption that particles retain their crys- tallinity even when their size is progressively changed. Such con- clusions were drawn early in the history of colloid-chemistry; and if the "reguline" state of a metal may be considered as crystal- line or cryptocrystalline, B. J. Richter (1862) may be regarded as the first to have urged the view that suspensoid phases have a crystalline constitution. By far the most convincing evidence in favor of the view that the disperse phase retains its crystalline il f <,:,; n tiuperoiU i llj'il li-ibo'iv 1 ':, e w.\ it: drjf.s '!, cmnamwm \k w til IP', IP' i< i{ 1:1 Cl :i ; t,j dew tlii , ; : nd c:il :ula.1 itui i<. h> :n )l ].] .[l;,T13J .ij (' cm tie is,::! :i" :i:t i J$DJ:I 'I-" 1 ll i| i Ullirn ; lij ::|H M tl"i ii'l: p 1|| 'i ii Bi- ll ;!' i <] II 60 GENERAL COLLOID-CHEMISTRY (b) Another point in favor of the crystalline constitution of suspensoid phases is their power of starting crystallization in supersaturated molecular-disperse solutions of themselves. Gener- ally speaking, only such solids have this power which are them- selves crystalline. Yet as von Weimarn himself found, highly disperse sols lose this power when their degree of dispersion is sufficiently increased. It is fair to attribute this to the law that the solubility of a substance is dependent upon its specific surface, that is to say, rises greatly with extreme subdivision (see p. 74). Highly dispersed particles would therefore not initiate crystalliza- tion in supersaturated molecular-disperse solutions, because the latter are still unsaturated with regard to them. Wilhelm Os t- wald's finding 1 that small quantities of salol, made highly disperse by trituration with an indifferent substance, are unable to effect the crystallization of supermolten salol, even though the salol is still demonstrable analytically, may also be thus interpreted. (c) That the particles of sols may coalesce to form micro- crystalline bodies and even definite crystals after long standing is another fact in favor of the crystalline ty of suspensoid phases. Thus P. P. von Weimarn 2 found silver crystals to form in a silver hydrosol after this stood a while. The ultramicroscopic observations of M. Traube-Mengarini, 3 of J. Amann 4 and of L. Pelet and A. Wild 5 who noted the direct formation of crystalline bodies by a simple coalition of ultramicroscopic particles in colloid lead (lead oxyhydrate), colloid iodine and colloid dyes are even more convincing evidence of the possibility of a "direct colloid crystallization," that is, a direct fusion of ultramicroscopic particles to form definite crystals. One is inclined to believe that only vectorial particles can have the power of growing into definite crystals, just as one believes that only such may produce crystalli- zation in supersaturated solutions. Yet we must point out even here that the crystalline character of these "crystalline elements " has been disputed by a whole series of investigators (see below) . From these and other facts we may conclude that most sus- pensoids, that is dispersoids having a degree of dispersion of 6.1 o 5 1 Wilh. Ostwald, Z. f. physik. Chem., 22, 289 (1897). 2 P. P. von Weimarn, Koll.-Zeitschr., 4, 317 (1908); 5, 62 (1909). 3 M. Traube-Mengarini and A. Scala, Koll.-Zeitschr., 6, 65 (1910). 4 J. Amann, Koll.-Zeitschr., 6, 235 (1910). 5 L. Pelet- Jolivet and A. Wild, Koll.-Zeitschr., 3, 175 (1908). GENERAL PROPERTIES OF COLLOID SYSTEMS 6 1 to 6. io 7 are possessed of a crystalline disperse phase. But serious objections may be raised to the assumption that all solid disperse particles are crystalline as P. P. von Weimarn, for example, has advocated. Thus, as mentioned above, the crystallinity of large masses of all solids under all circumstances has not been demon- strated experimentally. Even though most substances may be obtained in crystalline form, yet under many circumstances the "vectorial chaining together of the molecules" is so slight or so loose that vectorial properties are no longer observable. While it is true that proteins may be obtained in crystalline form, yet the solid precipitates from protein solutions, except as produced under special conditions, exhibit no trace of crystallinity. Under such circumstances it is therefore just as suitable to assume that the intensity of vectorial chaining is zero as to postulate a " latent" crystallinity. 4. Dependence of Crystallinity upon Size of Particles. There remains the possibility that the general assumption upon which all these indirect proofs of the crystalline nature of the colloid disperse phase depend, namely, the retention of vectoriality in extremes of dispersion, is not valid for the degrees of dispersion here under discussion. From the behavior of liquids in the proc- ess of solidification we are compelled to assume that solids have a positive surface tension even though its effects do not become clearly evident because of the great internal friction possessed by solid substances. But, as will be discussed later, the surface energy expressive of this surface tension increases markedly with every increase in the specific surface; in other words, a greater centripetal force acts upon the molecules of highly disperse particles than upon those of coarsely disperse particles. It seems not impossible that such a positive surface tension may produce a deformation in minute crystals, in other words, destroy their structural vectoriality by rounding off their corners and trans- forming them into spheroidal bodies. As shown by the behavior of liquid crystals, the optical vectoriality, for example, of such a particle need not be destroyed in such a process. It could there- fore be possible that the free surface tension of solid particles might attain values in extreme degrees of dispersion sufficient to destroy the vectorial chaining together of the molecules responsible for crystallinity. An investigation of the influence of pressure 62 GENERAL COLLOID-CHEMISTRY upon the optical properties of crystalline liquids would be of interest in this connection. Further, it might be possible that a relation exists between compressibility and vectoriality of such a nature that easily compressible substances lose their structural vectoriality at lower degrees of dispersion than less compressible ones, etc. It is of importance that such an influence of the free surface tension which increases with the specific surface is not only con- ceivable theoretically but is often demonstrable experimentally. In fact the influence of this factor has been repeatedly observed in that most striking expression of the vectoriality of any system, namely, its crystalline form. As long known from microscopic ob- servation of processes of crystallization, small spherical bodies (globulites) are first seen to appear which in no way resemble crystals. 1 It is only after these globulites have attained a certain size that they assume crystalline shape. Crystals with rounded edges are seen to appear, and so on. 2 According to Link, Franken- heim, Vogelsang, Behrens, Quincke, Biitschli, and many others, crystals are of ten formed by the coalescence of these microscopically isotropic globulites, from which there then result "margarites," " honeycombs," etc. 3 It would be of interest to determine whether other changes in the vectoriality of these primary crystals, more particularly changes in their optical properties, also develop as do the structural properties 4 or whether they exist from the first in even the smallest globulites. Such a microscopic investigation might perhaps be extended to ultramicroscopic refraction studies 1 See Wilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl. I, 1042. 2 See the beautiful microphotographs of P. P. von Weimarn in Koll.-Zeitschr., 2 (1908). 3 Splendid photographs of such honeycomb structures of cystalline materials are found in O. Biitschli, Untersuchungen iiber Structuren, Leipzig, 1898. See also the numerous, convincing observations of G. Quincke [Ann. d. Physik. (4), 9, i (1902)] as well as the earlier monographs of H. Behrens, Die Kristalliten, Kiel, 1874; H. Vogelgesang, Die Kristalliten, edited by F. Zirkel, Bonn, 1875. A. partial reprint of the early views may be found in O. Lehmann, Molekularphysik, i, 730, Leipzig, 1908. Especial reference should be made to the excellent observations on Asaron of C. Schmidt [Liebig's Ann. ,53, 171 (1845)] who observed a perfectly regular coalescence of four droplets. For a discussion of the vectorial arrangement of coarsely dispersed particles see R. Krulla [Zeitschr. f. physik. Chem., 66, 126 (1909)]. Wilh. Ostwald [Lehrb. d. allg. Chem., 2, Aufl. i, 1040, Leipzig, 1903] also recognizes the possibility of a "discontinuous" development of crystals from particles which were originally spherical. But in the end, the question of the state of these "crystal embryos" is still to be regarded as open (see p. 61, in the text). 4 It is important to note that we may not apply to all matter a vectoriality observed photographically in the finest precipitates of some solid substances. The degree of effect of positive surface tension upon form depends also upon the internal friction, etc., of the particles and this varies considerably in different cases as evi- denced by the so-called "liquid crystals." See in this connection the work of P. P. von Weimarn cited in the next footnote as well as the text on p. 61. GENERAL PROPERTIES OF COLLOID SYSTEMS 6 of colloids. If, for example, vectorial differences in the refraction coefficients of many crystals continue to exist when they become extremely small, then the same should be true of the corresponding refraction discs. An investigation of other properties of highly disperse solid particles, such as the thermal and electrical, would also be important. Attention should here be redirected to the conclusion reached above, that solid particles become more and more like liquids as their degree of dispersion increases ; and to the converse of this which P. P. von Weimarn among others has assumed to be the case. It therefore seems possible theoretically that a development of crystals may take place in that the "crystal embryos" are at first liquid and only later become solid as they enlarge, either be- cause of a "progressive " coalition of molecularly-dispersed particles or through a discontinuous union of submolecular phases. That such seems to be the case is evidenced by the investigations of the observers mentioned above. Wilhelm Ostwald (/..), in dis- cussing the analogous process of crystal formation in molten masses, even says : ' ' The precipitation of the insoluble from liquids seems always to occur primarily 1 in the form of, droplets, that is, in the state of an under-cooled liquid." If the dispersion in such a system were to become fixed at such, or more correctly, at a some- what earlier moment, a highly disperse system (among which colloid systems would be found) containing a liquid phase would result. In other words, at the beginning of crystallization a structural vectoriality would be lacking. Whether an optical vectoriality exists at this stage remains to be determined. Finally, it seems safe to assume that the form of development of crystals will also vary with the nature of the crystallizing substance. It is evident from all this that the question of the maintenance of crystallinity, in other words the question of a complete vectori- ality of the disperse particles, more particularly of the disperse particles of solids in high degrees of dispersion, 2 cannot yet be settled with entire certainty. 1 The italics are mine. This view is also held by G. Quincke (Ann. d. Physik., 9, 10, etc.). 2 P. P. von Weimarn, in a recent paper [Koll.-Zeitschr., 6, 32 (1910)] holds that an influence of the degree of dispersion upon the form of solid particles only becomes effective if their size is less than 5/iju, especially in the case of slightly soluble and difficultly fusible materials. The basis for this is derived from a "purely kinetic viewpoint" dependent upon kinetic views regarding the physics of the various " degrees of orientation" of molecules in the body and in the surface layers of a crystal. I confess that to me this argument is not convincing. 64 GENERAL COLLOID-CHEMISTRY 5. Crystallinity of Emulsoids. Since only a relatively small number of crystalline liquids are known, we may expect to encounter crystalline emulsoid phases but rarely. In fact, while a number of coarse emulsions having a crystalline disperse phase are known 1 not a single example of a crystalline emulsoid is known. This is in part due to the fact that it is rarely possible to make out optically the particles of a disperse phase and thus to investi- gate their vectorial properties, because of the slight difference of refraction between them and the dispersion means. It is of course not impossible that future investigators may demonstrate the existence of dispersoids having a crystalline emulsoid phase. In this connection the behavior of crystalline liquids when near their "clarification point 7 ' should be -borne in mind (see the literature cited in the accompanying footnotes). It must further be remembered that all degrees or grades of vectoriality may be demonstrated, particularly in liquids. 2 Not only do we find examples of different degrees of structural, optical, etc., vectoriality among liquid crystals and crystalline liquids, but as shown by O. Lehmann, many different external factors may influence the kind and the degree of vectoriality. Pressure and traction, the "adsorptive power" of solids, magnetic influences, changes in temperature or of the solvent, the presence of other substances, etc., are all of importance. There are liquids which assume vectorial properties only under the influence of powerful external agencies. Thus, A. Cotton and H. Mouton 3 showed that certain organic liquids of high molecular weight become doubly refractive in a strong magnetic field. Similar facts have long been known regarding many typical emulsoids, 4 such as concentrated gelatine solutions (jellies) when under the influence of pressure or traction. As is well known, all the contractile elements of living substance exhibit double refraction. 5 Here we deal with a temporary vectoriality which exists only when certain systems are under the influence of transitorily active 1 See the numerous examples in O. Lehmann, Fliissige Kristalle, Leipzig, 1906. 2 See the lecture of O. Lehmann, Fliissige Kristalle und die Theorien des Lebens, 29, Leipzig, 1906. 3 A. Cotton and H. Mouton, Compt. Rend., 141, 317, 349, etc. (1905). 4 See the numerous examples investigated by G. Quincke, Drude's Annalen d. Physik., 7, 9, 10, n, 12, 13, 25 (1902 to 1904). 6 A recent comprehensive presentation of these relations may be found in W. Engelmann, Ber. Berl. Akad. d. Wiss., 694 (1906). GENERAL PROPERTIES OP COLLOID SYSTEMS 65 agencies; or which is produced through the absorption 1 of submicroscopic aniso tropic particles. 2 These systems would therefore be classed as possessing the lowest possible grade of vectoriality both with regard to intensity and to number of vectorial properties. From all of which it becomes somewhat arbitrary whether we will follow O. Lehmann 3 and P. P. von Weimarn 4 in describing such systems as possessed of an "artificial vectori- ality" and as " liquid-crystalline," or not. 1 H. Ambronn, Ber. d. D. Botan. Ges., 6, 229 (1888); 7, in (1889); Koll.- Zeitschr., 6, 222 (1910). 2 Details regarding double refraction in emulsoids will be found in a forthcoming chapter on the Optical Properties of Colloid Systems. 3 O. Lehmann, Verh. d. D. physik. Ges., 10, 321 (1908); 10, 406 (1908). 4 P. P. von. Weimarn, Koll.-Zeitschr., 3, 166 (1908). CHAPTER III GENERAL ENERGETICS OF THE DISPERSOIDS 13. Surface Energies 1. Forms of Energy Characteristic of Dispersoids. The fore- going pages have dealt with the general and topographical characterization of dispersoid systems, more particularly colloid systems. It is our next problem to discuss the more important forms of energy which play a , role in these for like all physical systems, dispersoids exhibit phenomena which are attributable to changes in their thermal, radiant, electrical, chemical, etc., energies. Evidently, physical systems may be classified on the basis of the forms of energy which appear most frequently or most prominently in them. Thus, gases are best characterized by the behavior of their volume energies, while electrical phenomena seem to be especially characteristic of dilute salt solutions. The form of energy most characteristic of the dispersoids is directly deduc- ible from their definition. A development of much surface is the fundamental property of dispersoid systems. But the absolute value of this surface is a direct measure of the capacity factor of the so-called surface energies. One therefore antici- pates that the properties of these and of closely related forms of energy must play an important part in the dispersoids. Especially true is this of all changes in the dispersoid state which involve an increase or a decrease in the degree of dispersion; for according to definition every change in the magnitude of the surface must be regarded as the result of free surface energies or of their compensation by other energies. Wilhelm Ostwald pointed out the importance of the surface energies for the theory of colloid phenomena even before their dispersoid character was established on theoretical and experimental grounds. 2. Surface Energy of the First Order. Surface energy as usually discussed is made up of two components : a capacity factor as measured by the absolute surface, and an intensity factor as measured by surface tension. This type of surface energy en- 66 GENERAL ENERGETICS OF THE DISPERSOIDS 67 deavors to decrease the surface of a system if free energy is available. For reasons to be discussed in the succeeding para- graphs we shall call this, surface energy of the first order and its intensity factor, positive surface tension. Its most important properties are the following. If surface energy of the first order is freed in any way it is changed into other forms of energy, especially heat, the surface of the system decreasing at the same time. Conversely, if heat is introduced into a system capable of developing free surface energy of this order, the surface tension is decreased. Roughly, the decrease in surface tension is proportional to the increase in tem- perature. If an electric surface is produced, in that two phases having different electric charges which are not permitted to neutralize each other are brought in contact with each other, the surface tension of the phases decreases. Further, the value of the surface tension varies with the chemical character of the phases which are in contact with each other. General laws re- garding the relation between magnitude of surface tension and chemical character of the phases have not yet been discovered. The surface tension of a dispersion means may be lowered or raised by the molecular-disperse or colloid subdivision of a phase in it (for details see page 140). The value of the total surface tension of dispersoids is dependent upon the age of the surface. If the disperse phase lowers the surface tension of the dispersion means, the value of the tension decreases with time; but if the disperse phase increases the surface tension of the dispersion means, little or no change is observable. The ultimate value of the surface tension attained after a longer period of time is called the static surface tension, in contradistinction to the dynamic surface ten- sion observable in freshly produced systems. We shall discuss the reasons for such changes later. Details regarding positive surface tension and the many methods of measuring it with its correlated surface energy of the first order must be sought in text-books of physics and physical chemistry. 1 3. Surface Energy of the Second Order. For reasons which we are unable to discuss in detail here we are compelled to recog- nize the possibility of the existence of another form of surface 1 A recent and in part exhaustive presentation of the relation of positive surface tension to other physical and chemical factors may be found in H. Freundh'ch, Kapil- larchemie, Leipzig, 1909. 68 GENERAL COLLOID-CHEMISTRY energy, namely, surface energy oj the second order. As is well- known, two forms of volume energy are characteristic of gases: one which is transformed into other varieties of energy when the volume of the gas increases, and a second which is analogous to surface energy of the first order in that it also is converted into other forms of energy when the volume of the gas decreases. The intensity factor of this second, less well-known form of volume energy is the so-called " internal pressure." In liquids this attains a value estimated at several thousand atmospheres. Reasoning by anal- ogy we may suspect that a form of surface energy exists which has the tendency to change itself into other forms of energy whenever the surface of a system increases. The intensity factor of this type of surface energy might be designated expansive or negative surface tension. What evidence is there for the actual existence of such a second type and are we familiar with phenomena which may advantageously be explained through its properties? 1 As a matter of fact, certain phenomena are known which can only be explained by assuming the existence of such a surface energy of the second order an expansive surface tension. These are the increases in surface which occur in strictly diphasic systems. The simplest and clearest expressions of an expansive surface tension are observed when small volumes of liquid, such as drop- lets or streamlets, are electrified. The phenomena have long been known under the names " electric heart,' 7 " electric fountain/' etc. 2 The accompanying Fig. 9 taken from O. Lehmann illus- 1 We frequently encounter in the literature, as in the writings of Maxwell, Mens- brugghe, Wilh. Ostwald, Fuchs, van't Hoff-Donnan, M. Heidenhain, J. Perrin, L. Michaelis, F. Haber, etc., discussions of the possible existence of, and of the effects of the intensity factor of this kind of expansive surface tension. Since the begin- ning of 1905, partly without the knowledge of the studies of these authors and partly before their papers appeared, I have, occupied myself with this concept of surface energy of the second order. Since it led to conclusions which were somewhat surprising and far reaching, I did not dare to publish a monograph entitled "Unter- suchungen zur Theorie der Oberflachen- und Volumenenergien" even though the manuscript had been revised for the third time by the summer of 1905. It has been revised and enlarged several times since then and its contents subjected to rigid reexamination. Because similar views have been frequently expressed, and encour- aged by scientific friends, I have at last decided to publish these investigations, even though far from complete, under the title, "Die energetische Atomistik. Unter- suchungen zur Theorie der Oberflachen- und Volumenenergien" (Theodor Stein- kopff, Verlag, Dresden). Further details regarding the properties of surface energy of the second order and its r61e in dispersoid systems may be found there. 2 Regarding phenomena of this type see O. Lehmann, Molekula rphysik, I, 824, Leipzig, 1888; H. Freundlich, Kapillarchemie 212, 255, 260, Leipzig, 1909. [I do not, of course, agree with the theories of the latter which differ fundamentally from mine; see Wo. Ostwald, Koll.-Zeitschr., 7, 142 (1910)]. GENERAL ENERGETICS OF THE DISPERSOIDS 6 9 trates the "disruptive" surface increase against turpentine which liquid (molten) sulphur shows when electrified. The left-hand figure shows the effect of a weak, the right-hand that of a strong charge. The liquid sulphur surrounding the rod-shaped electrode first assumes conical shape at the tip of the electrode (this already means increase of surface) and then breaks up into individual droplets. Through strong electrification several such "points of discharge" all showing the same behavior, may be produced. When the electrode is placed in a vertical position, the charge is high FIG. 9. Increase in surface, when electrically charged, of melted sulphur against turpentine oil. (After 0. Lehmann.} The left-hand figure shows the effect of a weak, the right-hand, the effect of a stronger charge. and the liquid has little viscosity, the phenomenon of the "electric fountain" is produced (see the figure in O. Lehmann's volume). The "electric heart" is the name applied to the changes in form observed when the volume of liquid is weakly electrified (see the point of the electrode in the figure to the left). Such increases in surface have also been observed when solid phases are brought in contact with liquid ones even when no electric energy is available. Of recent investigations of this prob- lem those of M. Traube-Mengarini, A. Scala, 1 and J. Amann 2 1 M. Traube-Mengarini and A. Scala, Koll.-Zeitschr., 6, 65 (1910). 2 J. Amann, Koll.-Zeitschr., 6, 235 (1910). For other examples see the paragraphs on direct colloid solution in Part III. 70 GENERAL COLLOID-CHEMISTRY deserve special mention. These authors were able to observe microscopically and ultramicroscopically the breaking up of coarsely disperse particles of lead or iodine, in a suitable medium, into smaller but not amicroscopic (molecular-disperse) particles. J. J. von Kossonogow 1 found that electrification promoted these effects. Another striking illustration of an increase in the surface of a "solid" phase is seen in the production of lead sponge from lead plates when a suitable current is passed through them (see pp. 71 and 82). In complex dispersoids the phenomena characteristic of an expansive increase in surface remain essentially the same, but they are complicated through the simultaneously occurring changes in concentration and secondary chemical effects. Expan- sion phenomena are observed when fatty acids come in contact with alkaline solutions; when cholesterin, etc., come in contact with various pure solvents, etc. The so-called "myelin forms" produced under such conditions will be discussed later. If we bear in mind that all possible transitions exist between coarsely disperse, colloid, and molecular-disperse solutions, we are driven to the ultimate and, perhaps, most important conclusion of all, namely, that the process of molecular or "true" solution is also to be regarded merely as such a spontaneous and extreme increase in surface in a diphasic system. 2 1 J. J. Kossonogow, Koll.-Zeitschr., 7, 129 (1910) where earlier publications are listed. 2 Even the most modern textbooks of physics state that the only physical require- ment for solution resides in a reduction of the positive surface tension to zero. But this really tells us nothing concerning the character of solution, for to prove the absence of an energy potential gives no clue to the source of the work necessary for solution. Especial emphasis, therefore, must be laid on the experimental proof of a spontaneous increase in surface in two-pha.se systems. Surface increases due to three positive surface tensions have long been noted in three-phase systems (as in the spreading of oil on water). Regarding the view that solution is a chemical process consisting of the formation of compounds of solvent and solute in indefinite propor- tions, we need only remark that this assumption, even if correct, does not explain the extraordinary increase in surface which occurs in the process of solution. But this increase in surface is by definition a physical process which like all other physical phenomena depends among others upon the chemical properties of both phases but also upon their electrical, thermal, etc., properties, all of which influence the extent of the surface increase. No chemical conception of the process of solution, whatever its nature, is able to explain why a given solid (say tannin) dissolves as a colloid in one solvent (water) and as a molecular dispersoid in another (alcohol). If we regard the extensive "division" of a dissolved substance as the characteristic of both colloid and molecular-dispersoid solution then every process of solution becomes physical. We can only speak of "chemical" solution (with the exceptions noted above) when free surface energy of the second order is derived exclusively or mainly from chemical energy. The solution of metals in acids is an example of this sort. For details see the book announced on p. 68. GENERAL ENERGETICS OF THE DISPERSOIDS 71 Regarding the remarkable fact that separate particles are formed immediately in the expansive increase of surface in the case of solid phases (with the exception of lead sponge) while a progressive increase is often observed in liquids, and for further de- tails regarding the conditions for molecular subdivision, see p. 77. 4. The Relation of Surface Energy of the Second Order to Other Forms of Energy. Since the concept of expansive surface energy is an unfamiliar one, it is necessary to discuss briefly its relation to other physical and chemical factors. Theo- retically, many properties of this surface energy of the second order may be predicted, and this on the basis of the fact that the two types of volume energy show in most respects a reciprocal behavior. Thus, positive surface tension decreases as a rule with increasing temperature; conversely, expansive surface tension should increase when the temperature increases. This require- ment is satisfied by the general increase in solubility which sub- stances show with rising temperature. Lead forms spontaneously a colloid solution in distilled water at room temperature, while silver and platinum do so appreciably only when boiled (M. Traube-Mengarini and A. Scala, I.e.). Further, the positive surface tension of a system falls if a difference of potential is established at its surface; the negative surface tension should increase under such circumstances. That such is the case was repeatedly demonstrated in the earlier paragraphs of this book. An increase in surface may be effected very generally and often strikingly by different electrical means, as in the production of colloid solutions from non-disperse phases 1 (electric synthesis of the colloids) . As already mentioned, but few quantitative re- lationships have been established between the surface tensions of different substances. A similarly great variation should therefore exist in the values of the expansive surface tension. This re- quirement is satisfied in our lack of stoichiometrical generaliza- tions regarding both the molecular-disperse and the colloid solubility of substances, etc. These remarks may suffice to demonstrate the justice of assum- ing the existence of a surface energy of the second order with the described properties. We shall accordingly make use of this concept in the special parts of this book. 1 See Wo. Ostwald, Koll.-Zeitschr., 7, 132 (1910). GENERAL COLLOID-CHEMISTRY 14. Dependence of Surface Energies upon Specific Surface i. General Considerations. Relations exist between the surface energies and the shape of the phases at the boundaries of contact. These are extremely important. First, as regards surface energy of the first order: As is well known, its most striking effects appear in systems which have markedly curved surfaces or which, when possessed of plane boundaries enclose a relatively small volume. The so-called capillary phenomena in the strict sense of the word illustrate the influence of the markedly curved surface. The effect of the second factor is illustrated in the relation which exists between the FIG. 10. Capillary rise and specific surface. height to which a liquid ascends between two glass plates which are in contact with each other along one edge, and the thickness of the layer of the liquid and of the gas above it (see Fig. 10) . The thinner the layer, the more definite the capillary phenomena, that is, the higher the ascent of the liquid. The general effect of the influence of the curvature as well as of the thickness of the layer of the liquid upon the magnitude of the surface energy of the first order is expressed in the relation between the surface energy and the specific surface of the phases. Thin or markedly curved layers of a liquid are manifestly possessed of a relatively greater absolute surface than equivalent volumes of thicker or less markedly curved layers. 1 An increase in surface energy in any given volume is 1 The same is true of structures in which two dimensions are very small (threads, etc.). GENERAL ENERGETICS OF THE DISPERSOIDS 73 therefore produced whenever more absolute surface is devel- oped or the specific surface is increased. When we apply this conclusion to typical dispersoids we find that a given volume of the disperse phase, absolutely considered, contains more surface energy than the same volume of the same substance in a non-disperse state. But the total amount of surface energy of a single particle is also relatively increased. Thus, when volume and mass are decreased to Kooo by decimal subdivision of a cube (see Table i), the surface of one of the re- sulting cubic particles is only decreased Hoo- The greater the degree of dispersion, the more surface does the disperse phase "contain. 7 ' In fact we may say that when the disperse phase is so finely subdivided that the diameter of the individual par- ticles is only twice that of the sphere of action of molecular forces, it "consists only of surface." Evidently the shifting in any sys- tem of the relation of the different kinds of energy to each other in favor of those found in surfaces must have a fundamental influence upon the character of these systems. This growth of the surface energies with increasing subdivision, and their extraordinarily great importance in dispersoids having a high specific surface may be further illustrated as follows: 1 If the "internal energy/' that is, the total energy of a system minus the surface energy is designated by /, and the surface energy by S, then the total energy of the system equals I -{- S. The quantities of energy comprised under the name "internal energy" (for example, kinetic energy, chemical energy, etc.), are proportional to the volume v, while the surface energies are proportional to the surface s, in other words, / = iv and S = ts when i is the internal energy of the unit of volume and t is its surface tension. The total energy T of a system is therefore T = iv + ts. If now we consider T the total energy of the unit volume = T v , in other words, if we v is \ divide the entire equation by v we obtain TV = i +( --./). If \v J is small, that is, if the specific surface of the system is small, the second member is also small and may be neglected. This is the case in most of the physico-chemical reactions hitherto' investi- 1 Wilh. Ostwald, Grundr.d. allg. Chem., 4 Aufl., 531, Leipzig, 1909. The above is a somewhat modified presentation of the subject. 74 GENERAL COLLOID-CHEMISTRY gated in which interest has chiefly centered upon part i of the total energy. But 'if v is kept constant and 5 is increased, as in the subdivision of a given cube for example, the second member may grow tremendously in value. If the subdivision is very great, part i, which is proportional to the volume, may disappear alto- gether in comparison with the value of the second member which becomes infinitely great when v = s. Under such circumstances the total energy of the system consists almost entirely of surface energy and all its activities are characterized by the properties of the latter. 1 2. Surface Energy of the First Order and Specific Surface. Illustrations of the relations between surface energy of the first order and specific surface were given above. Another example of such a relation is the fact that the height of ascent of a liquid in a capillary tube is inversely proportional to the diameter of the tube; in other words, the product of the height of ascent and the diameter of the tube is a constant. This means that if the diameter of the capillary tube is reduced by half, the height of ascent of the liquid is doubled, and if the former is decreased to one- tenth its value, the magnitude of the latter becomes ten times as great. If we write the surface energy of a cube with an edge i cm. long as i, the surface energy of the same cube colloidally subdivided (so as to have cubes with io/*/* edges) amounts to a million when we assume that the surface tension remains unchanged. 3. Surface Energy of the Second Order and Specific Surface. Since the surface energy of the second order contains the absolute surface of a system as its capacity factor, we would imagine that its effects should also increase with increase in curvature, decrease in thickness, or increase in degree of dispersion. Is there any experimental evidence for this? It is found in what is called the influence of the size of the particles of solid substances upon their solubility. As Wilhelm Ostwald, 2 G. Hulett, 3 and others have shown, substances in a finely dispersed state, as produced by tritu- ration for example, are more soluble than those in a coarsely dis- persed state. Hulett found finely triturated mercury oxide to be more than three times as soluble as coarser pieces. The solubility 1 See p. 96 for the interesting conclusions deducible from this discussion. 2 Wilh. Ostwald, Z. f. physik. Chem., 34, 496 (1900). 3 G. Hulett, Z. f. physik. Chem., 37, 385 (1901); see also Hulett and Allen, Journ. Am. Chem. Soc., 24, 667 (1902). GENERAL ENERGETICS OF THE DISPERSOIDS 75 of a highly triturated powder as determined by conductivity measurements amounted to 0.694 millimols (150 mg. per liter). An especially interesting series of experiments of this kind were carried out by Stas in the year 1870 regarding the solubility of the different "precipitates" of silver chloride. 1 Stas found that, depending upon the experimental conditions under which it is obtained, silver chloride assumes the forms: i. "gelatineux; 2. caseeux, flocconeux; 3. pulverulent; 4. grenu, ecailleux, crystallin fondu;" and that the solubilities of these modifications, the degree of dispersion of which undoubtedly decreases in the order given below, was as follows: 1. Flocculent silver chloride 0.0140 gram per liter at 20. 2. Powdered silver chloride 0.0060 gram per liter at 17. 3. Granular silver chloride o.oooi gram per liter at 15. 4. Granular silver chloride 0.03 gram per liter at 100. The solubility of the granular preparation had to be measured at 1 00 because it is too slight at room temperature to be determined analytically. The solubility of the gelatinous chloride could not be determined because of the difficulty of separating it in this con- dition from the fluid in which it is precipitated and on account of its instability. A still older observation of this kind was made by Thomas Graham. 2 Graham found that silicic acid jellies of different concentrations have different (maximum) molecular solubilities. Thus, only two parts of the silicic acid of a i per cent, jelly formed a molecular-disperse solution in 10,000 parts of water, only one part of a 5 per cent, jelly, and even less of the more highly con- centrated jellies. But silicic acid is a typical emulsion colloid, that is, its degree of dispersion changes with variations in concentra- tion. Concentrated jellies are presumably less disperse than the more dilute and so have a lower molecular solubility. In harmony with the above-sketched conception of solution as a process of extreme increase in surface produced by a free expansive surface energy, it is evident that such influence must act by effecting an absolute increase in surface energy by increas- ing the specific surface. Such a relationship is rendered plausible by the fact that the " artificial" breaking up of a substance 1 See K. Drucker, Koll.-Zeitschr., 4, 216 (1909). 2 Thos. Graham, Journ. Chem. Soc., 1864; see also his collected papers, p. 618. 76 GENERAL COLLOID-CHEMISTRY preparatory for solution already represents surface work which is later saved in the process of that further surface increase which we call "solution." An interesting observation apparently contradicts this concep- tion of an increase in the surface energy of the second order of a system with its degree of dispersion. According to the concur- rent statements of R. Zsigmondy, 1 J. Donau 2 and The Sved- berg 3 colloid gold is only slightly amalgamated, if at all, by mer- cury. But this is really a question of solution velocity, not of maximum solubility. Besides, this case should not be compared with what was said above, for in the amalgamation of colloid gold by mercury we are dealing with a triphasic rather than a diphasic system; furthermore, an absolutely necessary preliminary condition, namely, contact of the two phases is absent. This must first be produced by shaking, etc., and is presumably hindered by the fact that surfaces, especially when markedly curved, are surrounded by liquid films having special properties such as great tenacity, etc., which must be broken before direct contact of the phases and solu- tion may take place (see later). 4. Dependence of Surface Tensions upon Specific Surface. Besides this influence of the specific surface upon the absolute and relative amounts of the surface energies, there exists another be- tween the latter and the shape of phases encountered when equiva- lent but differently constituted surfaces are compared. This relation depends upon the circumstance, which has both an experi- mental and a theoretical basis, that the direct effects of the sur- face energies extend to a certain depth on both sides of the mathe- matical surfaces of contact. In curved surfaces such subsurface effects of the surface energies may weaken or strengthen these, depending upon the convexity or the concavity of the curvature as well as upon the nature of the phase. Thus, in a surface which is convexly curved with regard to one of the phases and which has a positive surface tension, the subsurface effects may strengthen each other in the "convex" phase while they weaken each other in the "concave" phase. Since we must believe that these subsurface effects are produced by the surface energies or that, conversely, the latter are 1 R. Zsigmondy, Liebig's Ann., 301, 37 (1899). 2 J. Donau, Monatshefte f. Chem., 26, 525 (1905). 8 The Svedberg, Koll.-Zeitschr., 5, 323 (1909). GENERAL ENERGETICS OF THE DISPERSOIDS 77 the result of changes in the constitution of one phase produced by contact with another, this mutual weakening or strengthening of the subsurface effects must have a reciprocal influence upon the surface energies, more particularly upon their intensity factors, the surface tensions. If the simultaneous and opposite strength- enings and weakenings of such subsurface effects produced through curvature of the surface do not completely neutralize each other, the surface tension of one and the same surface may assume different values, depending upon its curvature. Special relationships are encountered when the curvature is so great or when the particles are so small or when a layer of one of the phases is so thin that the layers in which the effects of the sur- face energies still manifest themselves come very close to each other or into actual contact. As is demonstrable through molecu- lar physics 1 and on thermodynamic grounds, 2 the intensity factors of the surface energies change much under such circum- stances. This variableness of the positive surface tension in sys- tems having small dimensions has been demonstrated experimen- tally by the work of Reynold and Rucker 3 on soap films. Since we have no direct method of measuring negative surface tension in systems having small dimensions, experimental demonstration of its variableness has not yet been possible. For its indirect de- termination molecular dispersoids, or better, ionic dispersoids might be used. Special attention might be directed to the prop- erties of very dilute or extremely ionized solutions of electrolytes and their conductivity or viscosity peculiarities, and these might be correlated with variations in expansive surface tension. 15. Reciprocal Effects of the Two Surface Energies (Theory of Dispersion and Condensation) i. General Considerations. Ordinarily, only progressive varia- tions, that is to say, uninterrupted increases or diminutions in surface are considered when the phenomena of surface tension are 1 See Lord Rayleigh, Phil. Mag. (5), 30, 475 (1890). 2 W. Gibbs, Thermodynamische Studien, 274, Leipzig, 1892; van der Waals and Kohnstamm, Lehrb. d. Thermodynamik., i, 207, Leipzig, 1908. 3 Reynold and Rucker, Phil. Trans. Roy. Soc. London (2), 171, 447 (1881); 174 645 (1883); 177, 627 (1886); 184, 505 (1893). See also P. Drude, Ann. d. Physik, (3), 43, 158 (1891); Johanott, Phil. Mag. (5), 47, 501(1899); (6), n, 746 (1906); Schiitt, Ann. d. Physik. (4), 13, 712 (1904), etc.; also A. Pockels, Nature, 43, 437 (1891); Lord Rayleigh, Phil. Mag. (5), 48, 331 (1899). 78 GENERAL COLLOID-CHEMISTRY discussed. While the coalescence of liquid droplets when they come in close contact with each other is usually attributed to a sur- face tension effect, such processes are less satisfactorily explained on such a basis alone than is, for example, the contraction of a soap film. Conditions when droplets are in " close" contact are highly complex in character (see later). An analogous difficulty is encountered when a progressive increase in surface gives way to droplet formation. We have before us here the general problem: Under what conditions does a progressive variation in surface be- come discontinuous ? It is evident that this question is of special importance in the dynamics of the dispersoids, more particularly in that of the colloids, for these are produced either by increasing the dispersion of slightly disperse or non-disperse systems, or by condensing maximally disperse (for example, molecular) systems. 2. Discontinuous Increase in Surface. The simplest case of a progressive increase in surface is encountered when we observe the Q a- b c FIG. ii. Spontaneous changes in shape of drops on a plane surface. form of different sized droplets of a non-wetting liquid such as mercury resting on some solid support like a glass plate (see Fig. 1 1) . The smaller the droplet, the greater its relative (specific) surface and the more completely does it retain a spherical form (a). Larger droplets become flattened by their own weight (b), in other words, they increase their absolute surfaces spontaneously, for their smallest possible surface would also be spherical. If we attempt to enlarge the volume of the droplet on the plate by adding more liquid to it, the droplet becomes progressively flatter until at a maximum volume, different with different liquids, it breaks up into several smaller droplets. Thus, with ordinary materials it is not possible to make a coherent droplet, that is, a continuous layer of more than about 25 cc. of mercury on a glass or porcelain surface. 1 An analogous phenomenon is offered in the well-known fact that cylindrical deformation of a given volume of liquid can- not be produced after a certain maximum value has been attained, 1 It is not denied that thinner, continuous layers of mercury might be prepared by other methods or by using very pure materials. GENERAL ENERGETICS OF THE DISPERSOIDS 79 without having the liquid thread break. We need but call to mind the difficulties which must be overcome in the preparation and progressive deformation of fine mercury threads in the making of thermometers. Subdivision of the droplet of mercury may be facilitated by increasing its absolute (and specific) surface "arti- ficially" through the introduction of energy from without as by pressing upon it with a glass plate as shown in Fig. n c. This increase in surface, which is entirely analogous to that produced through gravity, leads to a dispersion of the drop into droplets which are at first irregular in size, but which approximate the spher- ical more and more as they become smaller. Analogous phenomena are observed when a drop of rancid oil is placed upon a very dilute alkaline solution in which it changes its shape " spontaneously" and finally emulsifies itself; or when, at a temperature of 4oC., a crystal of cholesterin is introduced into a solution of bile salts. 1 As soon as the progressive def orma- tion associated with increase in surface has attained a certain value it becomes discontinuous and the process called "dispersion" begins. This may also be clearly observed in the electric dis- persion of liquids. We need but recall the facts illustrated in Fig. 9. A weak electric charge produces only a deformation and enlargement of surface, which when a stronger charge is given becomes discontinuous and so gives rise to droplet forma- tion. That a progressive increase of surface may also take place in the "spontaneous" production of colloid and molecular dis- persoids is indicated by the appearance in them of "solution figures." 2 One can also easily see that the greater the positive surface tension of a drop of liquid as compared with that of the medium in which it is placed, the more easily will its dispersion be accomplishable. Thus, on the same glass or porcelain plate a drop of water, or better yet a drop of ether, may be spread into a much thinner continuous layer than a drop of mercury. The corresponding surface tensions are: ether, 16.5 (at 20), water, 70.6 (at 20), mercury 436 (at 15). A somewhat simpler method of demonstrating this relation between the discontinuous en- largement of a surface and its surface tension is to deform liquids by causing them to flow through a capillary tip (see Fig. 12). 1 H. Schade, Kolloidchem. Beihefte, I, 377 (1910). 2 See the earlier compilation in O. Lehmann, Molekularphysik., i, 481, Leipzig, 1888; where striking illustrations may also be seen. 8o GENERAL COLLOID-CHEMISTRY L J \ V While ether and water may flow through such a tip in a fine stream (a); mercury passes through in the form of droplets (b). It must further be pointed out that, as far as known, all phenomena of dispersion are connected with movement of the resulting disperse particles. It is evident that such spatial rearrangements of the disperse particles, in other words, these " dispersion movements" are to be separated theoretically from the process of dispersion itself, in other words, the increase in surface. They are to be considered as phenomena secondary to the transformations of energy which pro- duce dispersion. Regarding the more inti- mate relationships between the intensity of these movements and the dispersive forces, only suppositions may be made, for no exact investigations of them exist at present. It is of interest to consider the fate of an excess of free surface energy of the second order when the presence of a large amount of surface energy of the first order prevents its transformation into an increase of surface. It seems plausible to assume that such energy may then react upon the liquid dispersion means in such a way as to transform its ten- sion into a Pressure acting upon its surface and of great (6) surface layer (see page 01). This assumption is sus- tension when issuing . . ' .. . . ,- ,1 from a vessel. tamed by the well-known fact that the boundaries of a liquid surrounding another phase show those special properties which have given rise to the conception of "liquid films." We shall often refer to these (see page 87). 3. Theory of Dispersion. All increases in surface are regarded in this volume as expressions of surface energy of the second order. The work necessary for such transformations is made up of the product of the magnitude and of the tension of the surface. It is therefore by definition surface work. From this point of view all increases in surface, whether produced through gravity, through pressure or compression, or by any other means, in other words, all processes of trituration, pulverization, com- GENERAL ENERGETICS OF THE DISPERSOIDS 8l minution, etc., are only expressions of this surface energy of the second order and differ from each other only in the nature of the sources of the energy employed in bringing about the increase. As previously emphasized, not only mechanical energies but also heat and electrical energies may be transformed into surface energy of the second order, and by this means lead to an increase of surface. Since the increase in surface is always the same, inde- pendently of the nature of the energies employed to bring it about, it must remain the characterizing feature of these phenomena. If we consider one of the simpler effects of surface energy of the second order, as the deformation of a drop of liquid by its own weight, with regard to its possible effect upon the surface energy of the first order, we reach the important conclusion that the decrease in the free surface energy of the second order when converted into an equivalent of other energies through the in- crease in surface, increases the amount of surface energy of the first order in the system, for the quantity of surface energy of the first order in a system is proportional to the absolute surface when the intensity factor of tension remains constant. If tension is constant which is certainly the case in the non-disperse and coarsely disperse systems to be first considered the amount of surface energy of the first order increases with every decrease of the other surface energies. This is true for example when the surface of a liquid drop is progressively increased. But there is no reason for assuming that the increase in surface energy of the first order is always equivalent to the decrease of expansive surface energy. Experience shows (see the above-mentioned examples) that when certain increases in surface are brought about, the increase in surface energy of the first order is greater than the decrease in surface energy of the second order, for as soon as there exists an excess of contractile surface energy the surface of the given volume becomes discontinuous. The equilibrium between the two energies which is "dynamically" displaced by a slight deformation of the surface is destroyed as soon as the amount of surface energy of the first order produced, more than compen- sates for the decrease of expansive surface energy. Equilibrium will not be reestablished until the liberated amount of contractile surface energy has been transformed (into heat for the most part), a change which can be accomplished only by an accompanying 6 82 GENERAL COLLOID-CHEMISTRY diminution in surface. Since the expansile tension prevents a diminution of the volume as a whole this tendency toward dim- inution can only be satisfied by a subdivision of the volume into smaller parts, for then only can both requirements be fulfilled at the same time, on the one hand the increase in absolute surface as demanded by the expansile tension, on the other the decrease in absolute surface as demanded by the contractile tension. Sub- division is the only possible result; or to put it in another way, the reciprocal effects of these surface energies must lead to subdivision. Dispersion, or the conversion of a progressive increase in surface into a discontinuous one is characterized energetically by a liberation of positive surface energy brought about by an excessive development of absolute surface through the effects of expansile surface energy. 1 4. Consequences of the Energetic Theory of Dispersion. If the suggested conception of dispersion is correct, a number of deductions therefrom must be capable of practical support. It follows from what was said that, neglecting certain transi- tion phenomena, dispersion should set in suddenly as soon as a definite amount of deformation has been induced, for discontinu- ous increase in surface corresponds to an intersection point of two changes in energy. We should expect to encounter especially clear examples of such "critical" points when increases in surface are produced by the transformation of other energies into surface energy of the second order, for then a better control of conditions is possible than in the spontaneous increases in surface. As a matter of fact, such "critical" points have long been recognized, especially in the electric dispersion of liquids and solids (see above and later). There exists, for example, a so-called "disin- tegration tension" in all the known electric methods of making colloid solutions, 2 at which the dispersion of the previously non-disperse electrodes suddenly begins. Further, the critical point should vary with the value of the positive surface tension, in other words, with the value of the free surface energy of the first order of the substance to be dis- persed. As a matter of fact, the greater the positive surface tension of the substance to be subdivided, the greater is the 1 A mathematical formulation of the conditions necessary for dispersion on the basis of surface energy will be given in the new book I have announced. 2 See Wo. Ostwald, Koll. Zeitschr., 7, 132 (1910). GENERAL ENERGETICS OF THE DISPERSOIDS 83 amount of surface energy of the second order consumed, in other words, the greater must be the amount of electrical energy, for example, that must be introduced into the system. These deductions are supported by the well-known fact that progressive increases in surface which do not immediately yield disperse systems are observed more commonly in liquids than in solids. As a rule, large quantities of energy are necessary to produce an increase of surface in liquids and then they do not usually yield disperse systems at once. As already mentioned, only certain solids like lead show progressive increases in surface. We may explain this interesting difference between solids and liquids by the well-known fact that solid phases possess a greater positive surface tension than liquids as indicated by the pro- gressive increase in surface tension of cooling, molten substances. The transitional behavior of substances like lead is also in harmony with this view. When we apply this to the question of the dispersive effects of equal quantities of surface energy of the second order upon substances having different positive surface tensions, we find that the greater the positive surface tension of the substance to be sub- divided, the greater the degree of dispersion of the system. This brings up the question: under what circumstances can we obtain the highest degree of dispersion in one and the same substance? The answer to this is not that we must have present the greatest possible amount of free surface energy of the second order. Were this the case then the degree of dispersion of a dispersoid would have to be proportional to its solubility and this is by no means the case. To produce a maximum degree of dispersion a maximum of free surface energy of the first order must also exist in the system, either to begin with as in solids, or as the result of an especially great increase produced through an increase in surface. Hence, molecular-disperse systems will be formed when the two surface energies acting between solvent and solute attain the physical maximum. To the important consequences of this characteriza- tion of " molecules" in the terms of surface energies for our con- ceptions of the structure of matter we shall return later (see p. 96). Let it here be mentioned that F. G. Donnan 1 following a 1 F. G. Donnan, Z. f. physik. Chem., 37, 735 (1901); 46, 197 (1903). 84 GENERAL COLLOID -CHE MIS TRY suggestion of J. H. van't Hoff, has constructed a capillary theory of colloid solution in which he also uses the concept of "nega- tive" surface tension. He proceeds from the fact mentioned above that in very thin layers of a liquid the surface tension of a particle is no longer independent of the thickness of the layer. On the basis of theoretical considerations which originated with Gauss, he concludes that in layers of such critical thickness the thicker layers tend to spread and become thinner while the thinner layers tend to shrink and become thicker. The resultant consti- tutes an equilibrium yielding the stable " critical particle." It is evident that this interesting theory 1 differs fundamentally from that outlined above in that the sphere of action of the expansile surface tension is assumed to lie only within the " layers of critical thickness" or the "spheres of molecular activities." According to our view one may observe the effects of expansile surface tension macroscopically, just as one may observe the effects of positive surface tension in coarsely disperse systems, and all independently of the thickness of the layers of particles involved. Moreover, Donnan's view compels him to assume a qualitative difference between colloid and molecular-disperse solutions which is unnecessary in our conception. 2 It is also hard to conceive of the increases in surface until the sphere of molecular activities is . reached in Donnan's theory. "It is hard to conceive just what happens. Apparently the solid substance spreads into O (the solvent) in extremely thin layers or in the form of thin branching threads. It should be noted that the solid colloid is not in an explosive state, for dispersion takes place only in the thin surface layers so that the process of ' solution' of the colloid need not be a rapid one, etc." (Donnan, I.e., 1901, p. 738). The progressive, macroscopic, microscopic and ultramicroscopic increases in surface of diphasic systems discussed above show that the energetic theory is easily capable of filling this gap. 5. Discontinuous Diminutions in Surface. When one discusses discontinuous diminutions in surface one must bear in mind that we deal not with diminutions in the surface of the individual particles 1 It 'should also be noted that F. G. Donnan in his first paper (1901), outlined a more kinetic theory of colloid solution in that the state of dispersion was regarded as the result of two opposed "molecular streams" occurring in the surface. These proc- esses also take place only "within the molecular spheres of action." 2 See p. 134 concerning the "saturation point" of colloids postulated by D( )onnan. GENERAL ENERGETICS OF THE DISPERSOIDS 85 of a dispersoid but with the decrease in the sum of the surfaces of all the particles in the dispersion means. As a rule, such decreases in total surface are produced by approximation or coalescence of the individual smaller particles into larger ones. It is important to note that such decreases need not take place only through con- densation (agglutination, agglomeration, coalescence, etc.). Slight decreases in surface may be accomplished when for any reason elongated or flattened particles become more spherical. Tend- encies toward such progressive reductions in surface are en- countered when dispersoids are cooled. So far as known, the positive surface tension between two (homogeneous) phases always increases with decrease in temperature. Under such conditions irregularly shaped particles would therefore tend to become more spherical with fall in temperature. Yet the amounts of such internal diminutions in surface would at all times be small. The "condensation" type of diminution in surface is important in determining the properties of disperse, more particularly of col- loid systems. It shows itself in coarsely disperse systems in the coalescence of emulsified particles, in the formation of threads and flakes from microscopic precipitates, etc. It may be observed ultramicroscopically in colloid systems as a union of ultra- microns to form crystalline or non-crystalline particles. In molecular-disperse systems the process of "crystallization" is encountered, attained at times only after passing through an intermediate stage (see above). Generally speaking, such con- densations are produced by the same means which accomplish their dispersion, only different intensities, concentrations, etc., have to be used. Thus, while electric energy has a dispersive effect, removal of the charge leads to condensation, especially in colloids. On the other hand, under proper circumstances and with certain charges, condensing effects may be accomplished electrically, as in the coalescence of electrified droplets. 1 Changes in temperature within certain limits and additions of foreign, especially ionized, substances have similar effects. Mechanical treatment, like sudden one-sided pressure, may also bring about condensation, especially in coarsely disperse systems. When we study a simple case of condensation, as the coa- 1 See Lord Rayleigh, Proc. Roy. Soc., London, 28, 406 (1879); 34, 130 (1882). 86 GENERAL COLLOID-CHEMISTRY lescence of two liquid droplets, we find it hard to follow the transi- tion changes from the original state to the final. Coalescence usually takes place very rapidly so that the two droplets suddenly become one, though it may still be possible to observe the con- tractile effects of the positive surface tension in the movements of the surface. To study such processes of condensation in detail it is best to use drops of viscid material 1 which consume more time in the process. The intermediate phenomena, which we shall find of special theoretical importance, may then be studied to better advantage. Intimate contact of the droplets of a system with each other seems to be an absolute pre-requisite for coalescence (as well as for the union or clumping of solid particles). The droplets must be brought so close together that their surfaces have at least one "point" in common. To put it another way, condensation of two particles can occur only when their surfaces are continuous, O3 CO O O i 234 FIG. 13. Diagram of coalescence of two fluid particles. (According to L. Michaelis.) even though such surface continuity be limited to a single point. It should be noted that we mean a "physical" and not a " mathe- matical " point, in other words, a structure at present unmeasur- able but nevertheless of finite dimensions. Such a point, greatly "magnified" and fixed at the beginning of its development shows itself as a cylindrical or doubly coniform neck, as illustrated schematically in Fig. 13 taken from L. Michaelis. This con- necting piece broadens during coalescence until complete con- densation is attained. Sometimes (especially in viscid and in solid disperse particles) another type of contact may be encountered which does not correspond with the description just given. Here the particles also approach each other very closely but do not come in direct contact. In other words, while fixed to each other they are nevertheless still separated by a very thin layer of the dispersion means. The adhesion of iron filings to magnets or of powders to hot objects, etc., are macroscopic illustrations of such contacts. 1 See J. Loeb, Koll.-Zeitschr., 3, 113 (1908); L. Michaelis, ibid., 4, 55 (1909). GENERAL ENERGETICS OF THE DISPERSOIDS 87 The precipitation of coarse suspensions (of kaolin, quartz, etc.) illustrates the same phenomenon in a disperse system. It is probably characteristic of such "flakes" that the individual particles in them are separated from each other by a distance less than the diameter of the surface tension films. Figure 14 repre- sents the matter dia grammatically. Even though the individual particles in this type of contact are not in themselves continuous, the liquid membranes of their surfaces are (see Fig. 14). Be- cause of this difference it seems well to distinguish between the two and to designate the former as condensation while the latter is better called "aggregation." Evidently aggregation may often lead to condensation, and conversely aggregation may be assumed to constitute a precursor of condensation. FIG. 14. Diagram illustrating con- FIG. 15. Appearance preceding co- densation. agulation in a concentrated gold solu- tion. (According to H. Sidentopf.) As is well known, special means have to be employed to bring about such intimate contact or continuity of surfaces. One of the chief factors which tends to prevent this is the fact that the dispersion means (gas or liquid) exists at the phase surfaces in a denser state, has in other words a so-called surface viscosity. These envelopes act like the vapor envelopes about the drops of liquid formed when water is poured on a hot surface; they cause a "repulsion" of the particles when they meet acci- dentally and so tend to prevent their coalescence. These phenom- ena are closely related to the processes of "wetting" touched upon above. Stress was laid upon the importance of these envelopes in phenomena of condensation early in the history of colloid- chemistry. Thus, J. M. van Bemmelen wrote in 1888: "7 think 88 GENERAL COLLOID-CHEMISTRY it possible that the formation of the flakes which are precipitated in a liquid is dependent upon a change in the surface tension of the liquid membranes surrounding the colloid particles, of such type that these membranes between the particles are torn at some point, thus per- mitting the particles to form aggregates." 1 The condensation of disperse particles is connected with phenomena of movement just as is their dispersion. These "condensation movements" consist of a mutual approach of the particles and are also a necessary preliminary for their contact and coalescence. In fact these movements precede contact. The first demonstrable changes in a process of condensation are therefore kinetic in character. This fact is of importance for the theory of condensation. The appearance of such condensation movements is not a mere theoretical assumption but a necessary conclusion derived from the experimentally observed behavior of disperse systems before and after processes of condensation have occurred in them. Such condensation movements have actually been observed both microscopically and ultramicroscopically as illustrated in the accompanying Fig. 15 taken from Siedentopf. 2 6. Theory of Condensation. If we attempt to analyze the processes of condensation from an energetic standpoint as was done with the phenomena of dispersion, we discover that the former are more complex. In dispersion the phenomena of sur- face energy are the primary ones, while processes of movement, the formation of liquid films, etc., are secondary. But in the processes of condensation these different secondary phenomena must take place in reverse order before the surface phenomena proper come into play. Such considerations harmonize with the fact that phenomena of condensation and the means of initiating them are manifold in character as will appear later when we discuss the phenomena of coagulation. The theory of conden- sation therefore divides itself into two parts, first, to put it briefly, the means by which "intimate con tact " of the particles is brought about, and second, the analysis of the processes taking place after contact has been established. Since a discussion of the different means by which the intimate contact of the particles is 1 J. M. van Bemmelen, "Die Absorption" Gesammelte Abhandl., 22, Dresden, 1910. The citation is printed in italics in the original also. 2 See H. Siedentopf, Verh. d. Dtsch. Physik. Ges., 1910, 25. GENERAL ENERGETICS OF THE DISPERSOIDS 89 assured belongs to the field of special dispersoid and colloid- chemistry, this must be postponed. Subject to general discussion here are the changes which begin when the particles of a dispersoid begin to aggregate. This process is characterized by the formation of a common liquid film about the particles. Since this surface film grows smaller in the process of aggregation, 1 the whole seems to be produced through the action of surface energy of the first order. It is also clear that with any increase in the contractile surface energy the liquid film tends to push the particles closer and closer together until they come in actual contact. When we deal with liquids, coalescence of the particles then occurs as described above. In the case of solids (provided we are deal- ing with actual phenomena of condensation such as crystal formation) there is a coalescence of at least the solid surface layers. The action of the posi- tive surface energy in the latter FlG " case may be imagined as shown in Fig. 1 6. There results in all instances a decrease of the total sur- face separating the disperse phase from the dispersion means. The process of condensation is therefore to be regarded as the consequence of a transformation of surface energy of the first order. The greater the condensation, that is, the smaller the resulting absolute surface separating disperse phase from dispersion means, the greater the amount of surface energy of the first order that has' been transformed. Processes of condensation do not always yield coarsely disperse or non-disperse systems but may stop when very different de- grees of dispersion have been attained depending upon the con- centration of the reaction mixture, as shown in the formation of precipitates in chemical reactions (P. P. von Weimarn). This variety in degree of condensation is analogous to the above-discussed variety in degree of dispersion under different experimental condi- tions, and must therefore have an analogous energetic significance. The degree of dispersion in a condensing system depends upon the 1 See in this connection the citation of J. M. van Bemmelen, on p. 88. QO GENERAL COLLOID-CHEMISTRY amount of expansive surface energy present in it. The smaller the surface becomes by condensation, the greater must become the tendency of the expansive surface energy to counteract the diminu- tion of surface. The system becomes stable when an intermediate degree of dispersion has been attained, in other words, when the surface energy of the second order balances the surface energy of the first order which is producing the condensation. The in- fluence of the introduction of other forms of energy upon the degree of condensation is analogous to the influence of these as discussed for dispersion on p. 82. A relation between condensation in colloids and surface energies was first pointed out by G. Bredig 1 in explaining a special form of coagulation. At an even earlier -date P. Curie 2 pointed out the role of surface energy of the first order in bringing about condensa- tion in molecular-disperse systems in processes of crystallization. It is remarkable that the important suggestions of this investigator have received but slight (or one-sided) development since they were first expressed. P. P. von Weimarn (whose numerous papers appear in the Kolloid Zeitschrift and in the Kolloid- Chemische Beihefte) has also developed theories of condensation and dispersion which he believes to be so universally applicable that he would explain through them all known processes of con- densation and dispersion. A detailed account of his views cannot be given here. It should, however, be emphasized that no theo- retical conception of such processes can be formulated comparable in universality with the energetic one which must by definition always remain the broadest form in which natural phenomena may be described. P. P. von Weimarn in his theories of condensation and dispersion often makes use of moleculo-kinetic conceptions, in other words, he employs special expedients in the elaboration of his views. 3 1 G. Bredig, Anorg. Fermente, 15, Leipzig, 1901. 2 P. Curie, Bull. Soc. Min., 8, 145 (1885). * Objections may be raised against certain details of the argument of this author. According to his theory, electrical methods of pulverization are explainable only as condensation processes, which is obviously wrong in view of the dispersing effects of electrical energy, described and illustrated on p. 69. Furthermore, he formulates the basic idea of his theory thus: "When, for any reason, the intensity of the dis- solving forces increases on the surface of the dispersed particles, but does not exceed that value at which the velocity of crystallization or of solution becomes considerable, then the dispersed particles are peptisized (dispersed) by the dispersion means." [Kolloidchem. Beih., 1, 398 (1910)1. I can see in this only a "translation," and not an GENERAL ENERGETICS OF THE DISPERSOIDS 9 1 16. Influence of the Specific Surface upon the Relations be- tween Surface Energies and Other Forms of Energy 1 . Specific Surface and Volume Energy ; Capillary Pressure. The relation between surface energies and volume energies plays an important role in the phenomena observed in dispersoids. If the surface energies are not confined to a plane surface, in other words, if we deal with structures having a spatially defined surface or one which is curved, then the two surface tensions exert pres- sure. To put it more correctly, the surface energies in such bodies, more particularly in curved surfaces, readily change into volume energies when opportunity for such change offers. Thus, in the positive surface tension of a markedly curved system the centripetally directed capillary pressure may bring about a change in the pressure. If we assume the particle to be spherical, the value of this pressure is inversely proportional to the radius and directly proportional to the surface tension. Analogous phenom- ena are encountered when the curved surfaces have a negative surface tension. As indicated above 1 these relations between surface and volume energies may be demonstrated experimentally and are of course of great importance in dispersoids. It will be shown later that an increase in density due to positive capillary pressure may be demonstrated experimentally. 2. Specific Surface and Changes of State. The surface energies which dominate the behavior of disperse systems are also much influenced by temperature (and corresponding herewith, by pressure) . Thus the vapor pressure of small droplets or particles is found to be greater, at a given temperature, than that of the same substance in larger masses. Smaller drops therefore tend to evaporate more easily than larger ones, wherefore, in a closed system these recondense 2 upon the larger ones. A lowering of the melting point of solid bodies occurs when their specific surface is increased, just as does a decrease in the evaporation temperature analysis of the process of dispersion, for the assumption of "dissolving forces" and of a relation of these to other processes constitutes the problem of dispersion but does not solve it. These objections, however, are not valid if von Weimarn's theories are limited to condensation and dispersion phenomena produced by chemical means. As will become later evident, the theories of P. P. von Weimarn agree throughout with the phenomena observed in this particular field. 1 Wilh. Ostwald, Grundr. d. allg. Chem., 4 Aufl., Leipzig, 1909, p. 533. 2 Wilh. Ostwald, Lehrb. d. allgem. Chemie, 2 Aufl. II, 2, 362; Z. f. physik. Chem. , 22, 289 (1897). Q2 GENERAL COLLOID-CHEMISTRY with increasing specific surface. Thus P. Pawlow 1 found dusts of salol, antipyrin, etc., to melt at a temperature some 7 lower than larger particles. He calculates that in the case of salol, a depres- sion of the melting point of 2.8 about corresponds to a hundred- fold increase in specific surface. A far-reaching influence of the specific surface or curvature is indicated also in the phenomena of solidification or freezing of homogeneous systems. According to Muller-Thurgau, 2 filter paper, moistened with distilled water, freezes at 0.1, while a clay sphere, moistened with water, freezes, according to Bachmetjew, 3 at 0.7. These figures are not simply so called under-cooled values for water, but indicate freezing temperatures after such under-cooling is eliminated. In these processes of evaporation, of melting and freezing, a number of energies change. Positive and negative changes in volume and density take place, solid bodies acquire, on melting, free surface energies of the first order, the optical properties change, etc. For these reasons it is, as yet, not possible to show the relations which- exist between single energy changes and the simultaneously appearing changes in the surface energies, the effect of which increases with increasing specific surface. 3. Specific Surface and Electrical Energy. The relations between electrical energy and surface energies must also change when macro-heterogeneous are compared with disperse systems. Th. Des Coudres 4 showed that in harmony with our theory, a difference of potential between curved and flat surfaces of mercury may not only be proved experimentally but its value be ap- proximately calculated. Of the influence of an electrical potential opposing the positive surface tension, O. Lodge 5 states that in a drop this influence increases inversely as the fourth power of the diam- eter of the drop. In this connection should also be mentioned the important study of H. von Stein wehr 6 who found that finely ground calomel, as used in the preparation of normal electrodes, shows a greater difference of potential toward its saturated solution than does the same substance when less highly dispersed. Further 1 P. Pawlow, Z. f. physik. Chem.,65, i, 545 (1909); 74, 562 (1910); Koll.-Zeitschr. 6, 37 (1910); 7, 265 (1910); P. P. von Weimarn, ibid., 6, 32 (1910); 7, 205 (1910). 2 Mtiller-Thurgau, Landwirtschaftl. Jahrb., 9, 176 (1880). 3 Bachmetjew, Z. f. wissensch. Zoologie, 66, 584 (1899). 4 Th. Des Coudres, Wiedem. Ann. d. Physik. 46, 292 (1892). * Wm. C. Me. C. Lewis, Koll.-Zeitschr., 5, 91 (1909); also E. Hatschek ibid., 7, 158 (1910). 8 H. von Steinwehr, Z. f. Instrumentenkunde, 25, 205 (1906). GENERAL ENERGETICS OF THE DISPERSOIDS 93 relations between the value of the specific surface of electrodes and electrochemical phenomena may be found in the paper of G. Bredig and J. Teletow. 1 We would expect, on general principles, that the relations between surface energies and electrical energy would play an especially important part in the case of dispersoids. The majority of electrical phenomena take place on the surface since electrical energy, in contrast to heat, for example, tends to reside on the surface of a homogeneous body. The electrical capacity of a hollow metal condenser is therefore about as great as that of a correspondingly large solid body. Electrical energy will therefore often enter easily into reciprocal action with the surface energies. The great importance of these electrical phenomena in colloid systems will become apparent in the special parts of this book. 4. Specific Surface and Chemical Energy. Since colloids belong to the heterogeneous systems, the general law of chemical kinetics governing such systems, may be applied to them. This states that the amount of chemical change in the unit of time is proportional to the absolute surface (Wenzel). 2 This leads one to suspect, because of the extraordinarily large absolute surface in colloids, that many reactions will occur more rapidly in them than in coarse heterogeneous systems. Such is, in fact, true. M. Raffo and A. Pieroni 3 found that colloid sulphur behaved toward silver salts like an energetic reducing agent; while non-colloid sulphur, even though finely divided and obtained by precipitation of a polysulphide, would not form silver sulphide in the cold. Even after prolonged boiling this occurred only partially. The reactions of precipitated metallic silver vary according to .the size of its particles. The coarsely dispersed "gray" silver, obtained by reduction with oxalates, is less sensitive to mercuric chloride than is the highly dispersed "black" silver, precipitated by sulphites, etc. (R. Liesegang, Liippo-Cramer). 4 Analogous rela- tions exist in the decomposition of hydrogen peroxide by platinum. While smooth platinum foil decomposes this compound slowly, a "platinized" foil (one covered with finely divided metallic platinum) does it more rapidly. When colloid platinum is 1 G. Bredig and J. Teletow, Z. f. Elektroch., 12, 589 (1906). 2 See Wilh. Ostwald, Grund. d. allg. Chemie, 4 Aufl., 328, Leipzig, 1909. 3 M. Raffo and A. Pieroni, Koll.-Zeitschr., 7, 158 (1910). 4 Liippo-Cramer, Koll.-Zeitschr., 3, 35 (1908). 94 GENERAL COLLOID-CHEMISTRY used, the effect is still observable, if there is but i gram-atom of platinum in 70 million liters of the reaction mixture (or i gram-atom of colloid palladium in 26 million liters; or i gram- atom of colloid gold in one million liters). 1 Still greater surface effects are naturally to be expected when, as in the last example, we deal with phases having different specific surfaces, that is, having different surface concentrations in space. From the existence of capillary pressure and from the changes in density which result from this pressure we would expect an in- fluence upon the velocity of chemical reactions, for the speed of a chemical reaction is primarily dependent on the density of con- centration of the reacting components. Therefore, we would expect that the phenomenon ' of catalysis would be especially marked in colloid systems. The distinguishing characteristic of a catalyzer resides in the enormous change which it is capable of bringing about in the velocity of a chemical reaction. Thanks to the brilliant investigations of G. Bredig, 2 his students and others, it has been shown that many catalytic effects may be brought about by highly dispersed surfaces of all kinds, and that the especially important catalytic reactions of the organic ferments may be closely imitated by various inorganic materials in the colloid state, such as the colloid metals. We need in illustration but recall the catalytic effects on gases of a trace of platinum sponge, or platinum black as compared with the effects of a piece of smooth platinum foil. The great part played here by the specific surface, that is, the volume concentration of the surface, is also self- apparent. 1 G. Bredig, Bioch. Zeitschr., 6, 315 (1907); G. Bredig and J. Teletow, Z. f. Elek- troch., 12, 581 (1906); J. Teletow (abstract), Chem. Centr., i, 793 (1908). 2 G. Bredig, Anorganische Fermente., Leipzig, 1901 ; further, the recent review of the author in Bioch. Zeitschr., 6, 283 (1907) ; here may also be found many references to the literature. The following according to Bredig, are the best connected presentations of the field. Bodlaender, Uber langsame Verbrennung, Stuttgart, 1899. W. Ostwald, Grundr. d. allgem. Chem., 1909; Leitlinien der Chemie, 1906; Uber Katalyse, Leip- zig, 1902; Natur-philosophie, 1902. Sv. Arrhenius, Immunochemie, Leipzig, 1907; Theorien der Chemie, Leipzig, 1906. W. Nernst, Theoret. Chemie, 1909. W. Herz, Lehre von der Reacktionsbeschleunigung., Stuttgart, 1906. R. Hoeber, Physikalische Chemie der Zelle u. d. Gewebe, Leipzig, 1906. E. Cohen, Physical Chemistry for Physicians and Biologists, Trans, by M. H. Fischer, New York, 1903; H. J. Ham- burger, Osmotischer Druck u. lonenlehre i. d. mediz. Wiss., Wiesbaden, 1904. G. Bredig, Elemente der chemischen Kinetik, in Spiro u. Ashers Ergeb. d. Physiol., 1902. Schade, Bedeutung der Katalyse in der Medizin, Kiel, 1907. M. Bodenstein Chem.-Zeitg. 26, 1075, 1902. J. W. Mellor, Chemical Statics and Dynamics, Lon- don 1904; H. Freundlich, Kapillarchemie, Leipzig, 1909. Comprehensive presenta- tions by Bredig appear in Oppenheimer's Handb. d. Bioch. as well as in Bredig's Handb. d. angewandt. physik. Chemie. GENERAL ENERGETICS OF THE DISPERSOIDS 9$ Closely connected with density changes of great surfaces are the so-called adsorption phenomena, which we shall consider in detail later. With these are also connected changes of a chemical nature and reaction accelerations. But since they cannot be discussed to advantage without a previous discussion of adsorp- tion itself, we must postpone the whole matter. Even now, however, we may point out that theoretically the amount of a reaction product ultimately obtained, in other words, the equilib- rium point in a chemical reaction, -may be shifted under the influence of great spatial concentrations of the surface energies, as obtaining in dispersoids, for example. 1 If a chemical reaction occurs in the zone of contact between two phases, in which, for example, a positive surface tension is present, either of two things may happen. The surface tension may be either raised or lowered by the chemical change occurring in the two phases. In the first instance, the " chemical resistance," that is, the speed of the oppos- ing reaction, would be decreased through the consumption of energy necessary for the increase in the surface tension; in the second, wherein the surface tension diminishes, an acceleration of the reaction would occur, for the free surface energy produced would now tend to change into chemical energy. Besides the increase in rate, there would also be an increase in the product of the reaction, since the amount of chemical energy available for its formation is increased by the amount resulting from the transformation of surface energy into chemical energy. A great specific surface will therefore be able to shift the equilibrium point of a chemical reaction just as does a rise in temperature. Wilh. Ostwald 1 has given a practical illustration of this. 2 If the solution of a salt of a fatty acid is brought in contact with a large surface, the fatty acid set free by hydrolysis tends to collect in the surface, that is, it concentrates itself there more than does the base. The hydrolytic equilibrium of the remaining solu- tion is thereby disturbed, and to reestablish it, more of the 1 J. J. Thomson, Applications of Dynamics to Physics and Chemistry, 203, 234, London (1888); see also the extensive discussion of this question but not one free from objection, by T. B. Robertson, Koll.-Zeitschr., 3, 49 (1908), and succeeding pages, especially, Part III. That the osmotic equilibrium between two molecular disper- soids, and that the distribution of a molecularly dispersed substance between two phases depends on the specific surface of the phases has been proved theoretically by F. Kaufler, Zeitschr. f. physik. Chem., 43, 686 (1908). 2 Wilh. Ostwald, Z. f. physik. Chem., 62, 512 (1908). 96 GENERAL COLLOID-CHEMISTRY salt must hydrolyze. Other phenomena of this class, especially as observed in colloid systems, will be discussed later. Finally, it should be noted that several exceptions have been noted to the general rule that substances with large specific surfaces react more rapidly than coarsely dispersed ones. Mc- Intosh 1 states that colloid silver dissolves very slowly in acids. Its solution can be greatly accelerated by the addition of small amounts of permanganate. One is inclined to suspect the presence of silver oxide coatings over the metallic particles in this case, which interfere with the action of the acid, rather than to suppose this to be an actual exception to WenzePs law. Occasionally in the literature of colloid-chemistry, we encounter the statement that colloid solutions react " sluggishly." In the light of our discussion, this statement is not correct when compari- son is made between colloidally and coarsely dispersed systems. But when comparison is made with the reactivity of molecular and ionic dispersoids, it is. It has been proved with any two substances composing a dispersoid that the reactivity decreases progressively with decreasing degree of dispersion. In molecular and ionic dispersoids in which it might be said that the dis- perse particles consist " almost entirely of surface," one would therefore expect an enormous development of surface energies. In this connection, one is, as a matter of fact, reminded of the old chemical saying, "Corpora non agunt nisi soluta." But the part played by the chemical energy resulting from the conversion of surface energies during chemical reactions in disperse systems, must also decrease with increasing degree of dispersion. When we come to deal with maximum degrees of dispersion, in other words, with. " indivisible" particles such as molecules, atoms or even electrons, one might develop a conception according to which chemical reactions, that is, the union and separation of molecules or atoms, etc., represent merely the results of decreases in the surfaces of the particles involved. The dynamics of molecules and atoms and especially the effects of chemical energy can in this sense come to be viewed as mere manifestations of the surface energies of maximally dispersed particles. The discontinuity of matter in which we have always believed and which has been proved in various ways then becomes synonymous with the ex- 1 Mclntosh, Amer. Journ. Physic. Chem., 6, 17 (1902). GENERAL ENERGETICS OF THE DISPERSOIDS Q7 istence of an immensely great absolute, as well as specific surface; and all changes in this discontinuity become connected with changes in the amount of the surface of, or of the degree of dis- continuity in the substance, in other words, with changes in the capacity factors as well as the spatial concentrations of the surface energies. 1 5. Specific Surface and Radiant Energy. The connection between specific surface and another type of energy, namely, radiant energy, is closely related to the chemical phenomena discussed in the previous division. Stas 2 found that the photo- chemical sensitiveness of silver chloride precipitates increased with their degree of dispersion. Corresponding to the series given on p. 75, the sensitiveness to light increased from the granular, through the powdered, up to the flocculent or cheesy. Interest- ingly enough, Stas emphasized that it is the latter type and not the "gelatinous" state of silver chloride which is most sensitive to light. Were we to assume, as does P. P. von Weimarn, that the gelatinous is only a continuation of the other varieties of pre- cipitates, in the sense that the precipitate in the gelatinous form represents merely a still finer division of the particles, but is otherwise of the same general character, is crystalline, for ex- ample, then the behavior observed by Stas would constitute a contradiction of Wenzel's law. But not only the improbability of such an exception but many other reasons indicate that in " gelatinous" silver chloride we are dealing with a system fun- damentally different from that characterizing the other solid precipitates. It is an emulsoid in contrast to the others which are suspensoids. These relations between photochemical sensitiveness and size of granules have often been observed since Stas's work and have 1 The history of science teaches that we have always held to the theory of the dis- continuity of matter, but that different kinds of energy were in turn made responsible for or associated with the elementary changes in the discontinuity. Distance energy (attracting and repelling forces) kinetic energy, and more recently, electrical energy have in turn been associated with the discontinuity. It is of interest to point out that this electrical theory of the structure of matter is closely allied with the concept that the surface energies are the forces responsible for the elementary changes in discontinuity, for, as pointed out above, electrical phenomena occur chiefly on surfaces. It seems, therefore, but a further step in the same direction, if we add surface tension and surface energies to the "forces" already considered, since both of them are as widely distributed and important as the discontinuity of matter itself. 2 Stas, see K. Drucker, Koll.-Zeitschr., 4, 216 (1909). 7 98 GENERAL COLLOID-CHEMISTRY attained great importance in the practice of photography and in the preparation of photographic films. 1 A more interesting, and perhaps more important discovery is the unusual one of E. Wedekind and H. Baumhauer 2 (together with Gockel) that the emanations of radio-active substances may be much increased if they are highly dispersed, as by being converted into colloid form. These authors succeeded in preparing radio-active thorium in colloid form. A comparison of the radio- activity of this thoriumsol with that of the metallic (coarsely dispersed) element, measured by the volt decrease per hour, showed the surprising fact that the radio-activity of a sol containing only 0.0235 gram was equal to that of a coarsely dispersed sus- pension containing o.iu gram. In other words, the radio-activity of the sol was 4.8 times as great as that of the coarsely dispersed element. The extraordinary significance of this discovery 3 lies in the fact it has not as yet proved possible to influence markedly the emana- tion from a radio-active substance by any other means 4 as by raising the temperature, 5 evacuation, electrolysis, etc. A more striking demonstration of the great effect of the surface energies which come into play with increase in dispersion could scarcely be found than this singular effect of degree of dispersion upon the radio-active dissolution of the elements. Furthermore, this fact seems to indicate that the surface energies will come to play not only an important, but, in comparison with the other kinds of energy, perhaps a dominant part in a general theory of matter. 6 1 See Luppo- Cramer, Kolloidchemie u. Photographic (Dresden 1908) as well as the numerous papers of this author in the Kolloid Zeitschrift. 2 E. Wedekind and H. Baumhauer, Koll.-Zeitschr., 5, 192 (1909). 3 It was not recognized by the authors themselves. 4 See the Textbooks on Radio-activity. 6 Recently an insignificant influence of temperature has been observed (Engler, etc.). 6 It would be a feat in colloid chemistry to carry out analogous experiments with colloid radium salts. Since colloid, especially suspensoid systems exhibit their characteristic properties with even minimal amounts of disperse phase, only small amounts of radium salts would be necessary. One might first test out their prepa- ration by using the physico-chemically similar barium salts, and after having dis- covered a suitable "micro-chemical" method apply it to radium. Gelatinous radium salts could perhaps be prepared by methods analogous to those used by C. Neuberg and his students (Koll.-Zeitschr., 2, 321, 354) on barium salts in alcoholic solvents. CHAPTER IV DISTRIBUTION OF THE COLLOID STATE AND THE CONCEPT OF COLLOID CHEMISTRY 17. The Fundamental Independence of the Colloid State of the Chemical Nature of the Phases i. Statistical and Experimental Development of the Idea of the Universality of the Colloid State. In the forthcoming historical portion of this work it will be shown that the number of known colloid systems has steadily increased as colloid chemistry has developed. In Graham's time (1861) and even later, colloidality was generally held to be characteristic of certain substances, but with the discovery of general methods of preparing colloid systems, it soon became clear that this was too narrow a viewpoint. At the present time, we may say that practically all solid substances have been, or can be prepared in colloid form by some method or other. P. P. von Weimarn, for example, has by a single method " con- verted" over two hundred different substances (salts, elements, etc.) into colloids. Of course, different substances are changed into the colloid condition with different degrees of ease, but no decisive effect of the chemical nature of the substance whose dispersion is attempted has as yet been discoverable. Nor is the chemical nature of a dispersion means of basic signifi- cance in determining its ability to maintain a second substance in the colloid condition. Even Graham knew that different disper- sion media could mutually displace each other without destroying the colloid state. He was able to replace the water of a silicic acid gel with alcohol, with sulphuric acid, etc. And while the first known metallic colloids were hydrosols, many metallic organosols (metallic colloids in various organic dispersion means) have recently been prepared. Among these are the sols of the alkali metals which cannot even exist in water (The Svedberg). Neither is the suspensoid or emulsoid. character of a colloid 99 100 GENERAL COLLOID-CHEMISTRY determined by the chemical nature of the disperse phase. There exist inorganic as well as organic suspensoids. Generally speak- ing, the emulsoid states are more common than the suspensoid, in the case of albumins, for example; but suspensoids are also found among these, as shown by their ready precipitability through traces of electrolytes, by their low internal friction, etc. (see pp. 12, 13). As P. P. von Weimarn has shown in his fun- damental researches, the same substance may be obtained either in the suspensoid or emulsoid state (as a jelly) depending upon the conditions of its preparation. One and the same substance may also exhibit either a suspensoid or an emulsoid charac- ter depending upon the nature of the dispersion means, as Freundlich and Neumann have found in the case of dyes (see P. 56). Finally, one and the same substance may appear under different circumstances either as a crystalloid or a colloid. We need but recall the crystallization of albumin or, on the other hand, the production in colloid form of materials usually known only as crystalloids, such as common salt. 1 P. P. von Weimarn (I.e.) recently, showed that mere change in the concentration of the com- ponents of a reaction mixture sufficed to precipitate them either in colloid or crystalloid form. These facts show clearly the fun- damental independence of the colloid (and crystalloid) state, of the special chemical properties of the substances involved. Obviously, the growing acquaintance of investigators with new colloid materials could not help but lead them gradually to recognize that colloid properties were not confined to specific chemical substances. The attempts of P. Rohland 2 in 1907 to tabulate colloid materials showed clearly how impossible was such a chemical viewpoint. The result was entirely unsatisfactory, for the table included not only a heterogeneous lot of chemical substances, but was incomplete. Conversely, however, it dem- onstrated the impossibility of coordinating satisfactorily chemical composition with colloid properties and brought home the fact that all materials may occur in the colloid state. But while this view was already beginning to be recognized in 1905 as a nec- essary conclusion to be drawn from the rapidly increasing list of 1 C. Paal, Ber. d. D. chem. Ges., 39, 1436, 2859, 2863 (1906). 2 P. Rohland, Koll.-Zeitschr., i, 201, 289 (1907); 2, S3 (1907)- DISTRIBUTION OF THE COLLOID STATE IOI colloid materials 1 it should be emphasized that P. P. von Wei- marn (1906) was the first to express clearly and emphatically on the basis of these findings that the colloid, like the crystalloid, is a universally possible state of matter. Although experiment shows the colloid state to be independent of the chemical composition of the phases, this does not of course mean that the properties of the dispersoids may not change with varying chemical composition of the phases. Examples have already been given which show that one and the same chemical substance may assume different types of dispersion with dif- ferent kinds of dispersion media. The usual view of this be- havior which holds the " chemical nature" of the phases respon- sible for the observed changes, may easily lead to error, for it is not the chemical properties, in other words, the analytical composition and the reactivity which determines that a sub- stance dissolves as a colloid or molecular dispersoid, but rather the different physical properties such as different " solubility" values, etc., in other words, the free surface energies which bring about the variations in degree of dispersion. Of course, these physical properties, like other properties are in good part de- pendent upon the chemical composition of the phases, and so change with chemical changes in these. Obviously the sta- bility, reactivity, etc., of a colloid must therefore vary with changes in the chemical composition of the phases concerned. But the chemical relations between disperse phase and dispersion means characterize the dispersoid just as little as the absorption or liberation of heat which always accompanies chemical processes completely characterize these, even though, as is well known, temperature influences them greatly. 2. Universality of the Colloid State as a Necessary Con- sequence of Characterizing Colloid Solutions as Disperse Systems. If it is granted that colloid solutions are merely repre- sentatives of disperse systems and that their properties are deter- mined through a degree of dispersion which has both an upper and a lower limiting value, it becomes self-evident that almost any desired material may be prepared in the colloid condition. For as certainly as all substances have not an unlimited solubility in every solvent, 1 In this connection see Wo. Ostwald, Koll.-Zeitschr., 6, 184 (1910) ; R. Zsigmondy, zur Erkenntnis der Kolloide, pp. 170, 171, 175, Jena, 1905. 102 GENERAL COLLOID-CHEMISTRY equally certainly can these substances be gotten into a disperse form provided a proper dispersion means is chosen. For every substance a second may be found in which the first is "insoluble" or only slightly "soluble." All special problems in "colloid synthesis" consist in finding the experimental means of obtaining the average values in degrees of dispersion that are characteristic of colloids, or of fixing such in passing through a series of progressively changing degrees of dispersion. The question of the "possibility of all substances existing in a colloid state" stands and falls with this recognition of colloid solutions as mere examples of dispersed heterogeneous systems. It is evident that this classification of colloid and dispersed systems accepts from the start the fundamental independence of the colloid state of the special chemical nature of the phases, for to classify disperse systems according to their degree of dispersion and the state of their phases makes use of no chemical conceptions whatever. On the contrary, it expressly departs from them. In this sense, the assumption of the universality of the colloid state must be termed the first and most essential generalization that may be made regarding this class of dispersed systems. It was therefore included in the conception proposed by Wolfgang Ost- wald in 1907 and developed independently of the investigations of P. P. von Weimarn. It should be emphasized, however, that even before the latter's investigations, the accepted inductive recognition of the essential independence of the colloid state of chemical composition constituted one of the essential steps which made possible the characterization of colloids as disperse hetero- geneous systems. 1 18. Isocolloids The view that the colloid properties of a substance represent only properties of state may perhaps be most strikingly demon- strated by considering a class of colloids in which both disperse phase and dispersion means have the same chemical composition. Among these remarkable colloids are found such as consist of but one chemical substance, in other words, their disperse phases and their dispersion means both consist of the same substance but in different "states"' We deal here with systems which are often 1 See P. P. von Weimarn and Wo. Ostwald, Koll.-Zeitschr., 6, 183 (1910); P. P. von Weimarn, ibid., 7, 155 (1910). DISTRIBUTION OF THE COLLOID STATE 103 referred to as "colloid" or "colloidally amorphous," but whose place and relation to the normal or more common colloids has not yet been clearly determined. We shall term these structures in which disperse phase and dispersion means are chemically iso- meric, isocolloids (isodispersoids) . For those cases in which a single element (in allotropic forms) makes up the colloid system we may reserve the name allocolloids (allodispersoids). 1 Examples of isodispersoids, more particularly of isocolloids are common in both inorganic and organic chemistry. As mentioned in the practical introduction (p. i) liquid colloids are especially apt to belong to the group of liquids which behave " abnormally." In spite of agreement in elementary analysis these structures may, through distillation for example, be divided into several fractions; in other words, they have no "constant" boiling point. Their internal friction often shows a remarkably high temperature coefficient, in other words, varies greatly with changing temperature (mixtures of fluid polymers). Their so-called molar surface energy, that is, the value V^ .7 (V = molar volume = volume of the gram molecular weight; 7 = the positive surface tension) is less than that of normal fluids (associated liquids). 2 They can at times be separated through centrifuging or even filtration, into a solid or semi-solid phase and a liquid one. They betray their physical heterogeneity optically, for they are turbid, opalescent, give a positive Tyndall effect, and in the coarser dispersoids, a " struc- ture" can be recognized microscopically. Among these systems are found oils, waxes and different varieties of rubber, the higher fatty acids, the fractions of mineral oils which come off at high temperatures, 3 probably molten salts (which according to R. Lorenz 4 are strongly associated), and molten materials of other com- position, as phosphoric acid and arsenious acids, which on cooling give rise to glacial forms. The name "glacial" indicates that emul- soid types of isocolloids occur, and our present knowledge would seem to show that the emulsoid type is by far the more common. The so-called "meta forms" of molten materials (which are not 1 Of the available prefixes, eu, hylo, allo, auto, iso, etc., that of iso is perhaps the best. 2 For details see the textbooks on Physical Chemistry, for example Wilh. Ostwald, Grundr. d. allgem. Chemie, 4 Aufl., Leipzig, 1909. 3 For a discussion of their colloid properties see D. Holde, Koll.-Zeitschr., 3, 270 (1908); Z. f. angewandt. Chem 2138 (1908); J. Schneider and J. Just, Z. f. Wis- sench. Mikrosk, 22, 501 (1905). 4 R. Lorenz, Z. f. physik. Chem., 70, 236 (1910). IO4 GENERAL COLLOID-CHEMISTRY to be confused with the meta forms of dissolved materials, like the different stannic acids) are particularly apt to produce jellies and glasses on cooling, as do such typical emulsoids as gelatine and agar. An especially instructive and lucid example is the system styrol-metastyrol, recently investigated by G. Posnjak. 1 Styrol, a hydrocarbon of the composition CsHg polymerizes spontane- ously on standing into (C 8 H 8 ) n which, according to the degree of polymerization, has a jelly-like or glass-like consistency (metas- tyrol). The polymerization can be followed quantitatively by measuring the internal friction which increases as the polymer- ization progresses. Interestingly enough this particular example shows what is also true of other substances, that light as well as heat favors polymerization (photopolymerization). When polymeriza- tion is complete the product (metastyrol) is hard and glassy, may be pulverized, etc. But the intermediate stages in the polymeriza- tion are nothing more than colloid solutions of solid metastyrol in liquid styrol. "If one adds to pulverized metastyrol an equal weight of styrol, the former gradually absorbs the latter. In the process the originally opaque powder becomes translucent and gradually changes into a homogeneous, gelatinous or jelly-like, viscid, transparent mass. If less styrol is added to the metastyrol , say only about a fourth as much of the former as of the latter, a transparent mass results which is not viscid, but glassy" (G. Posnjak, I.e., 14). These changes are entirely analogous to those observed in the swelling of emulsoids. That, on the other hand, fluid colloid solutions may be obtained by employing an excess of the liquid styrol, follows from the unlimited solubility of metastyrol in styrol 2 as observed by G. Lemoine. 3 Sulphur 4 is a typical allocolloid, as are phosphorus and selenium. 1 G. Posnjak, Das Metastyrol und die beiden Distyrole, Diss., Leipzig, 1910. 2 The different polymerization or dispersion states of styrol are preserved even when dissolved in a chemically heterogeneous solvent such as carbon tetrachloride. This is proved not only by the fact that, like a typical colloid, metastyrol when in solution, causes no rise in the boiling point, but also by the different reactivity observed in the different stages of polymerization, When a drop of permanganate solution, made alkaline with sodium hydroxide, is added to solutions of liquid styrol, gelatinous styrol and solid metastyrol, having the same percentage concentrations, it is decolorized respectively in 10 seconds, in 40 seconds and 650 seconds (G. Posnjak, l.c., 16). 8 G. Lemoine, Compt. rend., 125, 530 (1897); 129, 719 (1899). 4 I have previously pointed out the great interest attached to a considera- tion of the allotropic forms of sulphur from a colloid-chemical standpoint, Koll.- Zeitschr, 7, 172 (1910). DISTRIBUTION OF THE COLLOID STATE 10$ As is well known, the soft, plastic, translucent to transparent 1 form of sulphur obtained by cooling it rapidly has long been called colloid or "colloidally amorphous" sulphur. As the result of many investigations, we have been forced to assume the existence of several more so-called allotropic modifications more especially of two liquid forms of sulphur, designated as S x and S M . When pure sulphur is heated one obtains the modification S x which is a bright yellow, labile liquid, from the time the sulphur first melts up to i6oC. If the temperature is further raised the system again becomes viscid and its surface tension again increases. If this melted sulphur is cooled to 100 then as Malus, F. Hoffmann and R. Rothe, A. Smiths, etc., 2 have shown a second liquid sulphur phase, S M makes its appearance which gives rise to the so-called "insoluble" sulphur when the sulphur solidifies. Between 160 and about 270 in the range of the physical irregularities men- tioned above, the system is therefore allodispersed and it seems logical to assume that at certain temperatures, a colloid condition is traversed. The physical characteristics observed in the be- havior of sulphur remind us in many respects of the behavior of emulsoid colloids. 3 1 Concerning perfectly "transparent" sulphur see P. P. von Weimarn, Koll.- Zeitschr., 6, 250 (1909). 2 An extended discussion of the work done on sulphur up to 1902 may be found in Wilh. Ostwald, Lehrb. d. allg. Chem. 2, Aufl. II, 2, 449. See also the recent extensive papers of A. Smiths and his coworkers, Zeitschr. f. physik. Chem., 42, 469 (1903); 52, 602 (1905); 54, 276 (1906); 57, 685, 692 (1907); 61, 200 (1907); F. Hoffmann and R. Rothe, ibid., 55, 113 (1906); 59, 448 (1907); H. R. Kruyt, ibid., 64, 513 (1908) where extensive references to the literature may be found; 65, 486 (1909), etc. 3 1 purpose publishing details regarding these analogies elsewhere. Here I would only point out that sulphur melts bring to mind the critical fluid mixtures such as those of butyric acid and water, a fact which seems not previously to have been noted. Thus the viscosity curves in the temperature ranges where an anomalous behavior is noted [see L. Rotinjanz, Z. f. physik. Chem,, 62, 609 (1908)] are iden- tical in form with the corresponding friction curves of aqueous critical fluid mixtures [J. Friedlander, ibid., 38, 430 (1901)]. Attention should also be called to the non- conclusiveness of the view that both fluid phases "cannot" exist in equilibrium above and below the "transition" point (about 160) because this would contradict the phase rule [see especially H. R. Kruyt, I.e.]. As a matter of fact the apparently unlimited stability of critical fluid mixtures as proved by Friedlander's careful studies seems to indicate that "real" equilibria and not simply "dynamic" retarda- tions exist in the case of sulphur also. Those investigators, who assert that the existence of true equilibria would contradict the phase rule, forget that this rule holds only if the nature of the phases is the same throughout their entire mass, or, to quote Gibbs himself, only "if the changes of the shares of energy and entropy which arise from the surfaces between the heterogeneous masses, are so small in comparison with those which arise from these masses themselves that they are negligible. In other words, we will exclude any consideration of the effects of capillarity." [Thermo- dynamische Studien, Leipzig, 1892, 75; see especially pp. 89 and 90 where the reasons for the limitation mentioned above are given in greater detail.] But this 106 GENERAL COLLOID-CHEMISTRY As shown by the photographic investigations of O. Biitschli 1 and A. Wigand, 2 highly dispersed solid systems of sulphur may also be produced. It would be of interest to extend these investi- gations into the ultramicroscopic realm of the colloids. This has been done by H. Siedentopf 3 for phosphorus which shows analogous separation phenomena under the influence of intense light. 4 White phosphorus, as is well known, is converted into red, by light. The intense light of a so-called " cardioid " ultramicroscope accomplishes this in a few seconds. Ultra-microscopically, a solidified drop of white phosphorus at first appears " empty," but almost instantly, upon illumination, white submicrons appear. These grow and combine through tendril-like extensions into a kind of network, and finally become red. According to these investigations white phosphorus, with a trace of red phosphorus undoubtedly represents a solid, at first perhaps, semi-solid allocolloid. 19. Multiplicity of the Colloid State of One and the Same Sub- stance. 'Example: Colloid Ice As a further evidence of the fundamental independence, of the colloid state of the chemical character of the substance, and as an assumption of Gibbs as used in his general considerations of the phase rule is, of course, not true as I have repeatedly emphasized, and as Gibbs himself has said, when we deal with disperse and more especially, colloid systems. Gibbs started with the assumption that we could neglect that part of the energy, etc., which originates in the surfaces between the heterogeneous masses. Such an assumption is fully justified in many cases and for many purposes, or whenever the masses are large; but when the masses are formed in or between materials of different nature or state, or are at the instant of formation infinitely small, such an assumption becomes unreliable, for now the surfaces become infinitely large as compared with the masses. In answer to the question as to whether the phase rule holds for colloids, we may say, that a phase rule which recognizes only concentration, pressure and temperature as variables, is not going to be valid, but an elaborated one in which the degree of dis- persion of the system is added will be. The same is true in dealing with electrically charged disperse phases, in which account must be taken of the influence of electrical energy upon the equilibrium of the system; for as is well known, the latter is not, or at least not always, a function of the mass of the charged phase. Regarding the val- idity of the phase rule in colloid systems see J. M. van Bemmelen, Die Absorption, Ges. Abhandlgn., 347; Dresden, 1910; A. Mittasch, Zeitschr. f. physik. Chem.,34, 495 (1900); G. Galeotti, ibid. ,54, 727 (1905); P. Pawlow, ibid.,fs t 48 (1910). In the last- named paper which appeared while these paragraphs were in press, the phase rule is broadened as suggested above. 1 O. Butschli, Untersuchungen iiber Strukturen, Leipzig, 1898; Die Mikrostruk- turen des estarrten Schwefels, Leipzig, 1900. 2 A. Wigand, Zeitschr. f. physik. Chem., 72, 752 (1910). 3 H. Siedentopf, Ber. d. dtsch. chem. Ges., 43, 692 (1910). 4 As is well known, sulphur also suffers an allotropic change under the influence of light (Daguin, Lallemand, Berthelot, Rankin, etc.; see H. R. Kruyt, I.e., 543 (1908). DISTRIBUTION OF THE COLLOID STATE 1 07 example of the multiplicity of colloid phenomena that may be exhibited by one and the same substance, we shall consider the possible colloid forms of water, and those which have actually been prepared. As follows from the chapters on the influence of the degree of dispersion and of the type of the dispersed material upon the colloid state, the same chemical compound may give rise not only to one, but to a large number of colloid systems. If our view is correct, that only a certain intermediate degree of dispersion gives the property of a colloid, then it should be a matter of indifference, in the case of water, with which form we begin. From it we should be able to prepare all its various colloid states. That such is possible seems proved by recent investiga- tions (G. Quincke, 1 P. P. von Weimarn and Wo. Ostwald, 2 H. Schade 3 ). 1. Isocolloids of H 2 O. Let us commence with the isocolloid state. The following possibilities exist : (a) Solid ice (dispersion means) + water -vapor bubbles of colloid dimensions (disperse phase). Coarsely dispersed systems of this character, as the turbid or milky- white ice (milk ice) produced by refrigerating machines, are well known. It is probable that a more exact study of the relation between degree of turbidity or diameter of the bubbles and the conditions pre- vailing during preparation of the ice will acquaint us with systems showing a colloid degree of dispersion. (b) Solid ice (dispersion medium) + liquid droplets of colloid diameter (disperse phase). Considerations analogous to those discussed in the preceding paragraph hold for this case. G. Quincke (I.e.)' regards it as the most general structure of solid, " amorphous" ice. (c) Solid ice (dispersion medium) + solid ice of a colloid degree of dispersion (disperse phase). That such systems exist is proved by the investigations of G. Tammann, 4 who, as a matter of fact, recognizes three or four different modifications of solid ice. According to G. Quincke (I.e.) the forms of ice described in 1 G. Quincke, Drude's Ann. d. Physik., 18, n (1905). 2 P. P. von Weimarn and Wo. Ostwald, Koll.-Zeitschr., 6, 181 (1910). 3 H. Schade, Koll.-Zeitschr., 7, 26 (1910). See also the extended discussion of this theme in Trans. Faraday Soc., 6, 71-129 (1910). 4 G. Tammann, Zeitschr. f. physik. Chem., 69, 569 (1910); 72, 609 (1910); Zeit- schr. f. anorgan. Chem., 63, 283 (1909) where may be found further references to the literature on the different modifications of ice. 108 GENERAL COLLOID-CHEMISTRY the previous paragraphs, may, by lowering the temperature, by freezing at different rates, etc., be converted into the system dis- cussed here. (d) Fluid water (dispersion medium) + water-vapor bubbles of colloid dimensions (disperse phase). Highly dispersed systems of this type are represented by the chemically homogeneous fluids near their critical vaporization temperatures. Their fine turbidity or strong opalescence, their varying density, etc., etc., betray their colloid character. 1 (e) Fluid water (dispersion medium) + fluid water droplets of colloid dimensions (disperse phase). As is well known, water belongs to the strongly associated liquids, so that the formation of complexes and of polymeric molecules (polyhydrols) more especially at lower temperatures has often been called upon to explain its anomalies in behavior. 2 Since mo'dern theoreticians hold that this polymerization may be so great that they speak of the formation of "droplets" of the polymer in the non-asso- ciated liquid, it is easy for us to assume that colloid degrees of dispersion may be reached. H. Schade (I.e.) has, as a matter of fact, developed such a colloid-chemical theory of the Con- stitution of water, according to which, its behavior in many directions is shown to be analogous to that of well-recognized colloids. The observation of P. P. von Weimarn (I.e.) that sud- denly cooled water (induced by mixing with liquid air) is at first soft and viscid, brings to mind the behavior of melted sulphur, when this is cooled to iooC. (/) Fluid water (dispersion medium) + colloidally dispersed solid ice (disperse phase) . It cannot be definitely decided whether highly dispersed examples of this type really exist. Snow in water would represent a coarsely dispersed system of this type. (g) Water vapor (dispersion medium) + water droplets of colloid dimensions (disperse phase). Examples of this are found among the critical phenomena as in the liquefaction of water vapor. Such structures, usually called mists or fogs, play a great part in cosmic physics, where they are known as clouds, or when in a coagulated state, as rain (P. Pawlow). 3 1 See, for example: J. P. Kuenen, Die Zustandsgleichung, 34. Braunschweig, 1907. 2 See H. Schade for references to the literature on this subject. 8 P. Pawlow, Koll.-Zeitschr., 8 (1911). I also pointed out that these cosmic structures are examples of dispersed or colloid systems [Koll.-Zeitschr., I, 291, 331 (1907), and in the first edition of this book, 96 (1909)]. DISTRIBUTION OF THE COLLOID STATE 109 (ti) Water vapor (dispersion medium) + colloidally dispersed ice (disperse phase). Such systems are obtained as fine snows when fogs are rapidly cooled below their freezing point. Their optical properties, which remind us of the Tyndall effect of col- loid systems, are observable on winter nights, when the moon has a halo. The fact that in winter we deal with fine particles of solid ice explains why the halo is then more marked than in sum- mer when the mists are liquid in character. 2. Chemically Heterogeneous H-,0 Colloids. When only the disperse phase consists of water, while the dispersion means is another substance, then as in the case of the water-isocolloids eight colloid systems are possible. The water must of course be insoluble in such systems, or at least but slightly soluble, otherwise molecular dispersoids result. It is an easy matter to take up these eight possibilities and parallel them with what was said before, and to find examples for the different types. Of especial interest are the ice colloids of the composition liquid + gas, liquid + liquid and liquid + solid, in other words, the aqueous colloid "foams," the aqueous emulsoids and the ice suspensoids. In the known examples of the first two of these classes, we seem again to be dealing as a rule, with complex dispersoids, in that the gas bubbles obtained, say, by sudden diminution of pressure in an alcohol-water mixture (champagne or charged water constitute crude examples), usually consist of a mixture of several gases (water vapor, alcohol vapor, air, carbon dioxide, etc). More decisive experiments could, of course, be arranged. 1 The commonest illustrations of highly dispersed water are also found in complex systems, as in phenol-water, mineral oil-water, etc. Such sys- tems, as a rule, are rather unstable, their instability increasing with the purity of the liquids employed. The stability may be greatly increased by adding substances like saponin, soap, gelatine, etc. Finally, highly dispersed colloid systems of the composition liquid + solid, in other words, ice suspensions and ice suspensoids, have recently been prepared. Such systems are obtained when organic liquids in which water is only slightly soluble in molecular 1 For some observations on highly dispersed aqueous foams see Wo. Ostwald, Koll.-Zeitschr., I, 333 (1907). IIO GENERAL COLLOID-CHEMISTRY form are rapidly cooled while thus saturated with water. Ether, xylol and especially chloroform have been found suitable for this purpose. Depending upon the amounts of water dissolved, the rate of cooling, etc., systems of varying degrees of dispersion are produced, the turbidity of which is the less, or the opalescence (yellowish-blue) of which is the greater, the more highly dispersed the ice phase which separates out. Under favorable conditions, these mixtures coagulate spontaneously after standing some 40 minutes, 1 in the form of snow-white flakes or " cream," which rise to the surface of the liquid. The addition of certain sub- stances like resins, the salts of fatty acids, etc., greatly increases both stability and degree of dispersion. By rapidly chilling chloro- form saturated with water to 30, ice dispersoidsmaybe obtained which pass through filter paper (Schleicher and Schiill, 602, extra hard). This indicates that the ice particles are less than i/* in diameter, in other words, that they are already within the colloid range of dispersion. The possible colloid forms of water are not exhausted by these sixteen types. It was pointed out on pp. 35, 44, etc., that colloid systems assume different properties with differences in the relative proportions of disperse phase and dispersion means. This is especially true when very dilute colloids are compared with concentrated ones. Thus while many colloids with a small content of disperse phase show suspensoid properties, the more concentrated systems often have emulsoid properties, they are, in other words, jelly or glass-like. One suspects that he is here dealing with a general law and so looks for such in the extreme concentrations of ice colloids. Thus J. Alexander 2 has shown that "ice creams" consist of numerous highly dispersed ice crystals in the dispersion medium, cream, to which a trace of gelatine is often added. The bulk of the ice crystals is so great as compared with that of the dispersion means, that as in the case of fine, moist sand, the dispersion medium covers the solid dispersed particles with a thin but coherent envelope. So far as structures of the composition liquid + liquid are concerned, we may direct attention to the experiments of Wa. Ostwald 3 which indicate the possible existence of emulsions in 1 See P. P. von Weimarn and Wo. Ostwald, Lc. 2 J. Alexander, Koll.-Zeitschr., 4, 168 (1909); 5, 101 (1909). 3 Wa. Ostwald, Koll.-Zeitschr., 6, 103 (1910). DISTRIBUTION OF THE COLLOID STATE III which the disperse (aqueous) phase is present in excess. Foams may also occur in these two modifications. We may have fluid structures made turbid by minute bubbles (see above) or "stiff foams" consisting chiefly of vapor, held together by a small amount of fluid dispersion means, as in well-whipped saponin, albumin, beer, etc. It is characteristic of most of these systems that they are fairly stable only when certain third substances are present. In concluding these paragraphs it should be noted that even with this long list all the theoretical possibilities are not yet exhausted. For example, there are several solid modifications of ice known, any of which may appear, theoretically at least, as disperse phase or dispersion means; and when we consider com- pounds or elements whose polymorphism is still greater, as in the numerous solid, liquid and gaseous modifications of sulphur, the number of disperse and colloid states of one and the same substance, which are theoretically possible, becomes almost limitless. 20. The Concept of Colloid-chemistry The most important deduction from the previous para- graphs is that it is no longer appropriate to contrast colloid substances with crystalloid substances as though the condition were dependent upon the specific chemical properties of the material. Colloid-chemistry is not the study of colloid materials but that of the colloid state of materials (Wo. Ostwald, 1908). " Colloid" is not a chemical entity like salt, acid, base, oxidizing or reducing agent, but is expressive of certain physical elements like mechanical heterogeneity. The concept " colloid" does not even correspond to that of " precipitate " since only special forms of precipitates may be termed " colloid." Nor may "col- loid" substances be discussed as we discuss "radio-active" sub- stances, for radio-active properties are more closely associated with certain chemical compounds showing definite properties (high atomic weight, etc.), than are the colloid. Like considerations hold when we try to parallel the colloid condition with the "liquid crystalline," though as our knowledge has increased we have found the latter state less and less directly connected with definite 112 GENERAL COLLOID-CHEMISTRY chemical compounds. 1 In the same sense " colloid phenomena" are not to be regarded as due to the properties of colloid materials but rather as characteristic of any material observed in the colloid state. The difference between these two definitions will perhaps be clearer if we compare colloid-chemistry with thermo-chemistry. Just as the latter is not a study of "warm" and "cold" materials, but a study of the thermal condition of the material and its changes, so colloid-chemistry is not a description of individual colloid materials but treats of the properties of which colloid systems are but examples. Colloid-chemistry deals with the relations of the surface energies to other kinds of energy as shown in an 'espe- cially characteristic way in dispersed heterogeneous systems. Thus viewed, colloid-chemistry appears as a branch of physical chem- istry coordinated with electro-, thermo-, photo-, radio-chemistry, etc., in other words, with sciences which also treat of the relations of one kind of energy to others. Attempts have been made to express this by calling it capillary chemistry (Freundlich), strato- chemistry (Drucker), micro-chemistry (Wilh. Ostwald), etc. Since philological and practical objections may be raised against most of these terms the historically justified and useful one of "colloid- chemistry" will be retained in this work. 1 We need but recall the "inorganic" liquid crystals recently discovered. See H. Stoltzenberg and M. E. Huth, Z. f. physik. Chem., 71, 641 (1910), etc. PART II SPECIAL COLLOID-CHEMISTRY A. THE GENERAL .PHYSICO-CHEMICAL PROPERTIES OF COLLOIDS CHAPTER V MECHANICAL PROPERTIES OF COLLOID SYSTEMS I. RELATIONS OF VOLUME AND MASS IN COLLOIDS 21. Volume and Density Relations in Colloids i. Volume Relations of Colloid Systems. Compressibility In one of the previous sections (14) reference has been made to the fact that the surface between two phases (for example the surface between a liquid and a gaseous phase) has properties distinctively its own. *These peculiar surface properties do not extend much below the surface, not deeper than the " sphere of molecular attraction." But in spite of the thinness of this layer it does not resemble a mere shell, in other words, it is not sepa- rated from the rest of the phase by a sharp line. Mathematical considerations 1 would seem to indicate that there is a continuous change of properties within this layer, extending asymptotically into the depth of the phase. In addition to the fact that the surface of contact is the seat of surface energy we must also assume that its hydrostatic pressure and the related properties of volume and density differ from these same properties as ex- hibited by the interior of the phase. We have also pointed out how in coarsely disperse systems these differences can be demon- strated only with great difficulty. But the slight differences ob- served in coarsely disperse systems add up to considerable values in systems which have great areas of " surface contact." Especially is this true in the disperse heterogeneous systems where a great specific surface is found with great absolute surface. The relation between specific surface and volume is established by the capillary or curvature pressure (Krummungsdruck). Consideration of the progressive change in the properties of the surface and the effects of capillary pressure lead to the con- clusion that the volume of the dispersoid need not be equal to the arithmetic mean of the dispersion means plus the disperse phase. 1 See for example van der Waals and Kohnstamm, Lehrb. der Thermo-dynamik I, 64, as well as Hulshof from whose works we have quoted. Il6 SPECIAL COLLOID-CHEMISTRY The observed values are usually less than the arithmetic mean. This is in harmony with the assumption that the effect of positive capillary pressure generally outweighs that of the negative. 1 The amount of this contraction besides being a function of the positive capillary pressure is evidently a function of the com- pressibility or expansibility of the two phases in the sense that a great coefficient of compression or expansion will favor change in volume. It seems important, therefore, to consider the compression coefficients of colloid solutions. Theoretically, three compressi- bilities must be considered in a colloid (or in any dispersoid) : the compressibility of the dispersion means, the compressibility of the disperse phase, the compressibility of the system as a whole. The first and third of these can be measured by physical methods, the value of the second may be calculated from the other two. Of greatest immediate interest is a comparison of the compressi- bility of a colloid solution with that of the pure dispersion means. One anticipates that the compressibility of the colloid solution will be smaller than that of the pure dispersion means, and that it will decrease as the concentration of the colloid increases. That these relations will be more complicated in emulsoids than in suspensoids is also to be expected. The curve expressing the relation between concentration and compression of emulsoids will probably not be a straight line as in suspensoids. The fact that it is not a straight line in molecular- and ionic- disperse systems already indicates this; for as the investigations of Rontgen and Schneider, 2 H. Gilbaut 3 and others have shown, this function approximates a hyperbolic curve diverging more and more from a straight line with increasing concentration (see Fig. 20). Fig. 17 shows the relation between compressibility and concentration of NaCl according to the experiments of H. Gilbaut. A similar behavior is to be expected for the emulsoids because of their close relation to the molecular dispersoids. One would also expect that electrically charged or ionized colloids would decrease the compressibility of the pure dispersion means more than such not so charged, for according to Guinchant 4 1 See p. 91. 2 W. Rontgen and Schneider, Wiedemann's Ann. d. Physik., 29, 165 (1886). 3 H. Gilbaut, Z. f. physik. Chem., 24, 385 (1897). 4 Guinchant: Compt. rend., 132, 469 (1901). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 117 the compressibility of water is less reduced through addition of a non-electrolyte than of an electrolyte. Of the available determinations on colloids those of G. de Metz 1 merit special attention. Table 3 gives a selection from his careful measurements. For comparison the values of water and some other non-colloid solutions are appended. The table shows that at least as far as the emulsoids which have thus far been studied are concerned the compressibility of colloids is not essentially different from that of other non-colloid liquids. The compressi- ConcenlraHon *~ FIG. 17. Compressibility of NaCl solutions in concentrations varying between o and 26.22 per cent. (According to H. Gilbaut.) bility of collodion is twice that of water, but crystallized benzene has also an abnormally high coefficient. In the case of hydrosols we find, with the exception of setting gelatine, that the coeffi- cient of compressibility is lower than that of the pure dispersion means which again is analogous to the behavior of molecular dis- persoids. This also seems to apply to benzene-sols, as comparison of the values for pure benzene and for a solution of Canada balsam in benzene indicates. 1 G. de Metz, Wiedemann's Ann. d. Physik., 35, 497 (1888); see also G. Quincke, Ibid., 19, 401 (1883); E. H. Amagat, Ann. chim. et phys. (5), n, 535 (1877)- n8 SPECIAL COLLOID-CHEMISTRY TABLE 3. COMPRESSION COEFFICIENTS OF COLLOID SOLUTIONS (From G. de Metz) Substance Specific gravity Compression coefficient (absolute) k.io- Non-gelatinous glue 1.053 (l4-8) 44 337 (l2 18) Gum arable in water Gelatinizing glue* Canada balsam in benzol .... Duplex collodion. ... .... 1.041 (14-0) 1.005 (18.2; 0.950 (15.0) o 807 (15 o) 44-593 (14-84) 48.388 (11.67) 57.205 (14-90) 07 433 (lA 8 FIG. 24. Effect of time upon the viscosity of serum albumin to which alcohol has been added. (According to /. Simon.} It seems possible, therefore, that under the experimental conditions chosen by this investigator his solutions coagulated, in which case their behavior would naturally be irregular. Their method of preparation (swelling, trituration, nitration through glass wool) may also have influenced his findings. His solutions may have contained " undissolved " particles which at first caused a high viscosity, but which, later, after their " solution," led to 1 H. W. Woudstra: Kolloid-Zeitschrift, 5, 33 (1909). 2 Dr. Brauer of Leipzig has also observed that filtered solutions of purified rubber which are originally entirely clear show flocculi after standing some weeks. Since then I have been able to make analogous observations on benzol rubber sols. 158 SPECIAL COLLOID-CHEMISTRY decrease in the viscosity. Further experiments are needed on this point. In accord with Woudstra's observations are those of K. Schorr and H. Handovsky (I.e., 1910) who found that albumin solutions first show a gradual increase in viscosity but later a slow de- crease on the addition of alkali. Chemical changes (hydrolytic cleavage, etc.) which produce secondarily a decrease in viscosity, somewhat analogous to the hydrolytic action of ferments, are undoubtedly active here (see p. 160). 7. Effect of Mechanical Treatment on Viscosity of Emulsoids. It is a remarkable fact that the viscosity of emulsoids is affected by mechanical treatment. If they are shaken for a period or simply pressed several times through a capillary, as in a viscosimeter, their viscosity decreases. Such phenomena have been observed by Gokun (I.e.) and W. Biltz (I.e.). They show that even in such apparently perfect liquids there is present a kind of "structure" which is destroyed by mechanical treatment. This structure seems closely allied with the oft-mentioned liquid membranes of the dispersion medium which surround the disperse particles and which we used above to explain the first maximum viscosity observed in suspensoids (seep. 146). One may imagine that in higher concentrations these membranes unite, somewhat as repre- sented in Fig. 14 on p. 87, and that mechanical treatment pulls the individual envelopes apart again. In favor of this view is the fact that, according to W. Biltz and H. Steiner, this phenome- non is particularly marked in concentrated solutions ; and that the viscosities of solutions of different ages may be reduced to the same value by sufficient shaking (see Table iS). 1 The matter will be taken up more fully later. TABLE 18. INFLUENCE OF SHAKING ON THE VISCOSITY OF A 2.7 PER CENT. SOLUTION OF NIGHT-BLUE (According to W. Biltz and H. Steiner) Without shaking After shaking a b 151.5 118.2 117.0 143.4 118.0 117.0 139-9 118.4 II7-4 8. Influence of "Inoculation" on Internal Friction of Emul- soids. A remarkable phenomenon has been observed by H. Gar- 1 In passing it may be mentioned that H. Zangger observed ordinary milk to show this behavior. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 159 rett (I.e.) in solutions of gelatine and by W. Biltz and H. Steiner (I.e.) in solutions of night-blue. They found the spontaneous in- creases in viscosity which such show to be markedly accelerated through the addition of small quantities of aged or gelatinized solutions. TABLE 19. INFLUENCE OF INOCULATION ON THE INTERNAL FRICTION OF A SOLUTION OF TECHNICAL NIGHT-BLUE AT 25 (According to Biltz and Steiner) Time of outflow Without inoculation After inoculation Per cent. At once After i day After 6 days At once After ^hr. After i hr. After 2hr. After i day 0.90 1-35 i. 80 2.25 77- 2" 79-3 77-6 85.2 78.5" 82.0 85.6 Qi-3 78.5" 81.6 79-2" 82.2 83-2 88.9 78.8" 82.0 85.9 8 5 .2 88.9 86.1 91.6 85.6 103-3 IO2 .6 As the table shows, this behavior is best observed only in colloid solutions of high concentration. It should be emphasized, as P. von Schroeder (I.e.) has shown, that this phenomenon depends upon a chemical change in the gelatine, probably upon its hydrolytic cleavage. This is proved not only by the fact that the decrease in viscosity, with pro- longed heating, is irreversible, but also by the fact that it follows the laws of chemical mass action. Furthermore, after prolonged heating, precipitates appear in the solution, which I hold to be the products of this chemical reaction. 1 Analogous considerations apply to the changes in viscosity which silicic acid, etc., show when heated or otherwise treated (W. Fleming, I.e.). This view is also supported by the fact that long heating decreases the vis- cosity of many emulsoids, though by no means all. W. Biltz and H. Steiner (I.e.), for example, found that emulsions of night- blue do not alter their viscosity even after heating 7 hours. What has been said under headings 3 to 8 must always be borne in mind when making viscosity determinations on emulsoids. For this reason the discussion entered into there needed to pre- cede a consideration of the relations between internal friction, concentration, temperature, etc. 9. Influence of Thermal History on Viscosity of Emulsoids. When such typical emulsoids as gelatine, agar-agar, etc., are sub- 1 Wo. Ostwald, Pfliiger's Arch., 109, 277 (1905). i6o SPECIAL COLLOID-CHEMISTRY jected to the influence of heat their viscosity is affected in the same way and as markedly as when they are treated mechanically. Prolonged heating decreases the internal friction of these solu- tions. By prolonged boiling, it is possible to so change a solu- tion of gelatine or glue that it will no longer solidify when cooled. When alcohol is added to a gelatine solution thus altered by pro- longed boiling, a yellow precipitate is thrown down, which is easily soluble in water. A precipitate similarly produced in normal gelatine only " swells " when thrown into cold water. This was observed as early as 1867 by Moritz Traube. 1 Traube called the modification which would no longer gelatinize, /3 gelatine or ]8 glue in contrast to the normal, gelatinizing a form. Table 20 and Fig. 25 copied from P. yon Schroeder (I.e.) illustrate what has been said. S. J. Levites 2 has made further experiments on purified gelatine (gluten), agar-agar and on the sodium salt of thymonucleic acid with entirely analogous results. TABLE 20. INFLUENCE OF HEATING ON THE VISCOSITY OF GELATINE SOLUTIONS (According to P. von Schroeder) Internal friction of gelatine about 100 i per cent. 2 per cent. 3 per cent. 0.5 .29 1.75 i.o .23 1.55 1.5 .20 1.49 .... 2.0 .17 1.47 1.76 2-5 -IS 3-0 -J4 i-37 1.68 3-5 -!3 4-0 .13 1-32 1.56 4-5 - 11 5-5 I-" 6.0 1.28 1.50 7.0 i .26 47 8.0 .... 1.25 47 9.0 44 10. o .... 1.24 42 12.0 1.23 .40 I4.O .... 1.22 39 l6.O .... 1.22 39 1 M. Traube, Reichert and Du Bois Reymond's Arch., 87 (1867). 2 S. J. Levites, Koll.-Zeitschr., 2, 239 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 161 io. Influence of Concentration on Internal Friction of Emul- soids. The influence of concentration upon the viscosity of FIG. 25.- 5 10 Number of hours heated -Effect of prolonged heating on the viscosity of a gelatine solution, (According to P. von Schroeder.) 3,00 Z50 2,00 V50 1,00 Concentration FIG. 26. Influence of concentration on viscosity of gelatine solutions at 35. (According to S. J. Levites.) emulsoids simulates its effect upon suspensoids. This is clearly evident on comparing Figs. 26 and 22, in doing which it is well to ii 162 SPECIAL COLLOID-CHEMISTRY limit oneself to comparison involving the same temperatures. Examples of the effects of concentration are given in the follow- ing Tables 21 and 22. TABLE 21. INFLUENCE OF CONCENTRATION ON VISCOSITY OF GELATINE SOLUTIONS (According to S. J. Levites) a Gelatine, at 35 Gelatine, at 35 Per cent. Viscosity Per cent. Viscosity 0.25 1. 10 0-5 .186 0-5 I .22 I .O .262 0-75 1.32 i. 5 332 1 .0 1.46 2.0 432 i. 5 1-75 3-0 .603 2.0 2.05 4.0 .856 3-o 2.96 Emphasis should be laid on the fact that the above measure- ments refer either to low colloid concentrations or were obtained at higher temperatures. As every one who has experimented with gelatine or agar-agar well knows, there is, for every typical emulsoid, an optimum concentration and an optimum temperature at which the solution gelatinizes. Thus solutions of night-blue above 1.575 per cent, are so thick at o that they no longer flow through a viscosimeter. We wish here merely to point out that the influ- ence of concentration on viscosity in typical emulsoids is very great. Thus the viscosity of an agar-agar solution (at room temperature) varies within the first 2 per cent, from that of pure water to that of a solid. If one compares molar instead of per- centage concentrations, the great absolute increases in the value as well as the abruptness of the viscosity changes appear still more striking. The effect of temperature on the concentration influence is such that decreasing the temperature makes the ascent of the curve steeper, while increasing the temperature flattens it (see Fig. 27). This behavior is analogous to that observed in molecular dis- persoids and probably to that observed in suspensoids. Added substances like salts increase or decrease the slope of the curve as do temperature changes. Purification of the technical night-blue MECHANICAL PROPERTIES OF COLLOID SYSTEMS 163 TABLE 22. INFLUENCE OF CONCENTRATION ON VISCOSITY OF NIGHT-BLUE SOLUTIONS (According to W. Biltz and H. Steiner) Technical night-blue Purified night-blue At 50 At 25 Ato At 25 Concen- tration, ' per cent. : Internal friction 1 ! Concen- tration, per cent. Internal i friction after 6 days Concen- tration, per cent. Internal friction Concen- tration, per cent. Internal friction 0.225 .007 0.025 0.985 0.225 1.009 0.25 1.008 0-45 .019 0.045 0.990 0.45 1.026 0.50 1.027 0.675 .027 0.090 0.994 0-675 1.042 0-75 -058 0.90 .041 0.145 0.997 0.90 1.068 I . 00 . 068 I.I25 .054 0.180 0.996 I.I25 I .IOI 1.25 .091 i-3S .071 0.225 1. 006 1-35 1.132 1.50 .106 i. 575 .090 0.270 .006 1-575 1. 176 1-75 145 i. 80 .097 0.315 .006 I. 80 1.180 2.OO .171 2.025 125 0.360 .008 2.25 .221 2.25 .142 0.405 .014 2.50 .263 2-475 157 0.450 .019 2.75 -334 2.70 .178 0.495 .O2O 3-oo .403 3.15 .240 0.540 033 3.60 .298 0.6075 .037 4-05 393 0.675 .042 4-50 455 0.7875 .054 0.900 .022 1.0125 .065 1.125 .080 1.237 .110 1-35 .105 1-575 139 i. 80 .182 2.025 .272 2.25 390 2-475 .480 2.70 .525 (which is ordinarily contaminated by about 43 per cent, sodium sulphate) decreases the slope of the curve, that is, has the same effect as raising the temperature. It is not impossible, however, that the addition of other salts, such as the chlorides, nitrates, etc., might have an opposite effect. Chemical changes in the colloid itself also change the character of the concentration curve, as is evident in the tables and curves referring to a and gelatine. A mathematical definition of the influence of the concentration 164 SPECIAL COLLOID-CHEMISTRY on the viscosity of emulsoids, in other words, an equation adequate for the whole range of concentrations has not yet been formulated. But this is also true of molecular dispersoids [see S. J. Levites (I.e.), where references to the literature may be found]. Yet the regu- larity of the Biltz curves (Fig. 27) indicates that a general, even though empirical, equation may be worked out. I have purposely inserted the numerous tables in the text to excite interest in this direction. 7-o 50 Cone enf-ration I i / 2 3 V 5% FIG. 27. Influence of concentration on the viscosity of night-blue solutions. (According to W. Biltz and H. Steiner.} The curve marked " G" shows the behavior of the purified night-blue. ii. Influence of Temperature on Viscosity of Emulsoids. Besides the influence of concentration on the viscosity of emulsoids, described in the previous paragraphs, there exists also a relation between temperature and viscosity which is observed when all other factors are kept constant. But systematic investigations of this type over a larger temperature range have not as yet been made. For reasons already given, only dilute emulsoids can be used for such study. Some approximate determinations of the average temperature coefficients of the viscosity of emulsoids are, however, at hand. Thus the internal friction of pure water changes about MECHANICAL PROPERTIES OF COLLOID SYSTEMS 165 18 per cent, between 21 and 3iC. In contrast to this, the viscosity of a 3 per cent, gelatine solution, within the same tem- perature range, changes from 1.42 to 13.76, in other words, almost 1000 per cent. (P. von Schroeder, I.e.). According to Biltz and Steiner (I.e.), the absolute viscosity of a 1.8 per cent, solution of night-blue rises from 10.5 to 32 between 25 and o, in other words, triples, while the viscosity of water merely doubles under the same conditions. With higher concentrations the changes in viscosity within very narrow ranges of temperature become extraordinary, for the existence of gelatination and melting points means nothing else but that, within a temperature change of a degree or less, the viscosity of such systems changes from that of a fluid to that of a solid. 12. Influence of Added Substances on Viscosity of Emul- soids. The influence of added substances on viscosity, when all other external factors have been kept constant, has also been thoroughly investigated. Of the mass of facts available in this field we shall mention only a few. For details the original papers should be consulted. So far as the important effect of salts upon emulsoids is con- cerned, the accuracy of most of the earlier measurements is vitiated because impure preparations, contaminated with electrolytes, were used. Only recently have Wo. Pauli and his co workers (/.c.), in a careful and searching series of investigations, shown what minute amounts of electrolytes suffice to cause substantial changes in the viscosity of organic emulsoids. Nevertheless older experi- ments with commercial preparations and those purified by ordinary laboratory methods are not valueless, for such colloids are used in many of the arts and for some scientific purposes. We must distinguish between the effects of salts on the vis- cosity of emulsoids which with time are either stable or un- stable. When of the latter class, as with gelatine, a distinction must be made between the initial value of the viscosity as ob- served immediately after the addition of a salt and the final value which is approached only asymptotically. According to the experiments of P. von Schroeder (l.c.), S. J. Levites (I.e.) and Gokun (I.e.), the first of these values follows the general rule of mixtures: salts which raise the internal friction of water affect colloid solutions similarly, and vice versa. The final value ex- i66 SPECIAL COLLOID-CHEMISTRY MECHANICAL PROPERTIES OF COLLOID SYSTEMS i6 7 hibited by gelatine solutions after the addition of salts is very different from this first. In Table 23 and Fig. 28, taken from P. von Schroeder (I.e.), are collected a series of such viscosity TABLE 23. A. INFLUENCE or SALTS ON INTERNAL FRICTION OF GELATINE (After standing i hour) Salt Concentration Salt Concentration H norm. ^ norm. }4 norm. M norm. y* norm. % norm i norm. Pure gelat. . l. 7 8 I .73 .72 .21 1.78 Pure gelat... 1.88 1.70 1-83 I.7I Na 2 SO 4 2. II I.Q7 i. 95 2 2 9.41 3-32 NaCl... . l. 7 6 1. 80 1-73 I.7I 1.6 7 1 .69 1.74 1. 60 1. 60 1-59 LSI I-SI K 2 SO 4 KC1 (NH 4 ) 2 S0 4 NH 4 C1 .... Pure gelat. . 1.68 1.68 1.68 Pure gelat . . 1.65 1.68 l. 7 6 1.70 Concentration NaNO 3 . . . 1.63 1.65 1.61 i. 57 1-53 1.52 1.56 1.52 1.49 KNO 3 1.48 i.45 H norm. M norm. X norm. M norm. NH 4 N0 3 .... LiCl 1-73 i. 7 6 1.92 2.12 I. 7 8 2.0C 2.15 2.42 1.66 1.88 MgCl 2 Li 2 S0 4 MgS0 4 I.8S I.QO B. DIFFERENCES BETWEEN INTERNAL FRICTION OF SALT-GELATINE AND PURE i PER CENT. GELATINE Na K NH 4 Mg Li SO A \4 R norm _|_Q Tly SO 4 % norm -J-O 77 -}-o oo 4~O 17 -\-Q A A -i-n 24. SO 4 J^ norm. . -j-T OI -4-n 4.8 -\~O QA. -|_O A*J SO 4 ]/2 norm -4-7 67 -l-i 6 A Cl % norm o i <\ -J-O TO -4~o o 1 * Cl ^ norm. O 12 o 08 o 02 Cl ^2 norm -j-o 01 o 03 O 27 -4-o 72 -j-Q 2O Cl i norm O OQ o 20 o 20 NO 3 % norm. O 12 o oo NOs 24 norm O O2 o i norm O 1 1 O 24. O 27 NO 3 i norm. o 20 O 72 O 2C v. ^ The plus sign means that the internal friction of the salt-gelatine is greater than that of the pure gelatine, and the minus sign the reverse. 1 68 SPECIAL COLLOID-CHEMISTRY values in gelatine solutions which have stood for an hour. Table 23, B, details the difference in viscosity between salt-gelatine and pure gelatine. If the difference is positive it means that the viscosity of the gelatine has been increased by adding the salt, while if it is negative it means that the viscosity has been reduced to below that of pure gelatine. It appears that sulphates in all concentrations increase the internal friction of gelatine, while chlorides and nitrates decrease it, with the exception of MgCl2 and LiCl in higher concentrations. The exact concentration of the salt, however, plays an important part, especially in the chlorides which in medium concentrations (about J normal) show a maximum of viscosity which sometimes exceeds that of pure gelatine. Further details may be found in the tables and curves. 1 If the anions of the added salts are arranged according to their effect we obtain the series: SO 4 >C1>N0 3 In the case of the kations variations occur with different concentrations. If we choose the values found for J^ normal solutions we find that the sulphates and the chlorides arrange themselves as follows: Mg>Na>Li>NH 4 >K Ample opportunity will be found later, to return to these "ionic series," which in honor of the investigator who discovered them are now known as the Hofmeister series. There we shall also find that the complicated influence of the concentration of a salt is not an accidental or an exceptional one, but an expression of general characteristics of the relation between any salt and a change in the state of the colloid system. P. von Schroeder (I.e.) has investigated the important in- fluence of acids and alkalies on the viscosity of gelatine solutions. His findings are detailed in Table 24 and Fig. 29. The influence of concentration is again complex, for at certain low concentrations (3^56 normal for HC1 and J^28 normal for NaOH) a maximum 1 It should again be emphasized that pure gelatine would, perhaps, show totally different results. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 1 69 viscosity is attained, while at concentrations above ^2 normal a viscosity below that of pure gelatine is observed. Pure ConcenTraTion 1 /2 normal FIG. 29. Effect of HC1 and NaOH upon the viscosity of gelatine. (According to P. von Schroeder.') TABLE 24. INFLUENCE OP HC1 AND OF NaOH ON VISCOSITY OF GELATINE (According to P. von Schroeder) HCl Concentration Viscosity Concentration Viscosity O .40 O .40 /^12 norm. 55 x^jl2 norm. 52 ^56 .76 M56 .60 H28 .68 H28 79 /^4 58 x^4 .62 Yzi .42 /^2 .38 He 25 He 25 M 17 .10 H I. 12 M .10 NaOH Similar effects of concentration, more especially of the alka- lies, on the viscosity of soap solutions have been observed by F. Bottazzi and C. Victorow (I.e.). 13. Effect of Added Substances on Internal Friction of Emul- soids; Behavior of Protein Solutions. Through the work of E. Laquer and O. Sackur (I.e.), W. B. Hardy (I.e.) and others, and especially through that of Wo. Pauli and his coworkers (I.e.), we 170 SPECIAL COLLOID-CHEMISTRY have become better acquainted with the behavior of various pro- tein solutions such as those of serum albumin, egg albumin, globulin and casein in the matter of their viscosity when subjected to the effects of added chemical substances. These solutions be long to the emulsoids. Time alone changes their internal friction, yet these changes take place so rapidly that the final viscosity value is reached within a few minutes. Because of this and because 1250: Monochloracetic Acid 0,0050,01 0,02 O03 0.04 OjOSn FIG. 30. Influence of acids upon the viscosity of serum albumin. Wo. Pauli and H. Handovsky.) 17 means viscosity. (According to the proteins can be isolated and better purified than gelatine, for example, they adapt themselves especially well to a study of this important problem. The most striking fact that the study of the influence of elec- trolytes on the viscosity of purified proteins has brought out is the enormous change in viscosity which is produced by traces of electrolytes. This is especially true of acids 1 and alkalies which 1 Regarding the effect of acids, more especially of acetic acid on protein, see the paper of L. Zoja, Koll.-Zeitschr., 3, 249 (1908). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 171 TABLE 25. A. INFLUENCE OF ACIDS ON VISCOSITY OF SERUM ALBUMIN (According to Wo. Pauli and H. Handovsky) 1 Concentration Internal friction HC1 Citric acid Oxalic acid o.oo norm. 0.005 o.oi 0.015 0.017 0.02 0.03 O.O4 0.05 I . 0409 ! 1.0832 I I. 1660 1.2432 1.2432 1.2323 1.1647 I.I3S6 I . 1206 I . 0409 I . 0409 Sulphuric [ Trichloracetic acid acid 1.0409 1.0442 I. 0688 1.0613 I .0661 I . 1002 I .1112 I . 1408 I.I337 I.06I3 1.1634 1.0604 1.1852 ! 1.0638 1.1700 i 1.0656 1.0409 1.0511 1.0725 1.0594 1.0525 1.0564 i . 0603 Acetic acid 1.0409 1.0456 1.0518 1.0658 1.0751 I . 0906 B. INFLUENCE OF BASES ON VISCOSITY OF SERUM ALBUMIN (According to Wo. Pauli and H. Handovsky) Base Concentration Sodium hydroxide I o . 01 norm. 0.02 0.03 Ammonia o.oi norm. 0.03 0.05 Triethylamine o.oi 0.03 0.05 Ethylamine o.oi 0.03 0.05 Methylamine o.oi 0.05 Diethylamine o . 01 0.03 0.05 Piperidine o.oi 0.03 0.05 Tetraethylammoniurn hydroxide o . 01 0.03 0.05 Friction increase in per cent. 78 ISI 195 19 23 28 20 28 33 37 65 83 40 76 52 103 146 53 109 116 221 230 Concentration, OH'.io-* 9 6o 1900 2805 49 82 108 85 148 196 214 390 465 204 442 308 564 800 334 627 825 922 2718 4490 1 See H. Handovsky, Koll.-Zeitschr., 7, 268 (1910). 172 SPECIAL COLLOID-CHEMISTRY show a behavior entirely analogous to that discussed in con- nection with gelatine on p. 169. Thus Wo. Pauli and H. Han- do vsky (I.e.) found that the addition of 0.015 normal HC1 suf- fices to raise the viscosity of a serum albumin solution from 1.0623 to 1.2937, in other words, more than 20 per cent. With alkalies, a concentration of Hoo normal tetraethylammonium hydroxide is enough to increase the viscosity 230 per cent. Table 25 and Figs. 30 and 31 may serve to illustrate these facts. O.OJn Concentration of the Base FIG. 31. Influence of bases upon the viscosity of serum albumin. (According to Wo. Pauli and H. Handovsky.} So far as the effect of salts is concerned, it is found that this is different depending upon whether neutral, acid or alkaline albumin is used (Wo. Pauli). The relations are complicated especially when the effects of different concentrations of acids and alkalies as well as of salts are considered. It remains for future investigators to give us a clear and comprehensive presentation of this subject. The following features deserve emphasis : Neutral salts always lower the viscosity of neutral protein (Wo. Pauli). This be- MECHANICAL PROPERTIES OF COLLOID SYSTEMS 173 havior is analogous to the effects of salts on the viscosity of sus- pensoids (see p. 151). When we deal with acid albumin it is found that the anions of the neutral salts play a greater role than do the cations. Salts usually lower the viscosity, though complicated concentration relations appear. With a common cation the anions decrease the viscosity in the following order: C 2 H 3 2 > S0 4 > SCN > N0 3 > Cl [E. Laqueur and O. Sackur (I.e.), W. Frey (l.c.), H. Procter, 1 L. Zoja (I.e.), Wo. Pauli (I.e.) and others.] The reverse is true with alkali albumin, where the cations play the chief part. From a qualitative point of view all the salts bring about a decrease in viscosity, but when the effects of equal amounts of salts are compared a greater decrease is noted in alkali albumin than in acid albumin. The salts of the alkali earth metals exert a stronger influence than those of the alkali metals. 14. Influence of Added Substances on Viscosity of Emulsoids. Effects of Non-electrolytes and Mixture of Dispersing Media. Non-electrolytes in low concentrations usually change the viscosity of emulsoids only to the extent in which they increase the viscosity of the pure dispersion medium (S. J. Levites, Wo. Pauli, etc.). Yet it is not impossible for non-electrolytes even in low concentra- tions to influence the viscosity somewhat. Thus Handovsky found that caffeine causes a very perceptible increase in the viscosity of acid albumin. 2 We need more experiments in this field. In greater concentrations the addition of non-electrolytes causes very perceptible non-additive changes in viscosity. J. Simon (l.c.), for example, found alcohols, acetone, etc., to increase markedly the viscosity of albumin solutions. In future studies of these phenomena it might be well to subtract from the observed changes in viscosity those increases which result from mere mixing of the alcohol with water. Only then will the true changes in viscosity due to the change in the colloids themselves be clearly evidenced. Several albumins, such as thezeinof Indian corn, are remarkable 1 H. Procter, Koll.-Zeitschr., 3, 307 (1908). 2 Morphine, alcohol in low concentration, etc., probably produce similar effects. 174 SPECIAL COLLOID-CHEMISTRY in that they dissolve neither in water nor alcohol, but in a mixture of the two. 1 It would be interesting to study the viscosity be- havior of such systems. The same is true of many dyes which although soluble in each of the pure solvents show different de- grees of dispersion and even different types of colloidality in the two. 2 15. Viscosity and Electrical Charge of Disperse Phase. Nothing is known as yet of the influence of the electrical charge of the disperse phase on the viscosity of suspensoids. It is probable, however, that more exact measurements will show the existence of such an influence. We suppose this because every electrically charged particle induces about it an electromagnetic field which hinders its movement whether such is "spontaneous" or brought about from without. On the other hand, E. Laqueur and O. Sackur (I.e.), W. B. Hardy (I.e.) and especially Wo. Pauli (I.e.) pointed out long ago that the electric charge of protein particles greatly affects the viscosity of those solutions. These investigators hold the elec- trically or electrochemically charged particles in these solutions to spring from an electrolytic dissociation similar to that observed in molecularly dispersed, slightly dissociated systems. As will become more evident in the chapter on the electrical properties of colloid systems, this assumption has proved both satisfactory and fruitful in explanation, for example, of the variations in viscosity caused by added substances. It may be said that when the viscosity of a neutral emulsoid rises on the addition of some substance, this is due chiefly to an increase in the number of dis- sociated (electrically charged) colloid particles. The correctness of this view is at once evidenced when we recall to mind the striking increase in the viscosity of gelatine, soap, or protein solutions when small amounts of acids or alkalies are added to them. The decrease in viscosity observed in higher concen- trations of the acids and alkalies follows the decrease in dissocia- tion. The effect of salts ; n lowering the viscosity of acid- and alkali- colloids corresponds with the effect of salts in depressing ionization when a common ion is introduced. 3 Table 25 (p. 171) may serve 1 See the detailed paper of G. Galeotti and G. Giampalmo, Koll.-Zeitschr., 3, 118 (1908), where references to the literature may also be found. 2 H. Freundlich and W. Neumann, Koll.-Zeitschr., 3, 80 (1908). 3 For a discussion of the electrochemical side of these views see the textbooks of physical chemistry. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 175 to show the general parallelism between concentration of OH ions and viscosity. Certain exceptions to the general rule are, however, to be noted, as in the case of piperidine. The well-grounded fact that ions are more strongly hydrated than electrically neutral undissociated molecules explains why increase in dissociation and increase in viscosity go hand in hand. As a result of the magnetic field about the charged particles, or at least through its increase, we may imagine the solvent to be held more closely in the solvent envelopes about the separate particles. Thus also will the internal friction be increased and the separate particles become less mobile for now the charged particles have larger envelopes of the dispersion means about them. But let us not fail to point out that it does not seem safe to say that this direct application of electrochemical laws will, in the future, show itself to be entirely adequate. But the ability of these laws to elucidate at least some of the complicated relations observed shows them to be at least partly active. Future investigators may reveal great discrepancies between the laws governing the behavior of colloid systems and the electro- chemical laws which apply to molecular and supermolecular dis- perse systems. Notwithstanding isolated analogies, colloid sys- tems may be found to be governed by electrochemical laws which are not subordinate to those governing molecular systems but coordinated with them. Great variations from normal electro- chemical behavior are already known in the case of suspensoids. 1 We can discuss these questions to greater advantage when we come to consider the electrical properties of colloid systems. 16. Viscosity and Degree of Dispersion; Viscosity of Coarse and Complex Dispersions. Only a few observations are available on the theoretically important relation between degree of disper- sion and viscosity, and no systematic study has as yet been made of any number of systems with progressively varying degrees of dispersion. Theoretically one would expect the viscosity of a dis- persoid to grow with every increase in the amount of contact sur- face, in other words, with the degree of dispersion. It is here as- sumed that the particles of the disperse phase move about with greater difficulty than do the particles of the dispersion means itself. The dispersion medium, held in the often-mentioned sur- 1 See Wo. Ostwald, Koll.-Zeitschr., 7, 132 (1910). 176 SPECIAL COLLOID-CHEMISTRY face membranes, must have in addition to its usual character- istics a decreased mobility. Experimental evidence can be cited to support this view. The experiments described on p. 151, deal- ing with the decrease in the viscosity on ageing or the addition of salts, show a distinct parallelism between decrease in degree of dispersion and decrease in viscosity. K. Beck and K. Ebbinghaus 1 found that coarse emulsions of castor oil in water did not greatly change the viscosity of the water, but after gum arabic or similar substances had been added which permitted the attainment of higher dispersion, the viscosity rose considerably above that of the oil or the pure gum solution. The increase amounted to 44 per cent. The fact that cellulose becomes slimy and viscous with long grinding indicates the same thing. G. Buglia 2 found milk to show a distinct increase in viscosity after being " homogenized," that is to say, after having its fat finely divided by being squirted against an agate plate. A. Martici 3 has studied the viscosity of oil emul- sions in soap water and found that their viscosity increases as the oil droplets become smaller. But observations can also be cited to support the opposite view. Cases are known in which the viscosity increases as the degree of dispersion decreases. In the case of molecular disper- soids, it is the rule that when the substances have a high mole- cular weight that they show a greater viscosity. We need but consider the salts (soaps) of the homologous fatty acids in water (see p. 143). While the lower members (acetates) change the viscosity of water but little, aqueous solutions of the higher members are solid. The association of changes in molecular weight with changes in the viscosity of colloid night-blue solu- tions under the influence of changes in temperature has been observed by W. Biltz and A. von Vegesack (I.e.)- They calculated from direct osmotic measurements the molecular weight of technical night-blue at o, 25 and 50 to be, respectively, 11,550, 5260 and 3550. A glance at the viscosity curves of Fig. 27 shows that the greatest viscosity coincides with the greatest molecular weight. The phenomena in critical fluid mixtures may also be used to show direct parallelism between viscosity and degree of 1 K. Beck, Zeitschr. Physik. Chem., 58, 409 (1907); K. Ebbinghaus, Diss., Leipzig, 1907. 2 G. Buglia, Koll.-Zeitschr., 2, 353 (1908). 3 A. Martici, Arch. di. Fisiol., 4, 133 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 177 dispersion. As first observed by J. Friedlander, 1 a mixture of butyric acid in water, which is completely miscible at higher tem- peratures, shows a great increase in internal friction when cooled, and this increase occurs in the region where the system begins to become turbid, in other words, where the components begin to separate. This separation must of necessity be highly dispersed at the beginning as evidenced by the fact that a bluish opalescence first appears when the solution is still perfectly transparent. The degree of turbidity and the viscosity at first increase steadily as the separation proceeds. It is not impossible that in this case there occurs not only an increase in the number of drop- lets but also an increase in their size, for ultimately a coarse separation of acid in water is obtained which of course cannot have occurred suddenly. We can also cite an example which shows the opposite of what was said above in discussing cellulose. Highly "masticized," that is to say, mechanically treated rubber, yields much less viscid solutions than the untreated. In an analogous manner the increase in the viscosity of emul- soids with time during their gelation indicates a decrease in their degree of dispersion. The conclusion to be drawn from these seemingly opposed facts would be that, other conditions being constant, a dispersoid reaches its highest viscosity at a medium degree of dispersion. The experi- mental verification of such a conclusion is a problem of the future. There is much practical as well as theoretical interest attached to a comparison of the viscosity of coarse dispersions with those of colloid systems. While observations on the viscosity of coarse suspensions 2 are few, much more is known regarding the behavior of coarse emulsions. 3 Of course in many of these experiments we 1 J. Friedlander, Zeitschr. f. physik. Chem., 38, 430(1901); V. Rothmund, ibid. , 63, 54 (1908). 2 Besides the well-known behavior of sand we may point out that M. Franken- heim [Journ. f. prakt. Chem., 54, 433 (1851)] details some observations on increase in viscosity caused by the taking up of solid particles. 3 Besides the works of K. Beck, K. Ebbinghaus, J. Friedlander, V. Rothmund, G. Buglia and J. Simon we may also cite M. Bose, Physik. Zeitschr., 83, 47 (1907); Z. f. Elektroch., 13, 499 (1907); R. Schenk, Kristall. Flussigkeiten, 32, Leipzig, 1905; Eichwald, Diss. Marburg, 1905; D. Holde, Koll.-Zeitschr., 4, 270 (1908) Emulsions of Water in Mineral Oils, etc.; Wo. Ostwald, Koll.-Zeitschr., 6, 103 (1910); E. E. Hatschek, ibid., 6, 254 (1910); 7, n (1910); T. B. Robertson, ibid., 7, 7 (1910); S. U. Pickering, ibid., 7, n (1910) where references to the old literature may be found; M. W. Beyerinck ibid., 7, 16 (1910), Emulsions Consisting of Two Colloids; F. G. Donnan, Zeitschr. f. physik. Chem., 31,42 (1899); Koll.-Zeitschr., 7. 208 (1910) with H. E. Potts. 12 I 7 8 SPECIAL COLLOID-CHEMISTRY deal with complex emulsions consisting of more than two phases. Still a comparison of the viscosity relations of these systems with those of the emulsoids shows so many and at times such surprising analogies that a short discussion seems valuable especially since it serves to support the belief that emulsion colloids are systems hav- ing the composition liquid + liquid. An excellent example of the increase in the viscosity of a liquid when a second insoluble one is emulsified in it, is offered by the so-called solid lubricants (engine grease). Even 0.75 per cent, of water when thoroughly mixed into liquid solutions of soaps in mineral oil will convert these into salve- like bodies of so high viscosity that they may be spooned out in 5 10 Con cen fra hi on FIG. 32. Influence of concentration upon the viscosity of a castor oil- water emulsion (According to K. Beck.} coherent masses (D. Holde, I.e.). The same example serves to demonstrate the influence of concentration on the viscosity of coarse emulsions, for this varies within the concentration limits of o to 0.75 per cent, water from that of a liquid soap to that of a " solid " lubricant. Another illustration of the latter has been found by K. Beck (I.e.) and his coworkers in their work on emulsions of acacia water and castor oil. While small amounts of emulsified castor oil but slightly increased the viscosity of the gum arabic solutions certain higher concentrations caused sharp increases. Fig. 32 illustrates this behavior which is fully analogous to that observed in emulsoids. Excellent analogies for the great effect of tempera- ture on the viscosity of lyophilic colloids can also be found in the MECHANICAL PROPERTIES OF COLLOID SYSTEMS 179 case of the coarser emulsions. J. Friedlander (I.e.) and V. Roth- mund (I.e.) found the viscosity of critical fluid mixtures to be very sensitive to temperature. The temperature coefficient of viscosity in these ranges is three to five times as great as in those in which the system has lost its emulsion nature. The machine oils already mentioned may serve as further illustrative material. Their decrease in viscosity with increase in temperature is so great that one may distinguish a softening point and a dropping point which may at times lie but one degree apart. This indicates that their viscosity may fall from that of a solid to that of a liquid within the space of a few degrees, a suddenness of change which is similar to that observed in the melting points of solids. Finally, attention should be called to a third system, namely, that of an alcoholic solution of rosin containing a little water, investigated by J. Friedlander. This also possesses a relatively large temperature coefficient, namely, one of 5 to 6 per cent, per degree of temperature against that of about 2 per cent, for water. 17. Viscosity and Type of Disperse Phase. We have thus far considered the viscosity relations of only the more common and important dispersoids, namely, those having the composition liquid + solid and liquid + liquid. It should, however, be remembered that remarkable increases in viscosity of a liquid dispersion medium may be caused by finely dividing a gaseous phase in it as illus- trated by the mechanical properties of foams which often have many of the characteristics of a solid. We need of course to take into account that strictly two-phase systems of the type liquid + gas are hardly known and that the stability of most foams is closely associated with their so-called adsorption phenomena by virtue of which the gas bubbles condense dissolved substances upon their surfaces with consequent formation of solid films. Yet such adsorption processes are, in many cases, completely reversible and the fluid nature of the membranes is preserved throughout. Thus saponin foam melts down to a homogeneous fluid perfectly free from coagula, and egg-white may be freed of the threads and coagula present in it in its natural state by beating it to a foam. The greater part of the foam subsequently melts down to a solu- tion perfectly free from flocculi. This is evidence for the fluid nature of the walls of the foam. The preparation and detailed l8o SPECIAL COLLOID-CHEMISTRY investigation of colloid foams would evidently be of great interest to general colloid chemistry. 1 If one compares the internal friction of the three typical dis- persoids having a fluid dispersion medium, it is found that a low initial viscosity of disperse phase by no means precludes the at- tainment of high viscosity values for the whole system. In fact, if colloid dispersoids are compared with each other, it is found that emulsoids usually exhibit a higher viscosity than the suspensoids having the same concentration and, in view of the great stability of highly dispersed foams, it even seems as though such when in a colloid degree of dispersion might show still higher viscosity values. We must of course distinguish between high viscosity and the value of other physical properties such as hardness. Paradox- ical as it may seem, it even appears as though viscosity of the dis- persion medium and viscosity of the disperse phase may be only of indirect significance, for it seems probable that the properties of the different surfaces (liquid-solid, liquid-liquid, and liquid- gaseous) and not the low viscosity value of the disperse phase itself are primarily responsible for the viscosity of the dispersoid as a whole. 26. Surface Tension of Colloid Solutions i. General Remarks.- A closed two-phase dispersoid has a series of surfaces. The most important is the one between the disperse phase and the dispersion medium. There is, in addition, the surface between the whole dispersoid and its surroundings, in considering which we must distinguish between the surface bound- ing the dispersoid and its vapor and that between the dispersoid and the walls of the vessel. If we remember that there are two surface energies in every surface, then we may distinguish six different surface tensions. If we consider that the disperse particles may also come in contact with both the gaseous boundary and the walls of the vessel (as is actually the case in the adsorption phenomena occurring in three-phase systems), the number of tensions to be considered is increased to ten, while in three-phase dispersoids the number rises to eighteen. We cannot say in ad- 1 For some observations on fine foams see Wo. Ostwald, Koll.-Zeitschr., I, 333 (1907). Systems belonging to this class are also described by Schroeder, Poggen- dorf's Ann., 137, 76 (1869); see also the patent of J.Weinmayr, described in Chem. Centralbl., 586 (1910). MECHANICAL PROPERTIES OF COLLOID SYSTEMS l8l vance that this or that surface tension is insignificant in determin- ing the characteristics of a dispersoid or a colloid. The expansile surface tension between the dispersion medium and vessel walls, for example, determines its ability to "wet" the surface; while the relation of positive to negative surface tension between the dis- perse phase and dispersion medium determines the degree of dis- persion (see p. 81). Other groups of tension are responsible for the processes of coagulation, adsorption, etc. At the present time, however, the sense and value of only a few of these tensions are known; in fact quantitative measurements are available of but a single surface tension, namely, that of the positive tension in the surface between the dispersoid and its vapor. 2. Experimental Facts. Investigations show that the positive surface tension 1 of a colloid solution at its free surfaces may be more, or less, or equal to that of the pure dispersion medium (Rayleigh, 2 A. Pockels, 3 W. Ramsden, 4 G. Quincke, 5 H. Picton and S. E. Linder, 6 L. Zlobicki, 7 W. Frei, 8 G. Buglia, 9 F. Bottazzi and C. Victorow 10 ). Usually the tension is less. The surface tension of water is increased by gum arabic, starch and plum gum. It is lowered by gelatine, glue, egg-albumin, dex- trin, cherry and sweet cherry gum. It is greatly lowered by fats, fatty acids, soaps, resins, tannic acid, etc. Tables 26 and 27 taken from G. Quincke and L. Zlobicki may serve in illustration. Both the increase or the decrease in surface tension follows the concentration of the colloid. Traces of fatty acids, of soaps, etc., suffice to lower greatly the surface tension of water as seen in Table 26. The surface tension of colloid solutions as of liquids in general decreases as the temperature rises but, as Table 27 shows, is much more marked than in the case of the pure dispersion medium alone. 1 The textbooks of physics and physical chemistry should be consulted for methods of measuring the positive surface tension. 2 Rayleigh, Proc. Roy. Soc., 47, 364 (1890). 3 A. Pockels, Nature, 46, 418 (1892); Drude's Ann. d. Physik., 8, (1902). 4 W. Ramsden, Engelmann's Arch. f. Anat. und Physiol. Abt. f. Physiol., 517 (1894); Z. f. physik. Chem., 47, 341 (1902); Proc. Roy. Soc., 72, 156 (1904). 6 G. Quincke, Wiedemann's Ann., 35, 582 (1888) Ber. d. Berl. Akad. d. Wissensch., 38, 493, 858 (1901); Drude's Ann. d. Physik., 7,631 (1901); ibid.,g, 969 (1902);^., 10, 507 (1903); ibid., ii (1904). 6 H. Picton and S. E. Linder, Journ. Chem. Soc., 87, 1924 (1905). 7 L. Zlobicki, Bull. Acad. Sc. Cracovie, Juli, 488 (1906). 8 W. Frei, Zur Theorie der Hamolyse, Diss., Zurich, 1907; Transvaal Medic. Journ., August, 1908. 9 G. Buglia, Biochem. Zeitschr., n, 311 (1908). 10 F. Bottazzi and C. Victorow, Rend. R. Ac. Line., 19, 659 (1910). 1 82 SPECIAL COLLOID-CHEMISTRY TABLE 26. SURFACE TENSIONS OF COLLOID SOLUTIONS AT ABOUT 20 (According to G. Quincke) Substance Specific gravity Surface tension against " air" Water I OOOO 8o< ? Egg-albumin I 036? Aqueous bile solution (9%) 1.0384 1 I .0384 1 I IOI33 yo4 j 5-370 to I 4.913 5076 Venetian Soap ^inno per cent.. . o 0083 2 681 /^oo P er cent. O OOO2 2 672 1 Yo per cent ."* Tannic acid, 10 per cent I . OOOQ I .0^12 2.563 "? 8<7 Gum arabic, 20 per cent.. Isinglass } Gelatine f very dilute 1.0708 I . OOOO I OOOO 7.603 6.790 7 272 Agr J I . OOOO 7.842 TABLE 27. SURFACE TENSIONS OF COLLOID SOLUTIONS (According to L. Zlobicki) a Grams gelatine in 100 cc. solution 2 Grams gum arabic in 100 cc. solution Temp. Surface tension in mg. /mm. Temp. Surface tension in mg. /mm. Solution Water Solution Water o.o "3 17.0 6.62 6.21 5.98 7.69 7.52 7-43 0.0 6.6 17.0 8.66 8.47 8.16 7.69 7-59 7.42 24-5 5-70 7.32 24.0 7.75 7-33 The type of the disperse phase is of particular importance in determining the change in the surface tension of the pure dispersion medium. This is indicated by the fact that all the above-men- tioned examples are emulsoids. Coarse suspensions and suspensoids hardly alter the surface tension of the dispersion medium. H. Picton and S. E. Linder (I.e.) found suspensoid arsenious tri- sulphide, even in concentrations of 2 per cent., and dilute iron hy- droxide to produce so minimal a decrease in the surface tension of pure water that it scarcely exceeds the experimental error. N. Sahlbom 1 obtained analogous results. L. Zlobicki (I.e.) found coarse aqueous suspensions of emery, mastic and gamboge and 1 N. Sahlbom, Kolloidchem. Beih., 2, No. 3 (1910). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 183 colloid suspensions of silver and platinum to have the same surface tension as the pure dispersion medium. The temperature coeffi- cient of the surface tension of these systems was also the same as that of the pure dispersion means. These results seem to show that only emulsoids decrease the surface tension of their dispersion media. This difference can, in fact, be used as a means for distinguishing the two classes of col- loids from each other. Here again is evidenced the close connec- tion between emulsoids and molecular-dispersoids in that the latter also always exhibit a surface tension different from that of their pure solvents. As already indicated (p. 55), the same chemical substance may assume either emulsoid or suspensoid properties in dif- ferent dispersion media. Thus soaps, many dyes, etc., form emul- soids in water while they form suspensoids in alcohol. One would expect this distinction to show itself in the surface tension behavior of the different solutions when compared with that of their pure dispersion media, which, in fact, it does, as H. Freund- lich and W. Neumann 1 have found. Table 28 illustrates this in- teresting fact in that it shows that the surface tensions of the aqueous dispersion media are noticeably decreased while those of the alcoholic solutions show no change, or if anything, a slight in- crease. TABLE 28. SURFACE TENSIONS OF COLLOID DYES IN WATER AND IN ALCOHOL (According to H. Freundlich and W. Neumann) * Water Substance Surface tension Alcohol Substance Surface tension Water 7erved con- ntration culated con- ntration ght of level erved con- centration culated con- centration ght of level served con- entration culated con- ntration B S 8 6 s A 8 O 8 1 S " O 3 1 Jc, o O 6 s I00/i IOO IOO 9M 12.0 ii. i 35M 10 9.4 24/i 305 280 75 116 119 60 22.6 23.0 25 22 21.0 18 530 528 So 146 142 30 47.0 48.0 15 43 45-0 12 940 995 25 170 169 o IOO.O IOO.O 5 IOO IOO.O 6 1880 1880 o 200 2OI 1 12. Validity of Stokes' Law for Highly Dispersed Particles. G. Stokes formulated a law in 1850 which has gradually become famous. It expresses the relationship between the velocity of small globules and the forces acting upon them, such as gravity. The law may be expressed thus: v = *D-d^ r2 9 v In this equation v is the velocity, D the density of the particle, d the density of the liquid, t\ the viscosity, K the constant of gravity and r the radius of the particle. In a given dispersion means, at constant temperature, etc., the velocity of a particle is therefore proportional to the square of its MECHANICAL PROPERTIES OF COLLOID SYSTEMS 205 radius. Stokes employed this law in calculating the speed of falling rain drops. What interests us is whether this law also holds when the degree of dispersion is very high as in dispersoids showing Brownian movements (J. Perrin, 1 J. Duclaux 2 ). J. Perrin was able to show that the law is still valid for particles with a radius of 0.14 to 0.45/4, in other words, for such as approach the maximum diameter (iju) of colloid particles. He measured the sizes of the particles of his suspensions, first, by calculating their sedimentation velocities from Stokes' law, second, by counting the particles in a known volume containing a known amount of disperse phase, and thirdly, by a micrometric method (for the details of which see his paper of 1910). The three methods gave results which agreed closely with each other as evidenced in Table 36. TABLE 36. DETERMINATION OF STZES OF PARTICLES BY DIFFERENT METHODS TO TEST APPLICABILITY OF STOKES' LAW TO HIGHLY DISPERSED SYSTEMS (According to J. Perrin) By counting ! According to Stokes' law Micrometrically 0.46^1 0-45M 0.45-S/* 0.30 0.29 0.30 . 21 2 O. 212 O Id. O . I 2 w Af Whether this law holds for systems of still higher degrees of dispersion has not yet been determined, though it is already ap- plied with remarkable results, not only to the theory of tnolecularly dispersed solutions (see for example W. R. Bousfield 3 ) but to the migration phenomena of gaseous ions. 4 13. Kinetic Theory of Brownian Movement. As mentioned before, the sources of energy for Brownian movement must be sought in some very general mechanical forces resident within fluid or gaseous dispersoids. In harmony with the old accepted and widespread kinetic views it was to be expected that Brownian movement would sooner or later be regarded as a direct result of the supposed collisions between the molecules of the dispersion 1 J. Perrin, Compt. rend., 146, 967 (1908); Kolloidch. Beih., I.e., 1910. 2 J. Duclaux, Compt. rend., 147, 131 (1908). 3 W. R. Bousfield, Z. f. phys. Chem., 53, 270 (1905). 4 See J. J. Thomson, Conduction of Electricity through Gases, Cambridge, 1903. 206 SPECIAL COLLOID- CHEMISTRY means. As a matter of fact, the early authors (Chr. Wiener, G. Gouy, etc.), saw in this its only possible explanation. Recently, A. Einstein 1 and M. von Smoluchowski, 2 in some exceedingly important papers on molecular physics have developed by some- what different methods a theory of Brownian movement resulting in two almost identical formulae. Their fundamental equation governing the kinetics of disperse systems reads: In this A is the average path length of the particle, K a constant, R the gas constant, T the absolute temperature, N the number of particles in a gram molecule of the disperse phase (Avogadro's constant), t the period of vibration, t\ the viscosity of the dispersion means and r the radius of the presumably spherical particle. The formula of M. von Smoluchowski differs from that given above only in having the factor -- = 2.37 preceding the root on the right side. The derivation of the formula cannot be detailed here. 3 It will only be shown how well this equation, deduced theoretically, agrees with the experimental results of The Svedberg and J. Perrin. It should be emphasized that the two laws formulated by Svedberg concerning the uniformity of Brownian movement and its dependence on viscosity were discovered before he had any knowledge of the Einstein-Smoluchowski formula. Discussion of the equation leads to the following conclusions. If we assume all the factors in the equation to be constant, except the path length, period of vibration and viscosity, the equation becomes A = KJ- or A 2 = K! - \n 7? The latter form states that not the path length but its square is directly proportional to the period of vibration and inversely proportional to the viscosity of the dispersion means. Svedberg, 1 A. Einstein, Ann. d. Physik (4), 21, 17, 549 (1905); (4), 19, 371 (1906); Z. f. Elektrochem., 13, 41 (1907). 2 M. von Smoluchowski, Ann. d. Physik (4), 21, 756 (1906). 3 See the original papers as well as the excellent pamphlet of W. Mecklenburg, Die experimentelle Grundlegung der Atomistik, Jena, 1910. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 207 however, found (see p. 197) the first power of the path length to be proportional to the period of vibration and inversely pro- portional to the viscosity. As a matter of fact, the above equation can be separated into two of the form: If one of the factors, say --, is constant, as demanded by the kinetic gas theory (which assumes uniformity of average velocity of the gas molecules) and as Svedberg found it to be experimentally, then the other factor AT? must also be constant. Conversely, assuming the validity of Stokes' law, AT/ becomes a constant, and therefore also. The formulas deduced from the kinetic theory therefore really cover the case in which both Svedberg laws are simultane- ously active. Experience therefore confirms the molecule-kinetic deductions of the authors named. With this equation it now becomes possible, conversely, to calculate the absolute value of Brownian movement when vis- cosity, size of particles, etc., are known. Svedberg (I.e.) and V. Henri (i.e., 1908) have done this. Their calculated and observed results do not agree absolutely, but they are of the same order of magnitude and the deviations are all of about the same proportion. Undoubtedly the arbitrariness or inexactness of some of the con- stants used may therefore be held responsible. Table 37 shows the more important of these calculations. ' TABLE 37. CALCULATION OF PATH LENGTH OF COLLOID PLATINUM PAR- TICLES EXHIBITING BROWNIAN MOVEMENT, AFTER THE EINSTEIN- SMOLUCHOWSKI FORMULA (According to The Svedberg) Dispersion means &7IO (in sec.) A observed (in/i) A calculated (in M ) A found A calculated Acetone 0.032 0.028 0.026 0.013 O.OOQ 2-3 4-6 5-9 10. 2 22.6 3-i 2.0 i-5 1. 1 0.7 0.71 0.44 0.38 0.20 O.II 4-4 45 4.0 5-5 6-4 Ethyl acetate Amyl acetate Water n Propyl alcohol 208 SPECIAL COLLOID- CHEMISTRY With due allowance for the large experimental error, the value of the rotational movement of disperse phases also agrees, as J. Perrin (I.e.) has shown, with that derived from the formula of A. Einstein. The law developed by J. Perrin governing the changes in concentration of a suspension at different levels (as discussed in 11) has also been deduced from considerations of the kinetics of gases. Only the constants of the formulas are different in the two cases. Thus while the density of the earth's atmosphere does not decrease by half until a height of about 6 kilometers is attained, the concentration of the dispersoids in- vestigated by Perrin often fell off this amount when the difference between levels was only about lo/j,. It should also be pointed out that the constant N of the Ein- stein- Smoluchowski equation, in other words, the number of particles in a gram-molecule, which is of such great importance in various fields in physics and physical chemistry, can be calcu- lated in different ways. The values thus obtained agree sur- prisingly well with those obtained by entirely different means. Indeed it seems as though these methods as applied to submo- lecular dispersed systems yield the most exact figures of this fun- damental value new obtainable. As this constitutes one of the brilliant achievements of colloid or dispersoid chemistry the following table taken from J. Perrin (I.e., 1910) is given in full. TABLE 38. DETERMINATION OF THE NUMBER OF PARTICLES IN A GRAM-MOLECULE (AVOGADRO'S CONSTANT N) BY DIFFERENT COLLOID-CHEMICAL METHODS (According to J. Perrin) Phenomenon Studied N. io~ 22 Average of volume in liquid state > 45 From the dielectric force of gases < 200 By using Van der Waal's equation. 60 . From distribution of a uniform suspension 70.5 From the average displacement in a given time 71.5 movement From the average rotation in a given time 65 Diffusion of dissolved substances 40-90 Mobility of ions in water 60-150 Radiance of the sky 30-7150 Viscosity of Direct measurements of atomic charge Emissions of a corpuscles Of droplets condensed upon ions 60-90 Of ions attached to dust particles 64 Total charge emitted 62 Time constant of radium 70.5 Helium produced by radium 71 Energy of the Infra-red spectrum 60-80* 1 See Perrin (1910) for details regarding other phenomena. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 2 09 These brilliant results fill one with admiration for the remark- able fertility of the Emstein-Smoluchowski equation, especially when it is remembered how many still purely hypothetical factors enter into its composition. Nothing better illustrates the daring, we might say, of this train of thought than the remark of Perrin, to whom, with Svedberg, science owes most in this field, anent the theorem of the equality of the distribution of energy which is the nucleus of all kinetic deductions. "The word theorem should deceive no one, for it is full of hypotheses as is almost every theory of mathematical physics." It is safest, perhaps, to hold that the future will preserve but a part of our present kinetic notions to work over into a more general, less supposititious theory. As a matter of fact, several of the laws governing Brownian movement may be deduced even without recourse to kinetic assumptions, as for example, the inverse proportionality of velocity to viscosity, from Stokes' law. 1 Possibly this purely inductive method will some day discover these same laws; in fact, consideration of the methods of science demands it, but when the day will come must remain a matter of opinion. 14. Determination of the "Molecular Weight" of Dispersed Particles from their Brownian Movement. Since N can be cal- culated, the so-called "molecular weight" of dispersed particles may also be determined from the formula of Einstein and Smo- luchowski. This can also be done from the logarithmic distribu- tion equation governing concentration in different levels. J. Per- rin 2 made such calculations and by this method found his gutta-per- cha particles to have a molecular weight of about 30,000,000,000. It must again be emphasized that these values cannot be compared with the molecular weights of molecularly dispersed particles. In the former, the diameter of the particles (or their volume) is under discussion, and this "molecular weight" becomes progressively less as the size of the particles decreases. The normal concept of molecular weight does not consider the size of the particles as at all variable, but deals simply with that single 1 The influence of electrical energy upon Brownian movement as postulated on p. 201 cannot be deduced from kinetic considerations, but is an inductive con- clusion. It appears in the Einstein-Smoluchowski equation as a factor analogous to the viscosity factor since the velocity would be approximately inversely propor- tional to the intensity of the induced field of force. Judging from the experi- mental results of Svedberg and Perrin these proportionality constants would, in many cases, have a very small value. 2 J. Perrin, Compt. rend., 147, 475 (1908). 14 2IO SPECIAL COLLOID-CHEMISTRY value which is observed at the maximum degree of dispersion. This obviously constitutes a fundamental distinction between the "molecular weights' 7 of differently dispersed systems. 28. Diffusibility of Colloids i. General Remarks. When one pours some of the pure dispersion means upon a molecularly dispersed system, the mo- lecularly dispersed phase wanders over into the pure dispersion means until uniform distribution throughout both phases is at- tained. This phenomenon is known as diffusion. In trying to explain what has happened it is natural to think of the influence of Brownian movement. In the irregular, particularly in the forward, movements of small particles, as observed, for example, by Zsig- mondy in colloid solutions, it is to be expected that an accidental wandering of the particles over into the pure dispersion means must take place. But such accidental migration cannot wholly explain all diffusion, the laws of which A. Fick formulated in 1855. In order that Brownian movement may lead to diffusion, it must become directive in character toward the pure dispersion means or toward the "more dilute" parts of any continuous system. As a matter of fact, the existence of such a directive movement in diffusion until uniform distribution of the dispersed phase through- out the whole system is attained can be foreseen, when the relation between degree of movement and concentration of dispersed parti- cles is called to mind. As noted above (p. 196), R. Zsigmondy observed less movement in dilute systems than in concentrated ones. Because of this, equilibrium cannot exist, so far as average velocity of particles is concerned, in a system consisting, say, of a colloid solution covered by a layer of the pure dispersion means. In places of greater concentration, the particles will be moving faster than in those of a lower one. The sources of energy for Brownian movement, whatever they be, must therefore have dif- ferent values in different parts of the system at the beginning of diffusion. But following the general laws of energy, equilibrium cannot be attained in a closed system until the energy intensities have the same value everywhere. 1 We need but call to mind the electrostatic charge on the surface of a metallic sphere. If the 1 Wilh. Ostwald, Lehrb. d. allgem. Chem., 2 AufL, 2, 35 (1903). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 211 energy intensities in a "diffusion field" are not everywhere the same the system is unstable and changes must occur of a directive character leading, in the end, to an equalization of the intensities in the whole system. Thus, if only a local charge were present on the metallic sphere, currents would emanate from this to all other points on its surface. In the case of diffusion, a movement of the dispersed particles toward regions of lower concentration would have to occur until the average velocity of the particles was the same in all portions of the system. The average absolute value of the movement would therefore become progressively less until the minimum is attained at the end of diffusion. It is in keeping with this general notion that in the molecular dis- persoids the diffusion coefficients (dis- tances traversed in centimeters per day) are greater at higher concentrations than at lower ones. 1 The influence of concentration on the diffusion of a colloid has not yet been studied. The experimental difficulties besetting such a study will become clear in the following paragraphs. FIG. 44. Apparatus for -r\'ff i -i the study of diffusion as ar- Diffusion experiments have a special ran ged by Thomas Graham. interest in colloid chemistry because its very foundations were built upon them by Th. Graham (1850- 1862). 2. Experimental Study of Diffusion of Colloids. The common method of determining the diffusion coefficient was originated by Th. Graham. 2 A wide-necked bottle is filled with the solution to be investigated and placed in a second vessel; the pure dispersion means is then poured with special care 3 into the second vessel until it covers the inner bottle to a depth of several centimeters. Fig. 44 is an exact copy of the sketch from Graham's original work. After a given time, the amount of dissolved substance which has escaped from the inner vessel is determined. The relation of this 1 The complicated diffusion phenomena observed in certain ionic dispersoids, as in hydrochloric acid, form exceptions to the general rule because electrochemical processes come into play. SeeWilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl., i, 686 (i9 3). 2 Th. Graham, Philos. Trans., 1-46, 805-836 (1850); 483-494 (i85i),e tc.; Liebig's Ann., 77, 56, 129 (1851); 121, 5, 29 (1862). 3 To prevent mixing of the two liquids at the critical moment Graham used a pointed sponge from which to express the second liquid. 212 SPECIAL COLLOID- CHEMISTRY to the time (at constant diffusion surface, temperature, etc.) is a measure of the velocity of diffusion. For a discussion of the more modern methods of using Graham's principle, as well as for the methods of calculating the absolute diffusion coefficients from the experimental data, the text-books of physics and physical chemistry need to be consulted. 1 It is difficult in Graham's method to bring the two liquids into contact with each other without disturbing their surfaces. Slight differences in temperature, vibrations, etc., may, moreover, in- troduce great experimental errors. But Graham already knew a remedy for this. He found that the velocity of diffusion was not much influenced if the experiment was carried out in a not too highly concentrated agar-agar,. gelatin or starch paste, instead of in pure water. Thus, when he placed in the diffusion cell a 2 per cent, agar solution containing 10 per cent, salt, and a pure agar solution of the same concentration in the outer vessel and allowed both to solidify, he found after 15 to 16 days that the latter con- tained 9.992 grams of diffused salt. Normal diffusion into pure water, after 14 days showed 9.999 grams, all other conditions, in- cluding temperature (10), being constant. These findings have often been verified. Thus F. Voightlander 2 observed 0.72 per cent, sulphuric acid to diffuse the following distances into agar jellies of different concentrations after i hour. Agar jelly, i per cent. = 8.5 mm. 2 per cent. = 7.8 4 per cent. = 7.0 The amounts that diffused were as follows: Into agar jelly, i per cent. = 1.08 mg. S0 3 2 per cent. = i.io 4 per cent. = 1.09 The absolute values for NaCl of the diffusion coefficients, amounts diffused in grams , are as follows : days Agar jelly, i per cent. = 1.04 2 per cent. = 1.03 3 per cent. = 1.03 1 See, for example, Wilh. Ostwald, Grundr. d. allg. Chem., 4 AufL, 194, Leipzig, 1909; Wilh. Ostwald-Luther-Drucker, Hand und Hilfsbuch, 3 Aufl. 1 F. Voightlander, Z. f. physik. Chem., 3, 329 (if MECHANICAL PROPERTIES OF COLLOID SYSTEMS 213 G. Hiifner 1 and others obtained similar results. But it should again be emphasized that diffusion is thus independent of the presence of gels only when these are there in low concentrations. Marked retardations appear at higher concentrations as even H. de Vries 2 knew. Diffusion is also influenced, of course, when chem- ical or colloid-chemical changes, as precipitations, are produced in the gels by the diffusing substances. A handy arrangement for demonstrating diffusion l\as already been described in the practical introduction on p. 9. Test tubes are half filled with colloid gels and the diffusing solution poured upon them. Figs. 2, 45 and 46 illustrate the results. Disturbance of the diffusion surfaces may also be avoided by stretching over the inner vessel a suitable membrane through which the dissolved substances pass freely. Hydrophane plates (G. Hiifner, I.e.), filter papers (S. Exner, see below), parchment papers (The Svedberg, see below), etc., have been used for this pur- pose. Or, the diffusing substance may be placed directly in cells entirely made of such substances. But the membranes used must be completely permeable to the diffusing substance and must not affect it, as through adsorption, etc. 29 on dialysis should be studied in this connection. 3. Experimental Facts Regarding Diffusion of Colloids. It follows from the relation between velocity of Brownian movement and size of particles discussed above that the velocities of colloid particles must be considerably less than those of molecularly or ionically dispersed ones. The compilation in Table 39 shows this clearly; additional facts regarding diffusion velocities are given below. As is clearly evident, the diffusion coefficients of typical col- loids average 34 o that f the slowly diffusing cane sugar and only 3/Loo that of the rapidly diffusing electrolytes such as acids and alkalies. The highly dispersed goldsol of The Svedberg which, for a colloid, diffuses exceptionally fast, takes an intermediate posi- tion. It should be remembered that the particles of the latter have a diameter of about i/*/*; in other words, this goldsol is OIL the boundary between molecular and colloid dispersoids. 1 G. Hiifner, Z. f. physik. Chem., 27, 227 (1898). 2 H. de Vries, Fittica's Jahresber. d. Chem., I, 144 (1884). 214 SPECIAL COLLOID-CHEMISTRY TABLE 39. DIFFUSION COEFFICIENTS OF DISPERSOIDS Molecular and ionic dispersoids. Specific area > 6 X IQ 1 Colloids. Specific area about 6 X loUo 6 X 10* Nitric acid (Voightlander) . . 2 . 10 (20) Sodium chloride (Voight- lander) i . 04 (20) Magnesium chloride (Voight- lander) 0.77 (20) Copper sulphate (Landolt- Barnstein) 0.47 (17) Urea (Scheffer 1 ) 0.81 (7.5) Cane sugar (Graham- Stefan 2 ) 0.31 (9) Mannite (Scheffer) 0.38 (10) Gold hydrosol (The Sved- berg 6 ) 0.27(11.7) Clupeinsulphate (Herzog) . 0.074 (18) Pepsin (Herzog 3 ) 0.070 (18) Rennin (Herzog) 0.066 (18) Egg-albumin (Herzog). . . . 0.059 (18) Albumin (Graham-Stefan). 0.063 (13) Caramel (Graham-Stefan). 0.047 (10) Ovomucoid (Herzog) 0.044 (18) Emulsin (Herzog) 0.036 (18) Invertin (Herzog) 0.033 (18) Diphtheria-toxin (Arrhe- nius and Madsen 4 ) 0.014 (12) Diphtheria-antitoxin (Ar- rhenius and Madsen) 0.0015 (12) Tetanolysin (Arrhenius and Madsen) 0.037 (12) Antitetanolysin (Arrhenius and Madsen) 0.0021 (12) Figs. 45 and 46 illustrate quantitatively the diffusion velocities of various dispersoids. They show what has happened after about 3 day's diffusion into solid 1.5 per cent, agar at 20. In Fig. 46 the supernatant liquids out of which diffusion has occurred have been poured off so that the diffusion phenomena may show up more clearly. To the left in this figure are found molecular dispersoids, to the right, typical colloids. The tubes are arranged, from left to right, according to the lengths of the diffusion paths. 6 The picric acid, cobalt nitrate and eosin of tubes i, 2, and 3 have wandered almost to the bottom of the agar column; benzo-pur- purin and congo red on the extreme right have scarcely moved. The dyes lying between these, show intermediate degrees of diffus- ibility. 7 Fig. 45 shows the results, after 8 days, of experiments on the diffusion of typical colloids (hydrosols of silver, gold, anti- mony sulphide, arsenic sulphide and iron hydroxide). The sharp- 1 G. Scheffer, Z. f. physik. Chem., 2, 390 (1888). 2 Graham-Stefan, Sitz. Ber. Ak. Wien, 77, II, 161 (1879). 3 R. O. Herzog (and H. Kasarnowski) Koll. Zeitschr., 2, i (1907); 3, 83 (1908); Bioch. Zeitschr., n, 172 (1908). 4 S. Arrhenius and Th. Madsen, Immunochemie, 16, Leipzig, 1907. 'The Svedberg Z. f. physik. Chem., 67, 107 (1909) The gradation is not as clearly shown in the photograph as it actually appears since the different (mostly o.i per cent.) solutions have different colors. A photo- graphic plate does not bring this out 7 Regarding the diffusibility of dyes see L. Vignon, Compt. rend., 150, 690 (1910). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 215 ness of the diffusion line between the diffusing substance and the gel should be noted. It is sharp in the case of typical colloids; but illy marked in that of the molecular dispersoids and systems of intermediate degrees of dispersion. 4. Influence of Degree of Dispersion on Diffusion Velocity. The diffusibility of a disperse phase is intimately connected with its degree of dispersion as shown in Table 39. Among molecularly FIG. 45. Diffusion of colloids into 2 per cent, agar-agar at the end of a week, i, Gold hydrosol; 2, silver solution (Credi}\ 3, antimony sulphide solution; 4, arsenic trisulphide solution; 5, iron hydroxide solution. dispersed substances, ions or electrolytes migrate most rapidly. Substances of higher molecular weight, or, more correctly, of greater atomic aggregation, follow. Last in the list, stand the colloids. This dependence of diffusion velocity on the size of the particles is of great interest. Of special importance is the possi- bility of procuring one and the same substance in different degrees of dispersion and therefore possessed of different degrees of 2l6 SPECIAL COLLOID-CHEMISTRY diffusibility. Thus S. E. Linder and H. Picton 1 were able to pre- pare the following four systems of arsenic trisulphide in water: c*As 2 S 3 ; particles microscopically visible, non-diffusible (coarse suspensions) , j8As 2 S 3 ; microscopically homogeneous, non-diffusible, 7As 2 S 3 ; diffusible, but unfilterable through porcelain cups, 5As 2 S 3 ; diffusible and filterable. After Wo. Ostwald 2 had repeatedly emphasized the great theo- retical interest attaching itself to a systematic and quantitative FIG. 46. Diffusion into a 2 per cent, agar-agar at the end of three days. i, Picric acid; 2, cobalt nitrate; 3, 0;i per cent, eosin; 4, o.i per cent, ponceau R. R. R.; 5, o.i per cent, new fuchsin O; 6, o.i per cent, vesuvin; 7, o.i per cent, safranin G.; 8, o.i per cent, benzopurpurin; 9, o.i per cent. Congo red. investigation of the relations between diffusibility (and other properties) and degree of dispersion, The Svedberg, 3 in the prose- cution of his experimental study of the Einstein-Smoluchowski formula (see above), attacked the problem. He determined the diffusion velocities of different gold hydrosols by pouring these into parchment cells having different porosities. His results are given in Table 40: 1 S. E. Linder and K. Picton, Trans. Chem. Soc. Lend., 61, 114, 137, 148 (1892); 67, 63 (1895); 71, 568 (1897); 87, 1906 (1905). 2 See, for example, Wo. Ostwald, Koll.-Zeitschr., I, 298 (1907). 3 The Svedberg, Z. f. physik. Chem., 67, 105 (1909). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 217 TABLE 40. DEPENDENCE OF DIFFUSION VELOCITY ON SIZE OF PARTICLES IN GOLDSOLS (According to The Svedberg) Size of particles in MM Concentration of in- Concentration of ner liquid in outer liquid in normality normality Relation of con- centrations to each other 14 20-30 I . 5 io- 4 1.5 io- 4 1.5 io- 6 1.3 io- 7 IOO 1 200 The gold content was determined colorimetrically. The reciprocal values of the concentration relations are measures of the diffusion velocity. In other words, D\ = k\ . Koo> when D is the diffusion coefficient and ki the proportionality constant. Similarly, D 2 = ^2^200- When the ratio of these diffusion coefficients is compared with the size of the particles (taking the latter to average 2.5 , 100 respectively 2.5 and 25/4) we observe, since ^-^ ------ that the diffusion velocity is approximately inversely proportional to the size of the particles, or D.r = constant. True it is, that we are basing these conclusions on studies in- volving but two degrees of dispersion. An investigation cover- ing a wider range would be of great interest. Finally, it should be mentioned that S. Exner 1 found coarse suspensions, such as clay silt, to show a distinct though slow diffu- sion. But whether pure dispersions made up of particles larger than 5/j and therefore free from Brownian movements are really capable of true diffusion appears doubtful (see below, p. 219). 5. Theory of Colloid Diffusion. The close relation between Brownian movement and diffusion was mentioned at the beginning of this paragraph. It seems natural, therefore, that the moleculo- kinetic considerations of A. Einstein and M. von Smoluchowski, 2 which proved so fruitful in the mathematical discussion of Brown- ian movement, should lead to similarly important results when ap- plied to diffusion. For example, the inverse proportion between size of particles and diffusion velocity is deducible from the equa- tions of Einstein and von Smoluchowski. For the diffusion co- efficient they developed the equation: N 1 S. Exner, Sitz. Ak. Wiss., Wien, 56, 116 (1867). 2 According to von Smoluchowski the right side of the equation contains the factor 2.03. 2l8 SPECIAL COLLOID- CHEMISTRY The symbols have again the meaning indicated on p. 206, r repre- senting the radius of the particles. If dispersion means, tempera- ture, internal friction, etc., are constant, the diffusion coefficients of two dispersed phases bear the following relation to each other: D 1 _ rt D 2 ~ ri ' in other words, they correspond to Svedberg's experimental find ings. This relation has much in common with the equation which expresses the connection between the diffusion of molecular dis- persoids and their molecular weight. The relation: D V m = constant has been established by S. Exner, for gases, and by L. L. Oholm 1 for (theoretically) infinitely dilute solutions of non-electrolytes. In this equation m is the molecular weight. If the square root of the molecular weight is made equal to the radius of the particles, this equation changes into that governing the diffusion of goldsols. Conversely, with the laws of Exner-Oholm and Einstein- Smoluchowski, we may calculate the size of the particles as well as their molecular weight. In this way R. 0. Herzog (I.e.) found an approximate agreement between the " molecular weights" of ov- albumin, hemoglobin, etc., thus calculated: and the figures obtained by other methods. For the values for toxins, etc., as calculated by Sv. Arrhenius and Th. Madsen (I.e.), control measurements are not yet available. The same objections may be raised against all these calculations which were raised in discussing the determi- nation of the molecular weight of colloid systems by freezing point, boiling point and vapor pressure methods. The calculations by R. O. Herzog and The Svedberg (I.e.) of the size of the particles by the formula of Einstein-Smoluchowski are less open to objection. Herzog, on this basis, calculated the size of the particles of ovalbumin to be 2. 86^- This figure about corresponds to the higher dispersion values obtaining within col- loid systems, and therefore agrees well with the general fact that the colloid properties of the albumins place them near the mo- X L. L. Oholm, Z. f. physik. Chem., 70, 378 (1910); this also includes the earlier literature. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 2IQ lecularly dispersed systems. Svedberg calculated, in a reverse manner, the size of the particles of the highly dispersed gold solu- tion of R. Zsigmondy, the particles of which, according to Zsig- mondy, had a diameter of i to 4w. He obtained, by Einstein's formula, o. 94/^/1, and by Smoluchowski's 2.16^, obviously a very good agreement. The calculated diameter of the particles of molecularly dispersed systems also agrees well with the values obtained by other methods. 6. Effect of Added Substances on Colloid Diffusion. Spurious Diffusion of Colloids. The effects upon diffusion of adding differ- ent substances are so complicated, even in molecular dispersoids, that general laws governing them have not been formulated. 1 It is to be expected that these relations will be still more complicated when phases having different degrees of dispersion are mixed. The more important phenomena observed when colloid systems are mixed with molecularly dispersed ones are the following: The effect of electrolytes on the diffusion velocity of colloids may be discussed under two headings the electrolyte may be added to the diffusing substance, or the diffusion of the colloid may be permitted to occur into the solution of an electrolyte. In either case, different results may be expected, depending on whether the electrolyte does not affect the degree of dispersion of the colloid (which is exceptional) or whether it increases or decreases it. Both an increase and a decrease in the degree of dispersion on adding substances from without have been described in the literature. An illustration of the latter is found in the common and well-known effects of electrolytes on colloids (aggregation, coagulation); an illustration of the former in the phenomena of peptization. The inhibiting e/ect of added substances on diffusion has been studied by E. von Regeczy. 2 He found pure albumin when placed in parchment-paper tubes to diffuse out of these in the course of 12 hours in sufficient amount to impart a decided albumin re- action to the outer liquid. But when some solid NaCl was pre- viously added to the albumin, no trace came out. S. E. Linder and H. Picton (I.e., 1905) noted a similar behavior in an inorganic colloid, arsenic trisulphide. They allowed a highly dispersed arsenic trisulphidesol to diffuse, on the one hand, into water, on 1 See, for example, Wilh. Ostwald, Lehrb. d. allg. Chem., 2 AufL, 674, Leipzig, 1903. 2 E. von Reg6czy, Pfluger's Arch., 34, 431 (1884). 22O SPECIAL COLLOID-CHEMISTRY the other, into an NH 4 C1 solution, so dilute that it caused no visi- ble coagulation. Their results are given in the following table: TABLE 41. DIFFUSION OF As 2 S 3 SOL INTO PURE WATER AND INTO NH 4 C1 SOLUTION (According to S. E. Linder and H. Picton) Time Diffused amounts in per cent, of the inner fluid Into pure water Into NH 4 C1 24 hours 48 72 96 10 per cent. 14 i per cent. 3 23 An antimony sulphidesol gave similar results when permitted to diffuse into water and into a solution of tartar emetic. An example of how the addition of an electrolyte may favor diffusion of a colloid is found in Th. Graham's paper (I.e.). He observed egg albumin, which in its natural state is slightly alkaline and diffuses but slowly, to diffuse more rapidly if it is carefully neutralized with acetic acid. While after a week but 0.63 gm. of native (alkaline) albumin diffused out, 0.94 gm., in other words, 30 per cent, more, came out when the albumin was neutralized. The neutralization increases the degree of dispersion, as proved by the observations of Wo. Pauli and others (see p. 169), who found neutral albumin to increase the viscosity of water less than that to which an acid had been added. H. Picton (I.e., 1892) made similar observations on suspen- soids of arsenic trisulphide. He found this to diffuse rapidly when still contaminated with the tartar emetic from which it was prepared. Whether the electrolyte serves to increase the degree of dispersion in this case remains a matter of question, though such an influence on suspensoids has been observed. It is more reasonable to assume that the electrolytes in their rapid diffusion simply drag the colloid particles along with them, a view held by H. Picton himself; or that the movements of the liquid, caused by the diffusion of the electrolytes, set up currents which bring about the observed results. The interesting experiments of W. R. Whitney and J. Blake 1 1 W. R. Whitney and J. Blake, Journ. Amer. Chem. Soc., 26, 1339 (1904). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 221 on the great velocity of diffusion of goldsols, produced by reducing ether solutions of gold chloride by means of acetylene, must, no doubt, be similarly explained. When they concentrated their colloid gold at the lower end of a vertically placed cylinder by electrophoresis and then carefully poured pure water upon it, they observed an unusually rapid and spontaneous upward movement of the gold which increased with the increase in the concentration of the gold. The observed velocities varied between o.oi cm. and 0.24 cm. per hour. When it is recalled that F. Voightlander (p. 212) found the rapidly diffusing sulphuric acid to cover only 0.85 cm. per hour in i per cent, agar while the finest goldsols of The Svedberg have a diffusion coefficient of only 0.27 (as compared with one of 2.0 for sulphuric acid on the same scale), it becomes impossible to believe that the experiments of Whitney and Blake deal with true diffusion of a colloid phase. The diffusion movements of the molecular dispersoids present in their preparations may have led to the high (apparent) diffusion of the colloid particles, as in the experiments of H. Picton. More probably still, the gold particles became loaded with gas through the electrical treatment to which the gold was subjected and this then led to their rapid rise. Suitable experiments could easily be arranged to test the validity of such an explanation. The favorable effect of electrolytes upon the diffusion of col- loids has again been observed when they are permitted to diffuse into solutions of electrolytes. Thus von Wittich 1 found, as far back as 1856, that albumin diffuses more easily into a salt solution than into pure water. Within certain limits, the diffusion is the more rapid the greater the concentration of the salt. E. von Regeczy (/.c.), M. Oker-Blom 2 and others have since studied this phe- nomenon. The paper of M. Oker-Blom is the source of Table 42. It is readily apparent that the amounts of diffused albumin in- crease with increase in the concentrations of NaCl, but in the in- termediate concentrations, from 0.56 to 1.30 per cent., a region of minimum diffusion is observed. What follows will show that this need by no means be due to experimental error. To explain these phenomena, 3 we need but remember that 1 von Wittich, J. Muller's Arch. f. PhysioL, 286 (1856). 2 M. Oker-Blom, Skandinav. Arch. f. PhysioL, 20, 102 (1904.) 3 Wo. Pauli, Koll.-Zeitschr., 3, n (1908). 222 SPECIAL COLLOID-CHEMISTRY albumin solutions are more strongly hydrated, in other words, swell more in many salt solutions than in pure water. We may assume that in this process the free, dispersed albumin particles wander into the strongly hydrating dispersion means just as the liquid wanders into the solid colloid to make it "swell." A suffi- ciently marked hydration of the dispersed particles must separate them from one another. TABLE 42. DIFFUSION OF SERUM ALBUMIN INTO NaCl SOLUTIONS (According to M. Oker-Blom) Concentration of NaCl in the outer liquid Amount of albumin, in grams, diffused after 24 hours about o . 28 per cent. 0.56 0.053 0.053 0.052 0.74 0.052 0-93 0.050 1.30 1.48 1.86 0.052 0.058 0.060 2.38 0.079 At the present time, we can only guess at what must be the influence of several colloids upon each other when they are mixed, and how they must affect each other's diffusion velocity. The influence of concentration and of temperature on the diffu- sion of colloids has not yet been studied. Judging from the find- ings of Th. Graham (7.C.), the rate of increase in diffusion velocity of egg albumin with the temperature is about as great as that of molecularly dispersed systems under the same circumstances, but exact figures on the subject are still wanting. 29. Dialysis of Colloid Systems I. General Remarks. The impeding effect of concentrated gels or membranes upon free diffusion was touched upon above. While ordinary electrolytes pass through parchment-paper mem- branes almost as rapidly as though they were not there, albumin and gum arabic cannot penetrate them. Th. Graham, who first investigated this phenomenon, called it dialysis (1861). He noted that all substances which, when allowed to diffuse in the open, do MECHANICAL PROPERTIES OF COLLOID SYSTEMS 223 so only slowly or not at all are also restrained by parchment mem- branes. On the other hand, those which diffuse rapidly are not markedly checked in their movement through the presence of membranes. This difference in behavior of " dissolved " substances toward parchment paper formed the basis of the whole concept of the colloids. Substances which do not dialyze (or pass through parchment paper) Graham called colloids, those which do, crys- talloids. The latter systems are today known as "molecular dispersoids." One can readily accomplish a separation of the different classes of dispersed systems by dialysis. As a matter of fact, Graham called his fundamental work "Liquid Diffusion Applied to Analysis." By using a constant type of membrane, systems of unknown degrees of dispersion may be classified into such as dialyze and such as do not (see the practical introduction). When, by any method what- soever, coarsely dispersed systems have been excluded, dialysis offers a convenient method of distinguishing between the colloid and molecularly dispersed systems. It must be emphasized that comparable results may be ob- tained only by use of one and the same kind of membrane. The precipitation membranes of copper ferrocyanide and tannic acid- protein, for example, are impermeable even to many molecular dispersoids and may, therefore, give rise to the phenomena of os- motic pressure (see the following paragraphs). 2. Methods of Dialysis. Parchment tubes, parchment dif- fusion capsules, reed tubes, fish bladders, urinary bladders, egg membranes and amniotic membranes are most used in the dialysis of colloids. 1 Membranes of collodion, as first used in col- loid studies by G. Malfitano, 2 are especially convenient in many respects. Their preparation is discussed in the practical introduc- tion (p. 10). Several forms of dialyzers were illustrated on page ii. 3 Because of their historical interest, Figs. 47, 48 and 49 are introduced, which are copies of the two types of apparatus which Graham used in the great work upon which colloid chemistry is built. 1 A detailed discussion of dialysis and its methods may be found in R. P. von Calcar, Dialyse, Eiweisschemie und Immunitat, Leipzig-Leiden, 1908. 2 G. Malfitano, Compt. rend., 139, 1221 (1904). 3 For a new form see R. Zsigmondy and R. Heyer, Z. f. anorg. Chem., 68, 916 (1910). 224 SPECIAL COLLOID-CHEMISTRY In dialyzing non-aqueous liquids, the effect of the dispersion means upon the membrane must be considered. A possible chem- ical effect of the substances subjected to dialysis must also be kept in mind, though such is rarely met with among the colloids. FIG. 47. Thomas Graham's disc dialyzer. FIG. 48. Thomas Graham's bell dialyzer. 3. Experimental Facts Regarding Dialysis of Colloids. Since the days of Graham, almost every student of the general properties of colloid systems has made use of dialysis. It is, therefore, not possible to review all the work that has been done in this field. Generally speaking, dialysis teaches the same facts as diffusion. FIG. 49. A second method of using Graham's bell dialyzer. Thus, S. E. Linder and H. Picton (Lc.) were able to distinguish between dialyzing and non-dialyzing metallic sulphides. Of the many groups of compounds studied, only one will be discussed here, that of the technically and theoretically important water soluble dyes. F. Krafft and G. Premier, 1 0. Teague and B. H. Buxton, 2 1 F. Krafft and G. Preuner, Ber. d. Dtsch. chem. Ges., 32, 1620 (1899). 2 O. Teague and B. H. Buxton, Z. f. physik. Chem., 60, 469 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 225 H. Freundlich and W. Neumann, 1 R. Hober, 2 L. Vignon, 3 W. Biltz and F. Pfenning, 4 have all studied these. In Table 43 are reproduced some of the findings tabulated by Biltz. In connec- tion with this table it should be noted that Krafft and Preuner used parchment tubes; Teague, Buxton, Hober and Vignon, parch- ment-paper capsules, manufactured by Schleicher and Schiill; Biltz and Pfenning, collodion membranes. The solutions used were usually o.i per cent.; Teague and Buxton used 0.02 per cent; Biltz and Pfenning 0.5 per cent. The abbreviations in parentheses after the names of the dyes mark their origin. TABLE 43. DIALYSIS OF DYES Typical Molecular Dispersoids Name Atomic number Molecular weight Dialyzes Observer Picric acid IQ 22O O Quickly Vignon Toluidin blue (Hoechst) Naphthol yellow 9 (Bayer, Hoechst) Chrysoidin Methylene blue Eosin 19 27 30 37 ?7 I43-S 355-0 214.0 317.5 602.0 Quickly. . Quickly. . Quickly.. Quickly. . Ouicklv. Biltz. Hober, Vignon, Biltz. Teague and Buxton. Krafft and Preuner, Teague and Buxton, Biltz. Teague and Buxton Erythrosin . 37 880 o Quickly Vignon. Biltz. Hober Biltz Bengal rose 37 1050.0 Ouicklv. Hober Quinolin yellow (Akt.) True acid fuchsin B (Bayer) . . Auramin o (Akt.) Safranin. 40 4i 43 44 477-0 467.0 303.5 2t;o cj Quickly.. Quickly.. Quickly. . Quickly Biltz. Biltz. Biltz. Teague and Buxton Wool violet S (Bad.) Brilliant crocein 36 Acid fuchsin S (Akt.) Methyl violet 4 6 51 52 56 445-0 556 o 572.0 JQ7 . e Quickly.. Quickly.. Quickly. . Ouicklv Vignon, Biltz. Hober. Hober. Vignon. Biltz Patent blue V (Hoechst) .... Guinea green B.. to 66 84 86 to 469.5 804.0 73O O Quickly.. Quickly Hober, Biltz. Hober Erioglaucin ne 782 o Quickly Hober 1 H. Freundlich and W. Neumann, Koll.-Zeitschr., 3, 80 (1908). 2 R. Hober, Koll.-Zeitschr., 3, 76 (1908); Bioch. Zeitschr., 20, 80 (1909). 3 L. Vignon, Compt. rend., 150, 619 (1910). 4 W. Biltz (with F. Pfenning), van Bemmelen-Gedenkboek, 108, 1910. IS 226 SPECIAL COLLOID-CHEMISTRY Transition Systems between Molecular Dispersoids and Colloid Solutions Name Atomic number Molecular weight Dialyzes Observer Neutral red 37 288.5 Slowly Teague and Buxton. True red A (Akt. Bayer) Ponceau 2 R (Akt.) Ponceau B O extra (Akt.) .... Victoria black B (Bayer) Nile blue . 41 45 5i 58 58 400.0 480.0 SS6.0 622.0 443 .4 Slowly Rather quickly Rather quickly Only in traces Slowly Hober, Biltz. Hober, Biltz. Biltz. Biltz. Teague and Buxton. Crystal violet rn 4O7 "? Rather Freundlich and Neu- Aniline blue 74 ' 565.5 quickly Very mann, Vignon. Teague and Buxton, Benzo blue 3 B (Bayer) Acid violet 6 B (Akt) 86 QI 960.0 733.O slowly Slowly Some- Hober. Hober. Hober, Biltz. what Typical Colloid Solutions Name Atomic number Molecular weight Dialyzes Observer Cloth red 6 A (Akt.) C7 482.0 Not at all Biltz. Congo brown 9 (Akt ) 68 682.0 Not at all Hober, Biltz, Teague, Congo red (Akt ) 7O 606.0 Not at all and Buxton. Vignon, Biltz. Azo blue (Akt.) 74 726.0 Not at all Krafft and Preuner, B enzopurpurin 76 724 O Not at all Teague and Buxton, Hober, Biltz. Krafft and Preuner, Congo blue B X 26 (Akt.)... Night-blue 80 84 860.0 575.5 Not at all Not at all HQber, Biltz. Biltz. Teague and Buxton, Heliotrope B B (Bayer) 88 SlO.O Not at all Freundlich and Neumann, Biltz. Hober. Chicago blue 6 B R W (Akt.) . 88 9Q2.0 Not at all Biltz. The table shows that, in general, dialyzability decreases with rising atomic number and increasing molecular weight. That the rule is only approximately true can be seen by comparing the tables horizontally. (The vertical rows are arranged accord- MECHANICAL PROPERTIES OF COLLOID SYSTEMS 227 ing to increasing atomic numbers.) In each of the three classes, some of the dyes have a low, while some have a high, atomic num- ber. Even substances with high molecular weights, as Bengal Rose may be found in the rapidly dialyzing class. The degree of dispersion of the dye is therefore dependent not alone on the atomic number or the molecular weight, but on other factors as well. It seems natural to have tried to explain the lack of parallelism through the chemical constitution of the dyes, as W. Biltz and others have done with a fair degree of success. A review of Biltz's results is beyond the limits of this book, but it should be noted that even so, a quantitative relation between chemical constitution and degree of dispersion does not appear even when only simple compounds in homologous series are considered. The absence of parallelism between size of particles and molecular weight demonstrates also the danger of trying to determine molecular weight from diffusion constants as discussed on p. 218. When the dialysis of non-aqueous colloids is discussed it must first be remembered that many dyes " dissolve" to form colloid solutions in water, but molecularly dispersed ones in other sol- vents, such as alcohol (F. Krafft, I.e., and others). Corresponding herewith, the alcoholic solutions dialyze better than the aqueous ones. Especially interesting results have been obtained with iodine dissolved in different organic solvents. J. Amann 1 has shown that iodine dissolves in benzene as a molecular dispersoid, in petroleum as a colloid. Corresponding to this fact, it dia- lyzes through a parchment capsule out of its solution in benzene but not out of that in petroleum. 2 4. Special Observations Regarding the Dialysis of Colloids. Colloids frequently pass through a dialyzing membrane /0r a short time immediately following their preparation. This is especially true of freshly prepared silicic acid as observed by Th. Graham and more recently confirmed by F. Mylius and E. Groschuff. 3 The explanation of this interesting fact is to be found in the instability of the degree of dispersion in colloid systems. When a colloid solution is prepared by condensation of a molecularly dispersed system, the desired product is not obtained at once, but only after hours or days. Sometimes, moreover, the condensation occurs 1 J. Amann, Koll.-Zeitschr., 7, 235 (1910); 7, 67 (1910). 2 According to the unpublished results of Prof. S. Suzuki and the author. 3 F. Mylius and E. Groschuff, Ber. d. Dtsch. chem. Ges., 39, 119 (1906). 228 SPECIAL COLLOID- CHEMISTRY unequally, in other words, a few colloid particles are first pro- duced but their number gradually increases with time, at the expense of the molecularly dispersed. It is to such changes that the behavior of silicic acid, of many albumin solutions, of humic acid, etc., must be referred. Another phenomenon of both practical and theoretical impor- tance is the chemical decomposition through dialysis of molecularly dispersed substances with formation of a colloid phase. It was known to Graham and belongs to the earliest methods of preparing col- loid systems. It is essential that the original material suffer hydrolysis in water, yielding an insoluble, or but slightly soluble, component. This is true of the chlorides, nitrates, acetates, etc., of the metals. Since the molecularly soluble product of the hy- drolysis passes through the dialyzing membrane while the " insol- uble" component remains behind in colloid form, a continual displacement of the hydrolysis takes place, favoring the forma- tion of the colloid. To obtain the corresponding colloid hydrate it is only necessary, therefore, to place the proper salt solutions in the dialyzer. From the abundant literature describing these phenomena we may cite the following example of the chemical changes exhibited by iron hydroxide-iron chloride solutions, during dialysis, as observed by S. E. Linder and H. Picton. 1 Table 44 shows the changes in composition of the outer liquid during the process. TABLE 44. CHANGB^IN COMPOSITION OF OUTER LIQUID DURING DIALYSIS OF IRON HYDROXIDE-IRON CHLORIDE SOLUTIONS (According to S. E. Linder and H. Picton) Time of dialysis in hours Relation of Fe to HC1 in outer liquid o 56 : 109.5 24 56 : 137.0 48 56 : 609 . o 120 56 : 1086 .o 168 | Not demonstrable: evident ! Toward the end of the experiment, as can be seen, only HC1 passed through the dialyzing membrane. The changes in composition of the inner liquid during the di- alysis is shown in Table 45. 1 S. E. Linder and H. Picton, Trans. Chem. Soc. Lond., 1909 (1905). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 229 TABLE 45. CHANGES IN COMPOSITION OF INNER LIQUID DURING DIALYSIS OF IRON HYDROXIDE-IRON CHLORIDE SOLUTIONS (According to S. E. Linder and H. Picton) Time of dialysis in Composition in grams per 100 cc. Calculated molecular days weight Fe Cl Formula 5 2303 0.1410 13 Fe(OH) 3 , FeCl 3 1767 n .2300 O.IIIO 2oFe(OH) 3 ,FeCl 3 2302 IO .7200 0.1250 25Fe(OH) 3 ,FeCl 3 2837 17 .5000 0.0773 36 Fe(OH) 3 , FeCl 3 4014 30 .2400 0.0550 42 Fe(OH) 3 ,FeCl 3 4656 37 .1800 o . 0490 45Fe(OH) 3 ,FeCl 3 4977 44 .1400 o . 0460 4 6Fe(OH) 3 ,FeC] 3 5084 61 .0400 o . 0430 4 5 Fe(OH) 3 ,FeCl 3 4977 210 0.6550 0.0150 82Fe(OH) 3 ,FeCl 3 8936 Gels separated out per ! O.OI2C1 i6 2 Fe(OH) 3 ,FeCl 3 17496 after 120 days. gram Fe The table shows plainly the relative increase in iron hydroxide content at the cost of the hydrochloric acid. The formulas of the iron compounds produced and their respective molecular weights, as calculated by Linder and Picton, are also given. The impossi- bility of isolating the compounds, coupled with the fact that they show a progressive change makes the chemical significance of the numbers assigned as molecular weights rather fanciful. On the other hand, they demonstrate very well the progressive transmu- tation to the colloid. The progressive decomposition during dialysis, with formation of colloid in such solutions can be shown in a striking manner, according to N. Sahlbom, 1 by " capillarizing " them, that is, by dipping strips of filter paper into them. If this is done every 24 hours to ferric chloride or ferric nitrate undergoing dialysis, pictures are obtained like those shown in Figs. 50 and 51. At the beginning of dialysis the molecularly dispersed solution ascends the paper without decomposition and concentrates high up, as shown by the dark bands at the tops of the colored columns. After i or 2 days, the upper concentrated salt zone begins to disappear while a sec- ond less-colored one appears below. The latter consists of col- loid iron hydroxide which when first formed is highly dispersed. 1 N. Sahlbom, Kolloidchem. Beihefte, 2, 79 (1910). 230 SPECIAL COLLOID-CHEMISTRY With progressing dialysis, the molecularly dispersed salt dis- appears entirely at the expense of the iron hydroxide, which gradu- ally acquires the properties of a typical, positively charged col- FIG. 50. Dialysis of a ferric chloride solution. (According to N. Sahlbom.) FIG. 51. Dialysis of a ferric nitrate solution. (According to N. Sahlbom.} loid and therefore ascends filter paper little, if at all, as described on p. 16. Finally, a third phenomenon often observed during dialysis MECHANICAL PROPERTIES OF COLLOID SYSTEMS 231 deserves mention. When the separation of the molecularly dis- persed or electrolytic components of a system from the colloid is far advanced, a radical change in the state of the system often occurs. It may coagulate. This fact, which was already observed by Graham, shows that the presence of a certain amount of elec- trolyte is necessary to insure colloid stability. An example of this behavior is offered in Table 45, when the ferric hydroxide has been dialyzed 120 days. 30. Osmosis of Colloid Systems i. General Remarks and Literature. During dialysis, an increase in the volume of the dialyzing liquid, in the interior of the cell is often observed. This is the phenomenon of osmosis, known for a century and a half. 1 Osmotic phenomena take place when- ever a dispersoid is brought in contact with a less-concentrated one or its pure dispersion means, under conditions which do not allow of the "free" diffusion described in 28. This may be accomplished by placing between them a so-called semi- permeable or, better expressed, a selectively permeable membrane, in other words, a device which gives passage to the dispersion means, but not to the dispersed phase. These devices are plainly nothing more than such as were used, for example, in the dialysis of colloid systems, as described in the previous paragraphs. In fact, osmotic phenomena may always be expected to appear during dialysis. Consideration of these osmotic phenomena discloses their close connection with the processes of diffusion and dialysis. Like the latter, osmosis represents an impeded diffusion. Osmosis, like free diffusion, tends toward the establishment of a uniform spatial distribution of dispersed phase and dispersion means. Since, in the presence of a dialyzing membrane, the dispersed phase cannot wander into the pure (or less concentrated) outer dispersion means, the reverse occurs and the pure dispersion means wanders into the dispersed phase. The result of this which represents the re- ciprocal of free diffusion, is an equalization, as far as possible, of the concentration of the dissolved substances in the different parts of the system. The intensity of the tendency to bring about a uniform dis- 1 For a history of the development of our knowledge of osmosis see Wilh. Ostwald, Lehrb. d. allg. Chem., 2 Aufl., 652, Leipzig, 1903. . 232 SPECIAL COLLOID- CHEMISTRY tribution of dispersed phase and dispersion means may be measured by opposing this osmotic leveling process by the hydrostatic pres- sure of a water column. The pressure thus made evident is called the osmotic pressure of the dispersoid. 1 To make osmosis possible it is immaterial whether the selective permeability of the mem- brane is brought about by its sieve-like action, which holds back mechanically the dispersed phase, or by its selective properties as a solvent in the sense that only the dispersion means is soluble in it. 2 Osmotic pressure and osmotic phenomena like Brownian move- ment and diffusion velocity are markedly dependent on the spe- cific surface of the dispersed phase. Colloid solutions, therefore, show but slight osmotic pressures, provided they are not contami- nated with molecular or ionic dispersoids. Most colloids can only with difficulty be rid of these impurities which enter these systems in the process of their preparation or are necessary for their stability. Such traces of impurities introduce great errors into pressure measurements which at the best yield but small values. 3 It cannot, however, be denied that many typical col- loids, especially when of high dispersion, possess some osmotic pressure of their own. This follows as a necessary conclusion from the existence in them of Brownian movement and diffusibility. Measurements of the osmotic pressure of colloids have been made and discussed at special length by W. Pfeffer, 4 H. Picton and S. E. Linder, 5 C. E. Linebarger, 6 E. H. Starling, 7 C. J. Martin, 8 A. Lottermoser, 9 B. Moore, W. H. Parker, H. E. Roaf, L. Adam- 1 In many textbooks, following the lead of W. Nernst, we find it stated that osmotic pressure is the "cause" or "force" producing diffusion. This way of putting it is incorrect as the above remarks on the relation of diffusion to osmosis show and as J. J. van Laar (Vortrage uber d. thermodynam. Potential usw. Braunschweig, 1906) has long emphasized. The concept of osmotic pressure stands and falls with the presence and absence of a selectively permeable membrane. It contradicts every correct view of osmotic pressure to assume its existence in the absence of such a membrane, as in the processes of free diffusion. It is, however, correct to hold that the phenomena of diffusion, of osmosis and of Brownian movement all spring from the same source of energy as clearly evidenced by the close relations and analogies between them. 2 For details regarding such and other properties of membranes see the compre- hensive monograph of H. Zangger, Ergebnisse der Physiologic, 7, 99 (1908). 8 With reference to the view that the admixed electrolytes may constitute integ- ral parts of the colloids see p. 143. 4 W. Pfeffer, Osmotische Untersuchungen, Leipzig, 1877. 5 H. Picton and S. E. Linder, Journ. Chem. Soc., 63, 148 (1892). 6 C. E. Linebarger, Silliman's Am. Journ. Sci., (3), 43, 218, 416 (1892). 7 E. H. Starling, Journ. PhysioL, 19, 312 (1805-6); 24, 317 (1899). 8 C. J. Martin, Journ. PhysioL, 20, 364 (1896). 9 A. Lottermoser, Anorg. Kolloide, Stuttgart, 1901; Z. f. physik. Chem., 60, 451 (1907). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 233 son, D. Bigland, 1 E. W. Reid, 2 J. Duclaux, 3 G. Malfitano, 4 R. S. Lillie, 5 G. Hiifner and Gansser, 6 W. M. Bayliss, 7 W. Biltz and A. von Vegesack 8 and others. Only the more important of their findings can be touched upon here. 2. Methods of Measuring the Osmotic Pressure of Colloids. From what has been said it is clear that any dialyzing apparatus may be used to measure osmotic pressure. As dialyzing mem- branes, the earlier investigators generally used parchment paper. More recently collodion capsules have been employed. C. J. Martin (I.e.) used clay cups impregnated with silicic acid gels; E. H. Starling (I.e.), the same impregnated with gelatine. For details the recent works of W. Biltz and A. von Vegesack should be consulted. Fig. 52, which represents a cell used for osmotic pressure measurements, is taken from their publications. Below is shown the collodion capsule. Of the two vertical tubes, one is used to fill the "osmometer," the other to record the pressure. The greatest source of error in the determination of the osmotic pressure of colloids lies in the disturbing effects of the presence of molecularly dispersed phases, especially electrolytes. Several schemes have been proposed to obviate the difficulty. Different investigators, especially B. Moore (with his collaborators) and J. Duclaux, have maintained that the accompanying electrolytes constitute integral parts of the colloid and are bound to it either chemically (see Duclaux) or at least through "adsorption." In other words, they hold the electrolytes to be essential to the maintenance of the colloid state. When they are removed the colloid is "denatured" and, as has been observed, "polymerized" into coarsely dispersed particles, even to the point of coagula- tion. That all this may occur, as in the case of the albumins, must be admitted, but it cannot be stated as a universal truth. As 1 B. Moore and W. H. Parker, A mer. Journ. Physiol., 7, 261 (1902); B. Moore and H. E. Roaf, Bioch. Journ., 2,34 (igc6); 3, 55 (1907) ; B.Moore and D. Bigland, ibid., 5> 3 2 ( I 99)> H. E. Roaf and L. Adamson, Bioch. Journ., 3, 422 (1908); Journ. Physiol., 39 (1909); Quart. Journ. Physiol., 3, 75, 171 (1910); in part available only in abstract. 2 E. W. Reid, Journ. Physiol., 31, 439 (1904); 33, 12 (1905). 3 J. Duclaux, Compt. rend., 140, 1468, 1544 (1905); Journ. Chim. physique, 5, 40 (1907); i, 407 (1909); see also the review in Koll.-Zeitschr., 3, 126 (1908). 4 G. Malfitano, Compt. rend., 142, 1418 (1906). 5 R. S. Lillie, Amer. Journ. Physiol., 20, 127 (1907). 6 G. Hiifner and Gansser, Engelmann's Arch. f. Physiol. 209 (1907). 7 W. M. Bayliss, Proc. Roy. Soc , 81, 269 (1909); Koll.-Zeitschr., 6, 23 (191). 8 W. Biltz and A. von Vegesack, Z. f. physik. Chem., 68, 357 (1909); 73, 481 (1910). 234 SPECIAL COLLOID-CHEMISTRY R. S. Lillie (I.e.) has emphasized, the presence of electrolytes is not essential to the existence of all metallic hydrosols, and no reason can be assigned at present why one phase cannot be divided into another to the point of colloid dispersion in the entire absence of any electrolyte. These remarks are not intended to deny the existence of colloid-electrolyte complexes. They are only made to emphasize that such discussion does not an- swer the question of what is the value of the os- motic pressure of pure colloids themselves and how it may be measured, for theoretically the col- loids must have some because they show Brownian movement and diffuse. The following measures have been proposed to attain this end. At first sight it would seem most satisfactory to use membranes which per- mit a sharp dialytic separation of colloids and molecular dispersoids. In the course of the dialy- sis the molecular dispersoids would then pass through the membrane while the colloids would remain behind. The end pressure would then be that of the pure colloid. It is well to empha- size, at once, that these final osmotic pressures have almost invariably been found to be very low. A second method consists in taking a limited volume of outer liquid and waiting until an equi- librium has been established between the concen- tration of the electrolytes in this and the con- centration of those contained in the inner liquid. In connection with this method it must be borne in mind that the equilibrium need not in any sense be synonymous with equality of concentration in the two liquids. A whole series of facts, one of which is the difficulty of "washing out" the last traces of electrolytes from precipitates, compels the conclusion that colloids tend to concentrate electrolytes upon themselves 1 and thereby to increase the possibility of de- veloping and exhibiting a greater osmotic pressure than is really due to the colloids themselves. Since such increases in concen- tration depend, as a rule, only on the concentration and not on the absolute amounts of the electrolyte present, they undergo FIG. 52 Os- mpmeter of W. Billz and A. von Vegesack. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 235 progressive variation as osmosis takes place because of the movement of the liquid, and thus further complicate the problem. The following procedure has also been used. After the electro- lyte content of a colloid has been determined by analytical means, an amount is added to the outer liquid to bring its concentration up to that assumed to exist within the colloid. The overplus of osmotic pressure exhibited by the colloid mixture is then re- garded as the osmotic pressure of the colloid itself. To get a proper outer solution the dialysate or outer liquid, rich in electro- lytes, is used against the original mixture, or a proper outer fluid is obtained by a nitration (see the following paragraphs) which separates the electrolyte solution from the colloid (J. Duclaux, I.e.). Finally, the maximal pressures observed in the osmosis of a colloid solution containing electrolytes has been taken as a convenient method of arriving at the osmotic pressure of the colloid itself. As W. Biltz and A. von Vegesack (I.e.) have pointed out, this is the resultant of two processes : of the osmosis directed toward the inner liquid (endosmosis) and of that directed toward the outer (exosmosis), which latter parallels dialysis. These remarks make it clear that the methods for the quanti- tative determination of the osmotic pressure of colloid systems are not as yet worked out entirely. If we do not wish to determine the osmotic pressures of highly purified colloids or their final values to the point of utilizing a microscope to make readings and a micro-osmometer, then employment of a constant volume of outer liquid, with attainment of an equilibrium between the elec- trolytes present in both liquids, seems most expedient. It would, of course, be well to determine also the distribution of the electrolytes between colloid and pure dispersion means, in order to work out from the obtained values a proper equilibrium curve 1 from which might then be exterpolated the osmotic pres- sure of the colloid when the concentration of the electrolyte equals zero. 3. Instability of Osmotic Pressure of Colloids. One of the first things to be noticed when the osmotic pressures of colloids are measured, even though every effort is made to keep constant 1 This would undoubtedly take the form of the adsorption isotherms. 2 3 6 SPECIAL COLLOID-CHEMISTRY all external conditions, is their inconstancy. Not only do prepa- rations of one and the same substance, prepared by different methods, show different osmotic pressures, but shaking, stirring, standing, etc., all cause considerable change in them. The follow- ing examples illustrate this behavior. TABLE 46. INFLUENCE OF PREVIOUS TREATMENT ON OSMOTIC PRESSURE OF ALBUMIN (According to E. W. Reid) Previous treatment Ash. per cent. Osmotic pressure of a I per cent, solution in mm., Hg. Ovalbumin, 4wice crystallized and once washed. . Ovalbumin, washed repeatedly 0.120 o 267 3.38 O OO Ovalbumin, precipitated and once washed O. 312 4.82 The same O 22O 1^.71 Precipitated bovine serum-albumin, repeatedly washed. The same, once washed 0.633 o 461 o.oo 4.2O These experiments of E. W. Reid (I.e.) show that the osmotic pressure of one and the same substance (egg-albumin) varies at the same concentration between the values zero and 15.71 mm. of mercury. They also betray the important fact that the ash content of a colloid is not fundamentally responsible for the value of its osmotic pressure. The osmotic pressure of a preparation hav- ing the greatest ash content is zero, for example. The following example, taken from R. S. Lillie (I.e.) is intro- duced to illustrate the influence of shaking. TABLE 47. INFLUENCE OF SHAKING ON OSMOTIC PRESSURE OF GELATINE AND OF EGG-ALBUMIN (According to R. S. Lillie) 1. 25 per cent, gelatine Pressure in mm., Hg. 1.6 per cent, egg-albumin -inri SU ?Ii? n Pure gelatine. . ' 42 Pure albumin 32.1 Pure gelatine shaken i 5-3 Pure albumin shaken. ... 31.3 Gelatine + *? NaCl 48 2.6 Albumin -f- NaCl 9.0 45 Gelatine + ~ NaCl, shaken 2.9 Albumin + ^ NaCl, 8.8 48 48 shaken Gelatine + Na 2 SO 4 2.4 Albumin + Nal 1 8.9 48 Gelatine + m Na 2 SO 4 2.6 Albumin + ^ Nal, 6.6 shaken 48 . . shaken MECHANICAL PROPERTIES OF COLLOID SYSTEMS 2 37 It is a remarkable fact that while the osmotic pressure of gelatine is increased by shaking, that of egg-albumin is decreased. Table 48 illustrates the influence of stirring on the osmotic pressure of colloid solutions. TABLE 48. INFLUENCE OF STIRRING ON OSMOTIC PRESSURE OF BENZOPUR- PURIN SOLUTIONS (According to W. Biltz and A. von Vegesack) A. Benzopurpurin low in electrolytes B. Benzopurpurin high in electrolytes Height of Height of Hours fluid Remarks Hours fluid Remarks column column 1 .0 9.41 Not stirred 5 1.22 Stirred. 2-5 9.62 Not stirred 15 1.25 Not stirred. 3-5 9-So Not stirred 18 1-34 Stirred. 4-5 9-68 Stirred 5 min 378 1.30 Stirred i hr. daily. 5.0 9.86 Stirred 5 min 5-5 10-07 Stirred 5 min 426 1.24 Stirred 7 hrs. pre- 6.0 10.18 Stirred 5 min viously. 7.0 10.40 Stirred 5 min 8.0 10.60 Stirred 5 min 4So 1.26 Stirred 7 hrs. pre- 9.0 10.64 Stirred 5 min viously. 10. o 10.66 Stirred 5 min 20. o 8.16 Not stirred 20.5 8.37 Stirred 5 min 21.0 8.92 Stirred 5 min o 21-5 9.08 Stirred 5 min IOO 1.14 Stirred during day. 22.0 9.16 Stirred 5 min 121 1.19 Stirred 6 hrs. 23.0 9.29 Stirred 5 min 145 1.09 Stirred 6 hrs. 24.0 8.98 Stirred 5 min I6 7 1. 10 Stirred 6 hrs. 25.0 8.99 Stirred 5 min 28.0 7.16 | Not stirred. 28 . 5 7 . 45 Stirred 5 min This table shows an increase in osmotic pressure with every stirring, even though the effect is but transitory. The increase occurred three times in the data given. It is also apparent that solutions containing small amounts of electrolytes are more sensi- tive to this influence than those richer in these which are scarcely affected. Gelatine behaves similarly, as shown in Table 47. In discussing the influence of time upon the osmotic pressure of colloids we need to distinguish between its variations when a colloid is simply left to itself in an osmometer and its variations if the same colloid is measured at different periods. The first 238 SPECIAL COLLOID-CHEMISTRY relation is evidenced in the left-hand column of Table 48. This benzopurpurin showed a rise to 1.21 cm. after 310 hours; while the capillary rise in a similar tube amounted to 1.12 cm. The osmotic pressure was therefore 0.09 cm. In illustration of the influence of age upon the solutions, these authors found a dialyzed solution of 0.00103 normal night-blue to yield a maximum osmotic pressure of 15.52 cm. of water after 2 days; after 6 days, 4.24 cm.; and after n days, 4.08 cm. When we survey these facts we are struck by the great in- constancy of the osmotic pressure of colloids as compared with that of molecularly dispersed solutions. The osmotic pressure of colloids is variable, being greatly modified by mechanical treat- ment, age, etc. Such sensitiveness is unknown in molecular dis- persoids. It is true, of course, that the experiments of W. Spring 1 have shown that even ordinary salt solutions, for example, are not absolutely stable in their conductivity, their optical properties, etc., but these variations are very small when compared with those exhibited by colloids. The reasons for this great variability are to be sought in the changes of state of colloids, such as varia- tions in their degrees of dispersion, states of aggregation, etc., for which many different causes may be responsible, as will be dis- cussed later. The osmotic pressure of colloids, more especially of emulsoids, varies therefore as does their viscosity. 4. Influence of Concentration on Osmotic Pressure of Colloids. The osmotic pressure of molecular dispersoids, as is well known, is governed by the important law of Pfeffer-van't Hoff : the osmotic pressure is directly proportional to the concentration. The rela- tions in colloid systems are not so simple. Examples are known, in which the law holds approximately, but there are also those in which the osmotic pressure increases faster than the concentration, or more slowly than this. Perhaps nothing better demonstrates the inappropriateness of applying without due consideration, the "solution laws" valid for molecularly dispersed systems to colloid systems, than this variability of the concentration function of the osmotic pressure of colloids. The following findings of W. Biltz and A. von Vegesack (I.e.) on purified congo red may serve to illustrate the first of the three 1 W. Spring, Koll.-Zeitschr., 7, 22 (1910), where references to earlier papers on this subject may be found. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 239 possibilities, namely, that wherein concentration and osmotic pressure are approximately proportional. W. M. Bayliss (Lc.) also noted this proportionality in concentrations ranging from 0.07 to i per cent, by weight. TABLE 49. RELATION OF OSMOTIC PRESSURE TO CONCENTRATION IN DIALYZED CONGO RED SOLUTIONS (According to W. Biltz and A. von Vegesack) Concentration C Osmotic pressure in cm. P ^r = const. 0.539 norm. 4.15 cm. 0.770 i. 08 8.15 cm. 0-755 1-44 10.24 cm - 0.695 i. 80 14.00 cm. 0.778 2.155 14.62 cm. 0.678 2.87 18.70 cm. 0.652 3.23 21.55 cm - 0.667 3-59 25 . 04 cm. 0.698 4-31 25.30 cm. 0.587 The constants are all of about the same order of magnitude. Gum arabic behaves similarly according to W. Pfeffer (Lc.). TABLE 50. OSMOTIC PRESSURE OF GUM ARABIC IN DIFFERENT CONCENTRATIONS (According to W. Pfeffer) Concentration Osmotic pressure in cm., Hg. P P C i per cent. 6-59 6.9 6 25-9 4.3 14 70.0 5-0 8 119.0 6.6 The observations of J. Duclaux (I.e.) on the same substance and given in Table 51 should be compared with these. Some illustrations of how the osmotic pressure may increase more rapidly than the concentration are given in Table 51. P As readily apparent, the relation increases greatly with rising o concentration. This is altogether different from the behavior of molecular dispersoids, in which, so far as known, the opposite occurs as the concentration rises. That the experimental methods 240 SPECIAL COLLOID-CHEMISTRY * w s M - - Q to ON 10 ^j . vo (- M g O O <> |. ^ +H ^ j_ ^ t, M H CS (M CO ^ C^ | o . -a O 1 a, B ON 10 ^- I jf pj y OH t^ 0* M 3| en 9 4) O f t>- PH O G 3 i| S vo co OO to O t^ > co g "3 1 O C^ ^f" vo ON O H 2 w i^3 erf o 2 1 - to vo tv. o O co g CM to ON t> t^ O O O O M o rj- ill ^ tiorium o O vo t>- to O co III m ."2 d 1 1 H cu O O O M w 4 l> o g C/J o " ^^ i-M ^ w . vo 00 00 O H W 10 +J O 413 PN Q-. a I IS SCO to t^. O . 2 A fe M S *Q PH M o | rt- t- t^ t-. -<4- oo co to M to M M IO 00 M +j H O H M d d W ^, o 3 1 1 I? v 5 5 ra 8 . O to g FIG. 53. Relation between concentration in colloid systems and the quotient of the osmotic pressure and concentration. sure of albumin, hemoglobin and congo red varies rectilinearly with the temperature, in other words, directly with the absolute temperature. This statement would make Gay-Lussac's law valid for these solutions. The findings of B. Moore and Roaf (I.e.), J. Duclaux (l.c.), W. Biltz and A. von Vegesack (I.e.), 1 The measurements of W. Pfeffer on gum arabic, given in Table 50, also show a minimum value for the quotient * MECHANICAL PROPERTIES OF COLLOID SYSTEMS 243 etc., contradict this. Moore and Roaf found the osmotic pressure of gelatine solutions to increase considerably faster than the absolute temperature. Technical night-blue solutions show an analogous behavior, according to the figures of W. Biltz and A. von Vegesack, contained in Table 53. TABLE 53. INFLUENCE OF TEMPERATURE ON OSMOTIC PRESSURE OF A 3.49 NORMAL SOLUTION OF TECHNICAL NIGHT-BLUE (According to W. Biltz and A. von Vegesack) Temperature, t r Osmotic pressure in cm. P 273 6.21 O.O22 25 298 10.81* 0.036 50 323 13-83 0.043 70 343 17.69 0.050 * Average of two experiments. J. Duclaux has observed the opposite to be true of iron hydrox- idesol. In this, the osmotic pressure decreases not only rela- tively, but even absolutely, with rising temperature. There exists no analogue for this in the field of molecular dispersions. Duclaux found the following: Temperature 2 (275) 25 (298) 70 (343) Osmotic pressure (cm.) 22.9 21.3 20.9 P j, 0.083 0.071 0.061 Figure 54 shows graphically how differently the osmotic pres- sure of different colloids varies with changes in the temperature. The dotted line represents the ideal case in which there exists simple proportionality between the two as is the case, at least approximately, in molecular dispersoids. It should now be pointed out that B. Moore and Roaf (I.e.) and R. S. Lillie (I.e.) observed interesting thermal after-e/ects or so-called hysteresis phenomena in gelatine solutions. Thus gelatine solution which has been heated continues to show a higher osmotic pressure for some time after cooling than when kept continuously at the lower temperature. The following Table 54 taken from R. S. Lillie illustrates this. It also shows that the differences first noted between the previously cooled and the pre- viously warmed gelatine become less with time. 244 SPECIAL COLLOID-CHEMISTRY TABLE 54. INFLUENCE or THERMAL HISTORY ON OSMOTIC PRESSURE OF i PER CENT. GELATINE (According to R. S. Lillie) Osmotic Pressure at Room Temperature in Mm. Hg. Age of the solutions | Previously chilled on ice Previously warmed to 65-70 i i day 5-0 6.4 5 days 5-0 5-3 2 days 4.9 (chilled 6.0 for long time previously) i day S-7 6.2 i day 5.6 6.0 Iron hydroxide sol Niqhh blue Temperature 25 50 70 FIG. 54. Relation between the temperature of colloids and the quotient of osmotic pressure and absolute temperature. This behavior also is unknown among the molecular dis- persoids. 6. Influence of Added Substances on Osmotic Pressure of Colloids. The influence of added substances upon the osmotic pressure of a given system, is, according to the classic theory of molecularly dispersed solutions, purely additive. In other words, the pressure exerted by the added substance is added to that of the original system. There exist exceptions to this rule, of course, and usually in the sense that the calculated osmotic pressures are found to be greater than those actually observed. The effect of added substances on the osmotic pressure of colloid systems is more complicated. Under this heading also, MECHANICAL PROPERTIES OF COLLOID SYSTEMS 245 concentration and temperature functions are encountered which not only do not correspond with any observed among molecular dispersoids, but which among themselves show great differences. The influence of added substances may be studied by adding them in equal concentration to both the inner and the outer liquid. The important experiments of R. S. Lillie (I.e.) were carried out in this way. Acids and alkalies may either increase or decrease the osmotic pressure of different colloids. Sometimes one and the same colloid may. show both types of behavior. Often very small quantities of hydrogen or hydroxyl ions are sufficient to cause noticeable effects. W. M. Bayliss (I.e.) found the osmotic pres- sure of very pure (and highly dispersed) congo red to fall from 207 mm. to 120 mm. when the outer water (conductivity water) surrounding his osmometer was replaced by the same water satu- rated with carbon dioxide. The stronger acids produce, of course, still more marked effects. The addition of alkali in- creases the osmotic pressure until a maximum is reached, beyond which it falls again. Table 55 gives a part of R. S. Lillie' s find- ings on gelatine. 1 TABLE 55. INFLUENCE OF ACIDS AND ALKALIES ON OSMOTIC PRESSURE OF 1.5 PER CENT. GELATINE (According to R. S. Lillie) Influence of HC1 Influence of KOH Concentration Osmotic pressure in mm. Hg. Concentration Osmotic pressure in mm. Hg. o 8.2 o 7-9 0/3100 HC1 6.8 n/3ioo KOH I4.I 11/2050 12.3 n/62o . 23.7 n/i5So 17.9 n/4i2 25.1 n/IO24 26.5 n/3io 2Q.O 0/770 32.4 0/620 34-9 n/4i2 39-3 As can be seen, low concentrations of acid lead to a slight but definite minimum of osmotic pressure. With higher concentra- 1 See also the analogous findings of H. E. Roaf (I.e.) on hemoglobin. 246 SPECIAL COLLOID-CHEMISTRY tions, there occurs a sharp increase in osmotic pressure which rises steadily for a time with increasing concentration. R. S. Lillie thinks it probable that beyond a certain point a decrease in os- motic pressure would again occur. Within the concentration range studied, alkalies led only to an increase in osmotic pressure. Figure 55 shows graphically this variation of the osmotic 0.00/1 .0020 Concen IraHon normal FIG. 55. Effect of acid and alkali upon the osmotic pressure of a 1.5 per cent, gelatine solution. (According to experiments by R. S. Lillie.) pressure of gelatine solutions with the concentration of the added acids and bases. Contrary to the findings in the case of gelatine, the osmotic pressure of egg albumin is always lessened by the addition of hydrogen or hydroxyl ions. Table 56 shows this. In the case of the acids a definite minimum again appears. The type of curve, at least for albumin, is therefore not so funda- mentally different from that for acid gelatine. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 247 TABLE 56. INFLUENCE OF ACIDS AND ALKALIES ON OSMOTIC PRESSURE OP 1.5 PER CENT. EGG ALBI MIN (According to R. S. Lillie) HC1 KOH Concentration Osmotic pressure in mm. Hg. Concentration Osmotic pressure in mm. Hg. 25.6 25.6 0/3100 HC1 20.7 n/3ioo KOH 24.1 n/I24O ii. S j 0/1240 22.6 n/62o 14.1 n/62o 20.2 0/412 20.4 n/4i2 18.0 n/sio 22.2 n/3io 17.9 0.0'09 .0)8 .0^6 normal Concenl-naHon * FIG. 56. Effect of acid and alkali on the swelling of gelatine plates. (According to experiments by Wo. Ostwald.) To illustrate the varied influence of salts the following examples may be given. Technical night-blue contains a considerable ad- 248 SPECIAL COLLOID-CHEMISTRY mixture of electrolytes. If they are removed by dialysis its osmotic pressure increases, as shown in the following table. The behavior of gelatine and albumin toward added salts has Acid 0.01 .02 .03 norms/ 'Acid Alkali 0.001 .002 Concentration 003 normal FIG. 57. Relation'between internal friction (upper figure) and osmotic pressure (lower figure) in albumin solutions when acid or alkali is added in different concentra- tions. (From experiments by Wo Pauli and his coworkers and R. S. Little.') Only the percentage increase in viscosity and not its absolute value could be given in the upper figure. been extensively studied (B. Moore and coworkers, R. S. Lillie, etc.). The following general truths are taken from the findings of R. S. Lillie: MECHANICAL PROPERTIES OF COLLOID SYSTEMS 249 TABLE 57. OSMOTIC PRESSURE OF PURIFIED NIGHT-BLUE AND OF NIGHT-BLUE CONTAINING ELECTROLYTES (According to W. Biltz and A. von Vegesack) Purified colloid Colloid containing electrolytes Concentration Osmotic pressure in cm. Concentration Osmotic pressure in cm. 1.30 5-8l 1.20 4.72 1.74 12.70 I.S8 S.IO 2.17 16.64 1. 60 S.3I 2.6l 21.99 1.96 6.24 3-04 20.24 2.36 7.90 3.QI 2S-32 2-73 9.42 4-34 32.18 3-49 11.19 5-21 37-24 5.76 14.10 6.08 43-94 6.12 20. 8l W. M. Bayliss (I.e.) obtained analogous results for congo red. The addition of salts always causes a decrease in the osmotic pressure of these colloids. The degree of this decrease varies with the concentration and with the nature of the anion and cation. Generally speaking, the neutral salts of the alkali metals cause the smallest decrease. The salts of the alkaline earths are more effective and those of the heavy metals most effective of all, though they vary considerably among themselves. With salts having a common cation the order of the anions, when that most effective is given first, is about as follows: S0 4 >Cl>N0 3 >Br>I>CNS 1 The cations similarly arranged follow the order: heavy metals > alkaline earths > alkali metals. Table 58 details some of the actual experimental findings. If the validity of the above-mentioned conclusions is to be tested, the data of the original papers must be consulted for the experi- ments differ considerably among themselves. Figures 58 and 59 also show the complicated effects of the con- centration of the added salts upon the osmotic pressure. The original paper (I.e., p. in) must be consulted for the detailed data upon which these figures are based. 1 For similar findings on hemoglobin see the work of H. E. Roaf (I.e.). 2 5 SPECIAL COLLOID-CHEMISTRY TABLE 58. INFLUENCE OF SALTS ON OSMOTIC PRESSURE OF COLLOIDS (According to R. S. Lillie) Salts of the alkalies Salts of the alkaline earths 1.25 per cent, albumin i. 25 per cent, gelatine i. 2 5 per cent, albumin 1.25 per cent, gelatine 5 a .5 .3 g a S g s a S k 1 S3 d 1 S3 1 M o g Is" o o o ti o O o O 21.6 o 7.9 O 21.5 5.9 m/24 NaCl S-o m/2 4 KC1 3.3 m/96 MgCl 2 7-3 m/96 MgCl 2 3.2 m/24 NaBr 4.6 m/24 KBr 3-7 111/96 CaCl 2 7-6 m/96 CaCl 2 2.7 m/24 Nal 4.0 m/24 KI 3.7 m/96 SrCl 2 7.2 m/96 SrCl 2 3.1 m/24 NaNO 3 4-8 m/24 KNO 3 3-5 m/96 BaCl 2 7.6 m/96 BaCl 2 2.7 m/24 NaCNS S-3 m/2 4 KC1O 3 3-7 m/24 Na 2 SO 4 4.0 m/24 KBrO 3 3-6 m/24 KCNS 3-75 m/24 K 2 SO 4 2.9 m/24 KCOOCH 3 3-4 m/24 K 2 C 2 O 4 3-4 Salts of the heavy metals Influence of different cations with common anion 1.25 per cent, albumin 1.25 per cent, gelatine 1.25 percent, albumin 1.25 per cent, albumin c c .H .9 a i a a 5 c 3 o (D .2 o I S I* S a PVI on a y s w Sif 8 |fi N 0) 6 6s 3 6 s 6 l d I s 21.5 5-4 o 20.8 o 5.4 m/96 MnCl 2 6. 9 m/i92 CoCl 2 2.0 m/48 LiCl 5.4 m/ 4 8 LiCl 2.9 m/96 CoCl 2 * 5-6 m/i92 CuCl 2 3-3 m/ 4 8 NaCl S-6 m/ 4 8 NaCl 2.6- m/96 CdCl 2 * 4.1 m/48 KC1 5-9 m/48 KC1 2.4 m/96 Pb- 2.8 m/ 4 8NH 4 Cl 4-5 m/ 4 8 NH 4 C1 2.6 (N0 3 ) 2 * m/96 CuCl 2 * 1.6 * A precipitate is formed. It is interesting to compare the behavior of the two colloids toward the same added substance. While the salts of the alkali metals produce about the same effect upon both (the sulpho- MECHANICAL PROPERTIES OF COLLOID SYSTEMS 251 cyanate having the least effect, the sulphate the greatest) almost opposite effects are produced on albumin and gelatine when other salts are used. Among the alkaline earths, SrCl 2 and MgCl 2 produce a greater effect on albumin than CaCl 2 or BaCl 2 . When gelatine is used the reverse is the case. Of the salts of the heavy metals, CuCl 2 affects albumin more than CoCl 2 . The opposite is true for gelatine. Such contrary effects are not so evident when the cations are compared. On the basis of the investigations of S. Posternak, 1 Wo. Pauli 2 A/a Acetate NaBr m /96 Concentration FIG. 58. Effect of salts upon the osmotic pressure of gelatine. (According to R. S. Lillie.) and R. Hober, 3 we are, no doubt, correct in referring these dif- ferences in behavior to the differences in the reaction of the two colloids. Fresh (native) albumin, such as R. S. Lillie used, has a slightly alkaline reaction, while commercial gelatine is always acid. The differences in the effects of an added salt upon an "acid" or an "alkaline" albumin so far as its internal friction was concerned was discussed in 25. It is of much interest that the osmotic pressure of colloid systems should also be so greatly dependent on the acid or alkaline reaction of the colloid. 1 S. Posternak, Ann. de PInst.-Pasteur, 15, 85 (1909). 2 Wo. Pauli, Hofmeister's Beitr., 5, 27 (1903). 3 R. Hober, Hofmeister's Beitr., n, 35 (1907). 252 SPECIAL COLLOID-CHEMISTRY The influence of electrolytes on the osmotic pressure *of colloids may show hysteresis. The after-effects of temperature were discussed on p. 243. If, in the osmotic study of a gelatine + acid mixture, the outer liquid is replaced by distilled water, the pressure column gradually sinks. But to attain its original level ConcenhnaHon FIG. 59. Effect of salts upon the osmotic pressure of albumin. (According to R. S. Lillie.) requires days, and maybe weeks, before the osmotic pressure of the pure gelatine is again reached, even when the acid which dialyzes out very rapidly, is constantly removed by frequent changes of the water (R. S. Lillie). Such lagging before equilib- rium is finally attained is unknown in the osmosis of molecular dispersoids. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 253 Our knowledge of the influence of non-electrolytes on the osmotic pressure of colloids is still limited. An investigation of this question would doubtless bring out many interesting facts. Table 59 reproduces some of R. S. Lillie's (I.e.) results, in which but small differences of both a positive and a negative nature appear. Obviously, higher concentrations of alcohol, acetone, etc., might cause a decided decrease in the osmotic pressure of these colloids. TABLE 59. INFLUENCE OF NON-ELECTROLYTES ON OSMOTIC PRESSURE OF COLLOIDS (According to R. S. Lillie) Egg Albumin 1 . 25 pe r cent. 1.6 pe r cent. Added substance Osmotic pressure in mm., Hg. Added substance Osmotic pressure in mm., Hg. o m/6 cane sugar m/6 dextrose 22.4 21-5 21.8 O m/6 glycerine m/6 urea 29.4 29-5 27.9 Gelatine 1.25 per cent. o 6.2 o 5.5 m/6 cane sugar 6.6 m/6 dextrose 5-7 m/6 dextrose 5-8 m/6 glycerine 5-6 m/6 glycerine 5-9 m/6 urea 6.6 m/6 urea 7-3 7. On the Theory of Osmotic Pressure of Colloids. In the classic theory of osmosis in molecularly dispersed systems, as formulated by J. H. van't Hoff, on the basis of W. Pfeffer's (I.e.) experiments, the absolute concentration, in other words, the number of molecules in the unit volume alone determines the amount of the osmotic pressure (at constant temperature). The osmotic pressure is directly proportional to the number of molecules and to the absolute temperature. Sv. Arrhenius assumed a dissocia- tion of the molecules into ions, in the case of the electrolytes in which a gram molecule in the unit volume shows a higher osmotic pressure than that calculated. On the other hand, when unex- 254 SPECIAL COLLOID-CHEMISTRY pectedly low osmotic pressures were observed, as in high con- centrations of different substances, it was held that there occurred association, polymerization, etc., of single molecules to larger aggregates, or that the dissolved substances combined with the dispersion means to form solvates, etc. But whatever the ir- regularities observed, they were uniformly reduced to either an increase or a decrease in the number of particles actually present in the unit volume as compared with their calculated number. The number of particles has, in other words, in this classic theory of solution, been regarded as the most important if not the sole variable. When the osmotic phenomena of dispersed systems are viewed in a more general way, especially in connection with other forms of movement, as Brownian movement and diffusion, it becomes evident that several other variables, not considered in the classic theory of osmosis, play an important part. They are the degree of dispersion and the type of the dispersed phase, together with such associated properties, as degree of hydration, etc. It makes no difference in the classic theory of osmosis what is the size of the dispersed particles, or whether we deal with molecularly and ionically dispersed phases or with coarse dispersions. Nor does the type of the dispersed phase matter, or its degree of hydration, except in so far as through hydration a portion of the solvent may be withdrawn, thereby causing an increase in the molar concen- tration. With any given substance in a given dispersion means, each particle, no matter what its type or size, behaves like a molecule, and if N particles (Avogadro's number) are present in the unit volume, the system will exert unit osmotic pressure. It is evident that we may not thus assume the independence of osmotic pressure, say of the degree of dispersion, when we come to deal with systems which have not a maximal degree of it, as in colloid solutions. To do so would be to deny the importance of the relations between osmosis, diffusion and Brownian movement. We cannot ascribe the small pressures exhibited by colloids to a low " molar" concentration of the colloid phase. Just as certainly as highly dispersed phases possess a greater Brownian movement and a higher diffusion coefficient, even independently of their concentration, equally certainly must they show a greater osmotic pressure than less dispersed ones, other conditions being equal. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 255 Only a theory of osmotic phenomena that considers the degree of dispersion of a system, in addition to concentration and tem- perature, can prove universally valid for all dispersed systems. It is not difficult to bring experimental proof for such theo- retical deductions. In fact, no one who tries to account for the great sensitiveness of the osmotic pressure of colloids to different influences, can escape considering changes in those characteristic variables of colloids, namely, their degree of dispersion and their type, as responsible for it. The variations in the osmotic pressure must be explained by the same kind of changes by which we explain, for example, the variations in their internal friction, namely, " changes in state." The influence of degree of dispersion upon osmotic pressure is very evident in congo red. W. M. Bayliss (I.e.) prepared a pure and highly dispersed congo red by allowing NaOH to diffuse into its free acid contained in an osmometer. 1 While the free acid is pronouncedly colloid, as betrayed by the fact that it is readily analyzable ultramicroscopically, congo red prepared in the manner described, cannot be thus analyzed. But it can be as soon as traces of electrolytes are added. Even the carbonic acid of the air suffices to do this. At the same time, the osmotic pressure of the system decreases. All factors which cause a decrease in degree of dispersion, as the addition of electrolytes, ageing, shak- ing, etc., decrease the osmotic pressure. Other factors which increase the osmotic pressure, as the addition of alkalies, also make the ultramicroscopically heterogeneous structure give way to an optically homogeneous one. The fact observed by J. Duclaux 2 that the osmotic pressure of a red gold hydrosol is considerably greater than that of a blue one also belongs here. We have every reason for believing that blue gold sols are not as highly dispersed as red ones. The view advanced here that changes in the state of a colloid, more especially variations in its degree of dispersion and its type, are of particular significance in determining its osmotic pressure, is perhaps most clearly demonstrated by the close analogies between the osmotic phenomena exhibited by colloids and their internal 1 The similar behavior of freshly prepared silicic acid is discussed on p. 227. * J. Duclaux, Compt. rend., 148, 295 (1909); for a description of the special method used by this author in determining the osmotic pressure see this paper and Koll.- Zeitschr., 3, 134 (1908). 256 SPECIAL COLLOID-CHEMISTRY friction and swelling. The close relationship between these pro- cesses is brought out not only by emphasizing that age, previous thermal history and mechanical treatment affect all of them in the same general way, but by the fact that they do this often down to the minutest details. This is clearly apparent when we compare the influence of acids and alkalies on the osmotic pressure (R. S. Lillie) with their effect upon the internal friction (Wo. Pauli, etc.). Still more striking, perhaps, is a comparison of the effects of acids and alkalies on the osmotic pressure of 1.25 per cent, gelatine solutions (R. S. Lillie) with those of these same substances on the swelling of gelatine discs (Wo. Ostwald). 1 Here the agreement is perfect even to details (see Figs. 55, 56, pp. 246, 247). 2 In connection with these facts the influence of added substances on the viscosity of gelatine solutions, as given on p. 169, should also be studied. As a matter of fact, the relation between osmosis and swelling is close even when the question is viewed from a theoretical standpoint. In the place of a selectively permeable membrane, we have the structure of the material undergoing swelling which hinders the movement of the dispersed phase into the dispersion or swelling means. The process leading to the highest attainable homogeneous (spatial) distribution of swelling substance and swelling producing medium, is possible only if the structure and the specific surface of the swelling body change simultaneously, while the spatial relationship of the two phases to each other remains. If this relationship is destroyed, as by increase of temperature above a critical value, then instead of swelling, solution occurs. Besides these analogies between the osmosis and the swelling of colloids (as well as between osmotic and swelling pressures), characteristic differences also exist between them. In the process of swelling, a radical change in state, namely, an increase in degree of dispersion takes place. In the osmotic processes of molecularly dispersed systems, the specific surfaces, 1 Wo. Ostwald, Pfliiger's Arch., 108, 563 (1905). 2 According to R. S. Lillie the acid minimum is about one-tenth that found by Wo. Ostwald in his experiments on swelling. But since the latter minimum is practically identical with that of the viscosity maximum of dilute gelatine solutions as found by P. von Schroeder (p. 169) and agrees fully with the acid maximum for albumin solutions (see H. Handovsky, Koll.-Zeitschr., 7, 192, 1910) Lillie's figure evidently represents an error either in measurement or calculation. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 257 etc., of the dispersed particles remain constant and only the re- lation of number of particles to unit volume changes. But when colloid systems are under discussion the processes of swelling and osmosis again agree; in fact, the osmosis of colloid solutions might well be termed a "swelling of liquids" in contrast to the usual swelling of solids. In considering the enormous effect of acids and alkalies on the osmotic pressure of colloids one might try to save the classic con- ception of osmosis by assuming an increase in the molecular con- centration of the albumin particles, caused, say, by hydrolytic cleavage. But examination of this idea leads to an exactly opposite conclusion, for, as Wo. Pauli 1 has emphasized, and as St. Burgarsky and L. Liebermann first showed, the observed freezing points of mixtures of acid and alkali with albumin are not as low as those obtained by adding together the effects which albumin and the added substance produce alone. A decrease in the molar concentration therefore occurs, either by chemical or adsorptive union of albumin with electrolytes. The addition of acids and alkalies as emphasized in the discussion of viscosity on p. 173 leads to the formation of a larger number of albumin ions which are capable of holding more water than the neutral albumin particles. The emulsoid properties of the system, originally relatively low, are, therefore, greatly increased, as be- trayed, for example, by the rise in its internal friction, indiffer- ence toward salt, etc. The remarkable effects of concentration and of temperature on the osmotic pressure of a colloid will some day, no doubt, be similarly explained through the changes in the state of the colloid produced by them. It need but be recalled that the degree of dispersion and the type of the dispersed phase are, at times, a function of the concentration and the temperature as discussed on p. 35. When the degree of dispersion decreases with rise in concentration, as in soap solutions, then the (relative) osmotic pressure must decrease. Actually this is found to be true not only for soap solutions but also for hemoglobin (see Table 52 on p. 241). Analogous considerations hold for the effects of temperature on the osmotic pressure of different colloid systems. 1 Wo. Pauli, Pfliiger's Arch. (1910). Festschr. f. E. Hering. Prof. Pauli was kind enough to place the proof sheets of this article at my disposal. 17 258 SPECIAL COLLOID-CHEMISTRY The many and complicated possibilities for great variations in behavior, especially among the emulsoids belonging to the number of the complex dispersoids, may be foreseen, especially when the additional variations which may be introduced through changes in the electrical properties are kept in mind. The suspensoids, which assume but one form, will show a simpler behavior. That this is so is borne out by the observations on dyes of the suspen- soid type, as congo red, benzopurpurin, etc., as studied by W. M. Bayliss (I.e.), W. Biltz, A. von Vegesack (I.e.) and others. The problem of the future is more the problem of analyzing the type of these various colloid changes than that of settling whether or not the observed peculiarities can be explained on the basis of the classic theory of osmosis. In a word, then, the osmotic pressure of most colloids is by no means only a function of the number of particles in the unit volume, but varies with the changes in the state of these sys- tems, more especially with the changes in the degree of dispersion and the type of the dispersed phase. The value of the osmotic pressure is therefore a more complex function in the case of colloids than in molecularly dispersed systems, and may not off- hand be made identical with the latter. In fact it seems im- possible, for these reasons, to assign absolute values to the osmotic pressure of colloidally dispersed systems. This is true of all emulsoid and complex dispersoids, while simpler relations, re- sembling those valid for the molecularly dispersed systems, seem to exist in the case of suspensoid systems (see the succeeding paragraphs). Perhaps future investigators will find it best to reserve the concept of osmosis for molecular dispersoids and to use another term like hydration (solvation) for the phenomena observed in colloid and coarsely dispersed systems. Such a term would constantly bring to mind the important difference be- tween the two kinds of phenomena. 8. Determination of the "Molecular Weight" of Colloid Sys- tems by Osmotic Means. As is well known, the molecular weight of a dissolved substance may be determined from the osmotic pres- sure of a molecularly dispersed solution, by the following formula : . M = (22.4 X 760)- , P-J- o MECHANICAL PROPERTIES OF COLLOID SYSTEMS 259 in which M represents the molecular weight sought, 22.4 the " normal" osmotic pressure of a gram-molecule of the molecularly dispersed substance at o, c the concentration (in per cent.), p the observed osmotic pressure in mm. of Hg., TI, the observed absolute temperature, and TQ, 273. Since J. H. van't Hoff first formulated this law the different investigators who have measured the osmotic pressure of colloids have, also, in good part tried to deduce therefrom their "molecular weight." Indeed, there exist but few publications on the osmotic pressure of colloids in which there is not a column devoted to their " molecular weight" as calculated from the osmotic pressures. Many examples could be given of this. The striking thing about these "molecular weights" of colloid systems is their great absolute value and their great variability under different conditions. The former seems obvious enough in view of the low values found for the osmotic pressure of colloids of even simple chemical composition. The second, however, accord- ing to which the molecular weight varies under different cir- cumstances is, strictly speaking, a contradiction in terms, for by definition, the molecular weight is a constant. But disparities between the molecular weights of substances as deduced from osmotic measurements and from analysis, have been observed in molecularly dispersed systems also. In other words, the simple proportion between osmotic pressure and concentration, as demanded by theory, has not always been observed to hold even here. Thus W. M. Bayliss (I.e., 1910) cites the fact that the molecular weight of alcohol dissolved in benzene rises from 50 to 208, is quadrupled, in other words, in passing from a con- centration of 0.494 per cent, to one of 14.63 per cent. It must be left to the students of the molecularly dispersed solutions to inter- pret these contradictions between their fundamental equation and its applications. But so far as the colloids are concerned, such a calculation of molecular weight from osmotic measurements can never be attempted with safety because it is wrong in principle. Not even the sense of the variations in the osmotic pressure of colloid solutions and their concentration need be the same in all cases. On p. 240 it was pointed out that, according to J. Duclaux, the osmotic pressure of iron hydroxidesol increases more rapidly than its concentration. Thus, while we generally observe an increase 260 SPECIAL COLLOID-CHEMISTRY in the molecular weight with an increase in concentration as expressed by the relatively smaller increase in the osmotic pres- sure, in the example just cited we would be dealing with a decrease in molecular weight, even to one-tenth the original. The " mo- lecular weights" of acid and alkali albumin would even be found to yield complicated curves with maxima and minima related to the concentration of the added electrolytes. In fact, two or three entirely different concentrations of acid or alkali would be found in which the molecular weights of the albumin, or its combination with an electrolyte, would be the same, thereby contrasting with the molecular weights observed in all other concentrations. Similarly, salts would affect the "molecular weight, " making it either rise or fall, depending solely upon the concentration of the added electrolyte. Depending upon the acid or alkaline reaction of the colloid, the "molecular weight" of a colloid might be either raised or lowered on adding a salt. With rising temperature, the molecular weight of some colloids would be increased, of others decreased. The "molecular weight" of a colloid would be changed by shaking or stirring, by ageing and by being warmed either slowly or rapidly. These illustrations will suffice to demonstrate the impropriety of apply- ing the ordinary concept of "molecular weight" to colloid solu- tions. 1 It is hard to see how a "constant" which varies between several hundred and infinity with concentration alone, as in soap solutions, can be of any value in the physico-chemical char- acterization of a system. In this condemnation of the value of "molecular weight'' determinations of colloidally dissolved substances by osmotic methods, 2 it is not maintained that there may not exist transition systems between colloidally and molecularly dispersed systems in which there is at least an approximate proportionality between osmotic pressure and concentration, and therefore a proper basis for the calculation of the molecular weight. In fact, W. M. Bayliss (I.e.) discovered such a system in congo red, freshly pre- pared by the method described above (see also W. Biltz and A. von Vegesack, I.e.). This dye when fresh and free from electrolytes, is highly dispersed, as evidenced by its ultramicro- 1 See in this connection J. Duclaux, Compt. rend., 148, 714 (1909). 2 For the determination of the " molecular weight " of colloids by indirect methods, as by measuring the vapor tension, the boiling or freezing points, etc., see p. 142. MECHANICAL PROPERTIES OF COLLOID SYSTEMS 261 scopic properties, its considerable osmotic pressure, etc. In this pure condition a 0.465 per cent, solution yields an osmotic pressure of 60 mm. of water. If now, by the formula given above, the moleqular weight of the pure congo red is calculated, the answer is a value of 90 to 95 per cent, of that obtained by analytical methods (696.47). W. Biltz and F. Pfenning obtained similar results. This shows that pure congo red behaves like a typical molecular dispersoid, at least in its osmotic relations. The applicability of the above formula to the determination of the molecular weight of this dye is also evidenced by the direct proportionality existing between concentration and pressure, in P other words, the constancy of the quotient -7^ as evidenced in Table 49, on p. 239. In cases of this type, and only in such, are molecular weight determinations by this method justified. Moreover, the considerable electrical conductivity of pure congo red solutions as studied by W. Biltz and A. von Vegesack further shows that we deal in this case with a molecular dispersoid rather than with a colloid, for high conductivity is not characteristic of typical colloids. 9. On the Moleculo-kinetic Theory of Osmosis in Colloid Systems. In view of the successful applications that have been made of moleculo-kinetic conceptions to the quantitative study of the phenomena of movement exhibited in colloid systems, it may be asked if they may not also be of service in the theory of the osmotic pressure of these systems. A. Einstein and M. von Smoluchowski 1 have considered this question. They conclude that the osmotic pressures of two equally concentrated but differently dispersed phases are inversely proportional to the cubes of the radii, of their particles.' 1 In other words, PI M 3 p 2 (ri)' This highly interesting conclusion has not yet been tested experimentally. It is of interest that the above conclusion was reached on the basis of considerations in which it was assumed that the Boyle- Gay-Lussac law (direct proportion between pressure and con- 1 M. von Smoluchowski, Boltzmann-Festschrift, 626, Leipzig, 1904. 2 See The Svedberg, van Bemmelen-Gedenkboek, 131, 1910. 262 SPECIAL COLLOID CHEMISTRY centration as well as absolute temperature) was valid. This assumption holds, of course, only at great dilutions. The Sved- berg 1 tried to determine indirectly the validity of the Boyle- Gay- Lussac law for colloids. An equation governing local changes in the motion of particles showing Brownian movement, so far as extent and frequency are concerned, may be derived from the equation of von Smoluchowski (I.e.). A detailed exposition of this second equation and the considerations leading to it cannot be given here. The Svedberg, however, found highly diluted gold and mercury sols to obey it. He concluded, therefore, that the Boyle-Gay- Lussac law used in constructing the formula would also have to be valid for greatly diluted colloid systems. It is perhaps too early to concur entirely in this conclusion, since the number of mathe- matical assumptions in the formula is exceedingly great. Be- sides, Svedberg's figures (see especially their graphic represen- tation on p. 555 of his paper, I.e., 1910) themselves show that the law holds strictly only at a transition point, for only in very dilute concentrations is there strict agreement between observa- tion and theory. Deviations from the rule, and therefore from the Boyle- Gay-Lussac law, begin to appear in the case of a mercury sol as soon as its concentration amounts to i/6.io~ 10 normal, or about 0.000,000,000,3 per cent, by weight. In view of the slight practical significance of the concentration range over which it is valid, the law appears to be an ingenious theoretical deduction more than a means of studying quantitatively the de- pendence of osmotic pressure in colloid systems on concentration and temperature. Addendum : Other Types of Movement in Dispersoids The phenomena of movement observed in colloid systems under the influence of an electric current will be discussed later. At this point we merely wish to mention the phenomena of move- ment which occur under the directive influence of heat and light. The botanists F. Stahl 2 and W. Sachs 3 observed such directed movements in small solid and liquid particles while attempting in 1876 to determine to what extent the thermal and helio tropic move- ments of unicellular organisms (such as zoospores) depended upon J The Svedberg (I.e.}, as well as Zeitschr. f. physik. Chem., 73, 547 (191)- 2 F. Stahl, Bot. Ztg., 715 (1876); Verb. d. phys.-med. Ges. Wurzburg, 14 (1879). 3 W. Sachs, Flora, 241 (1876). See also E. Strassburger, Jenaisch. Z. f. Naturw., 12 (1878). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 263 biological 'properties and in how far they were merely passive. The directive influence of light on the movement of dispersed particles was later studied in detail by G. Quincke. 1 W. R. Whitney and C. J. Blake 2 have studied such light and heat effects on the movement of colloid particles in colloid gold. The directive influence of light on crystallization and sublimation should also be mentioned here. 3 31. Filtration and Ultrafiltration of Colloid Systems i. Filtration of Colloid Systems. A property which dis- tinguishes colloid solutions from coarse suspensions is the ability of the former to pass unchanged through filter paper. It is by this means that we recognize the formation of a colloid solution when we wash a precipitate with pure water. While typical colloids pass through all filter papers, somewhat coarser systems begin to be held back by hardened filter papers and by clay and porcelain filters as those of Berkefeld, Reichel, Chamberland and Pukall. The filtrability of a dispersoid depends upon the size, shape and rigidity of its particles, upon the filtration pressure and the nature of the filter, more especially the size of its pores. 4 To determine the approximate size of the dispersed particles it is therefore well to know the average size of the pores of different filters. Such determinations, as made by H. Bechhold, 5 are given in Table 60. TABLE 60. SIZE OF PORES IN FILTERS (According to H. Bechhold) Filter Average size of pores (permeability to water) Size of largest pores (permeability to air) Ordinary thick filter paper .... 2 2i( Filter paper No. 556 (Schleicher and Schiill). . Filter paper No. 602 (extra hard, Schleicher and Schull). Chamberland filter I.7M 0.89-1.3;* l-I.S/z 0.23-0.41/4 Reichel filter 0.16 o. i7S/i 1 G. Quincke, Report Brit. Assoc. Advanc. Science, Glasgow, 60 (1901); Drude's Ann. d. Physik., 7, 701 (902). 2 W. R. Whitney and J. C. Blake, Journ. Amer. Chem. Soc., 26, 1347 (1904). 3 See the summary of J. M. Eder, Photochemie, 3 Aufl., 123, Halle, 1906. 4 Details regarding this question may be found in the paper of E. Hatschek, J. Soc. Chem. Industry, 27, 538 (1908); also Koll.-Zeitschr., 6, 254 (1910); 7, 81 (1910). 5 H. Bechhold, Zeitschr. f. physik. Chem., 64, 328 (198). 264 SPECIAL COLLOID-CHEMISTRY The original paper must be consulted for details of the methods used by Bechhold in arriving at the assigned values. As the table shows, typical colloids, with particles having a diameter of less than o.iju, must be just able to pass through the filter lowest in the list. But even with pores of this size by- effects, known as " adsorption" effects, often appear, due to the action of the filter itself upon the dispersed phase. These lead to retention of the dispersed phase and so to a clogging of the pores of the filter. At other times coagulation processes occur due to this surface action of the filter. Whenever any of these things take place, filtration cannot, of course, any longer tell us anything definitely regarding the size of particles in a dispersoid. 2. UltrafHtration of Colloid Systems. After W. Schmidt 1 and F. Hoppe-Seyler 2 found that solutions of albumin and of gum became more dilute by being filtered through animal membranes, C. J. Martin 3 discovered that colloidally dissolved materials could be completely separated from their dispersion means by being filtered through organic or inorganic gels. To give these a proper support he used Chamberland filters and impregnated them with gelatine or silicic acid. He could then filter liquids under 30 to 100 atmospheres of air pressure without breaking the filter. By using this method, he was able to separate from the albumin a clear fluid containing salt but entirely free of protein. Table 6 1 contains the more important of his results. Such filtration through gels was next used by French investi- gators (Borrel and Manea, 1904; G. Malfitano, 1904; J. Duclaux, I905) 4 to separate the dispersed phase from the dispersion means in different organic and inorganic colloids. They usually em- ployed collodian capsules as filters. H. Bechhold 5 took an important step forward in this problem of filtration when in 1906 he discovered the permeability of gels to be a function of their concentration. He found dilute gels to be 1 W. Schmidt, Poggendorf's Ann., 337 (1856). 2 F. Hoppe-Seyler, Virchow's Arch., 9, 245 (1861). 3 C. J. Martin, Journ. Physiol., 20, 364 (1896); see also E. W. Reid, ibid , 27, 161 (1903); A. Craw, Zeitschr. f. physik. Chem., 52, 569 (1898); Proc. Roy. Soc., 77, 172, 311 (1899). 4 For the history of nitration through gels see J. Duclaux, Koll.-Zeitschr., 3, 134 (1008); also H. Bechhold, ibid., 3, 226 (1908). 6 H. Bechhold, Z. f. Elektroch., 12, 777 (1906); Koll.-Zeitschr., I, 107 (1906); 2, 3 (1907); Zeitschr. f. physik. Chem., 60, 237 (1907); 64, 328 (1908). MECHANICAL PROPERTIES OF COLLOID SYSTEMS 265 more permeable than more concentrated ones. For details re- garding his methods his original publications must be consulted . TABLE 61. FILTRATION THROUGH CLAY CELLS IMPREGNATED WITH SILICIC ACID (According to C. J. Martin) Impermeable to Partially permeable to Readily permeable to Egg albumin. Serum albumin. Egg globulin. Serum globulin. Fibrinogen. Caseinogen. Nucleoalbumin. Hemoglobin. Glycogen. Soluble starch. Soluble starch (amylodextrin) Alkali albumin. i All albumoses. Acid albumin. Urochrome (pigment of Caramel. j urine). Biliverdin (bile pigment) . j All crystalloids. Dextrin. TABLE 62. ULTRAFILTRATION (According to H. Bechhold) Dispersoid Platinum sol (Bredig) Colloid iron hydroxide Casein (of milk) Colloid gold containing so- dium lysalbinate (Zsig- mondy). Collargol (v. Heiden) i per cent, hemoglobin solu- tion. i per cent, gelatine solution.. . Serum albumin . . The disperse phase is held back by a gelatine gel of the following concentration in per cent. 2 2-5 3 3-5 4 4 4-4-5 Protalbumoses. Silicic acid Deutero-albumoses A Deutero-albumoses B and C. Dextrin. . Remarks Average size of particles about 44n/j. (Zsigmondy) . About 4Ofj,/ji. About Molecular weight 15,000 down to 3000. All crystalloids. 4-5 8 10 TO Molecular weight about 2400. | Traces pass through. j Small amounts pass through; molecular weight about 965. I Pass through. 266 SPECIAL COLLOID -CHEMISTRY As a rule, he used ordinary filter papers as a foundation, impreg- nating them with various gels, as acetic acid-collodion, gelatine- formaldehyde, etc. Table 62 gives a survey of his results. It is evident that the filter becomes less permeable as the concentration of the gel rises. A hardened 10 per cent, gelatine filter holds back even molecules of the size of those contained in dextrin. A proper series of filters makes it possible to distinguish, within the realm of the colloids, between systems of different de- grees of dispersion, and these are then found to correspond with a differentiation between them made on optical grounds. For this reason H. Bechhold has named his method u Ultrafiltration^ Recently A. Schoep 1 has described a simple method of ultrafiltration, in which is eliminated the disadvan- tage of having to work with high pressures. 2 He found that filters of different degrees of permeability could easily be made by adding to collodion solutions different amounts of glycerine and castor oil. Dia- lyzing capsules may be made from such mixtures by the methods de- scribed in the practical introduction on p. 10. The dry collodion cap- sules become progressively more permeable (within certain limits) as the amount of glycerine or castor oil in them is increased. Fig. 60 illustrates Schoep' s simple method. We cannot advantageously discuss the theory of this variable permeability of gels of different concentrations until we have taken up their general structure. In conclusion, it must be mentioned that undesirable by- effects, such as adsorption of the disperse phase by the filter, occur in ultrafiltration, also. Ultrafiltration yields dependable results, therefore, only if checked up by other methods. 1 A. Schoep, Bull. Soc. Chim. Belg., 24, 354 (1910); Koll.-Zeitschr., 8 (1911). See also A. von Lebedew, Zentralbl. f. Physiol., 23, 767; 24,511 (1910). 2 Emulsoids may be separated from their dispersion means, with Schoep's filter, only when pressure is used. FIG. 60. A . Schoep's arrangement for ultrafiltration. AUTHOR INDEX Adamson, L., 232, 233 Albanese, V., 153 Alexander, J., no Alexandrow, N., 131, 142 Allen, 74 Amagat, E. H., 117, 120 Amann, J., i, 60, 69, 227 Ambronn, H., 65 Antonow, G. N., 184 Arrhenius, S., 94, 141, 214, 218, 253 Avogadro, 254 Axelrod, S., 153 B Bachmetjew, Z., 92 Barus, C., 118, 119, 120 Baumhauer, H., 98 Bayliss, W. M., 137, 233, 239, 242, 245, 249, 255, 258, 259, 260 Bechhold, H., 5, 263, 264, 265, 266 Beck, K., 176, 177, 178 Beer, i Behrens, H., 62 Beibl, 191 Bemmelen, J. M. van, 23, 87, 88, 89, 106, 124, 130, 136 Berkefeld, 263 Berthelot, 106 Berzelius, J., 52 Beyerinck, M. W., 138, 177 Bigelow, L., ii Bigland, D., 233 Biltz, W., n, 140, 154, 155, 156, 158, 159, 163, 164, 165, 176, 225, 227, 233, 235, 237, 238, 239, 240, 241, 242, 243, 249, 258, 260, 261 Blake, J. C., 131, 135, 220, 221, 263 Bodaszewski, L. J., 191 Bodenstein, M., 94 Bodlaender, 94 Borrel, 264 Bose, M., 177 Bottazzi, F., 131, 147, 149, 150, 154, 169, 181, 184 Bousfield, W. R., 205 Boyle, 261, 262 Brauer, 157 Bredig, G., 24, 90, 93, 94, 135 Broglie, M. de, 191, 193 Brown, H. T., 131, 132 Brown, R., 186 Bruni, G., 130 Bruyn, Lobry de, 27, 139 Bugarsky, St., 131, 142, 257 Buglia, G., 176, 177, 181, 184 Burnett, Th. C., 131, 141, 142 Biitschli, O., 62, 106 Buxton, B. H., 224, 225 Calcar, R. P. von, 223 Cavazzani, E., 153 Chamberland, 263 Chaudesaigues, P., 193, 194, 201 Cholodny, P. J., 121, 135 Cohen, E., 94 Cotton, A., n, 58, 64 Coudres, Th. Des., 92 Craw, A., 264 Curie, P., 90 Dabrowski, 201 Daguin, 106 D'Errico, G., 131, 147, 149* *5 Denning, Du Pre, 153 Doelter, C., 57 Doerinckel, Fr., 139 Donau, J., 76, 135 Donnan, F. G., 68, 83, 84, 177 Drucker, K., 54, 75, 112, 145 Drude, P., 77 267 268 AUTHOR INDEX Duclaux, J., ii, 131, 137, 205, 233, 235, 239, 240, 241, 242, 243, 255, 259, 260, 264 Diillberg, P., 131 Du Pre Denning, 153 Ebbinghaus, K., 176, 177 Eder, J. M., 263 Eduardoff, F., 140 Ehrenhaft, F., 43, 191 Einstein, A., 206, 208, 209, 216, 217, 218, 219, 261 Engelmann, W., 64 Erb, W., 56 Errico, G. d', 131, 147, 149, 150 Exner, F. M., 192, 196, 198 Exner, S., 213, 217, 218 Fano, G., 153 Faraday, M., 52 Fichter, F., 15 Pick, A., 210 Fischer, M. H., 94 Flemming, W., 153, 159 Frankenheim, M. L., 57, 62, 177 Free, E. E., 14 Frei, W., 153, 181, 184 Freundlich, H., 50, 55, 67, 68, 94, 100, 112, 174, 183, 225 Frey, W., 173 Friedenthal, H., 131, 132 Friedlander, J., 7, 55, 56, 105, 147, 177, 179 Fuchs, C., 68, 190 Galdi, F., 154 Galeotti, G., 106, 174 Gansser, 233 Garrett, H., 139, 153, 158 Gatin-Gruszewska, Z., 131, 132, 142, J43, 147 Gay-Lussac, 242 Geffcken, G., 136 Genthe, A., 146, 148 Giampalmo, G., 174 Gibbs, W., 77, 106, 184 Gilbaut, H., 116, 117, 120 Gladstone, J. H., 131, 142 Gokun, 153, 156, 158, 165 Goldsborough, 153 Gouy, G., 189, 190, 191, 192, 206 Graham, Thomas, 9, 24, 31, 39, 40, 75, 99, 145, 211, 212, 214, 220, 222, 223, 224, 227, 231 Groschuff, E., 227 Guinchant, 116 Guthrie, F., 130 H Haber, F., 68 Hamburger, H. J., 94 Hammarsten, O., 56 Handovsky, H., 153, 154, 158, 170, 171, 172, 256 Hantzsch, A., i Hardy, W. B., 153, 169, 174 Hartl, F., 139 Hatschek, E., 5, 92, 138, 152, 153. 177, 263 Keen, M. P. de, 127 Heidenhain, M., 68 Henri, V., 50, 153, 186, 187, 193, 200, 201, 207 Herz, W., 94 Herzog, R. O., 214, 218 Heyer, R., 223 Hibbert, W., 131, 142 Hober, R., 54, 94, 225, 251 Hoff, J. H. van't, 68, 84, 128, 133, 142, 144, 238, 253, 259 Hoffmann, F., 105 Hofmeister, 168 Holde, D., 46, 103, 177, 178 Hoppe-Seyler, F., 264 Hiifner, G., 213, 233 Hulett, G., 74 Hulshof, 115 Huth, M. E., 112 J Jahn, St., 192 Johanott, Phil., 77 Just, J., 103 AUTHOR INDEX 269 K Kasarnowski, H., 214 Kassel, R., 146 Kaufler, F., 95 Kohnstamm, 77, 115 Konowalow, D., 130, 141 KSrner, T., 131 Kossonogow, J. R. von, 70 Krafft, F., 129, 131, 142, 143, 224, 225, 227 Krulla, R., 62 Kruyt, H. R., 105, 106 Kuenen, J. P., 108 Laar, J. J. van, 232 Lacqueur, E., 153, 169, 173, 174 Lallemand, 106 Lalon, 153 Lebedew, A. von, 266 Lecoq, 200, 201 Lehmann, O., 57, 62, 64, 65, 68, 69, 79, 187, 189, 191 Lemoine, G., 104 Levites, S. J., 153, 154, i55, 160, 161, I 62, 164, 165, 173 Lewis, Wm. C. McC., 92, 184, 185 Liebermann, L., 131, 142, 257 Liesegang, R., 93 Lillie, R. S., 144, 233, 234, 236, 243, 244, 245, 246, 247, 248, 250, 251, 252, 253, 256 Linder, S. E., 34, 124, 130, 131, 136, 141, 181, 182, 216, 219, 220, 224, 228, 229, 232 Linebarger, C. E., 131, 232 Link, 62 Ljubavin, N., 131 Lobry de Bruyn, C. A., 27, 139 Lodge, O., 92 Loeb, J., 86 Loffler, B., 121 Lorenz, R., 103 Lottermoser, A., 131, 232 Ludeking, Chr., 122, 126, 130 Luppo-Cramer, 93, 98, 139 Luther, 145 M Malfitano, G., 131, 223, 233, 264 Maltezos, C., 191 Manea, 264 Martici, A., 176 Martin, C. J., 232, 233, 242, 264, 265 Massen, Th., 214, 218 Maxwell, 68 Mayer, A., 153 Mayer, H., 28 Mclntosh, 96 Mecklenburg, W., 31, 189, 199, 206 Mellor, J. W., 94 Mensbrugghe, G. van der, 68, 191 Metz, G. de, 117, 118 Meyer, W., 131 Michaelis, L., 35, 68, 86, 154 Michel, 131 Mittasch, A.. 106 Molisch, H., 189, 191 Moore, B., 232, 233, 241, 242, 243, 248 Morris, G. H., 131, 132 Moruzzi, G., 131, 153 Mostynski, B., 154 Mouton, H., n, 58, 64 Miiller, A., 52, 153 Miiller-Thurgau, 92 Mylius, F., 227 N Nernst, W., 94 Neuberg, C., 98 Neumann, W., 50, 56, 100, 174, 183, 225 Noyes, A. A., 50, 51 Oettingen, H. von, 7 Oholm, L. L., 218 Oker-Blom, M., 221, 222 Oppenheimer, 94 Ostwald, Walther, no, 138 Ostwald, Wilhelm, 3, 28, 60, 62, 63, 66, 68, 73, 74, Qi, 93, 94, 95, 103, 105, 112, 121, 127, 138, 142, 145, 210, 211, 212, 219, 231 Ostwald, Wolfgang, 23, 24, 25, 39, 47, 50, 52, 68, 71, 82, 102, 107, 109, no, in, 146, 148, iS9, X 76, 177, 180, 216, 247, 256 270 AUTHOR INDEX Paal, C., 100 Pappada, N., 130, 131 Parker, W. H., 232, 233, 241 Paternd, E., 131 Pauli, Wolfgang, 40, 153, 163, 165, 170, 171, 172, 173, 174, 220, 221, 248, 251, 256, 257 Pawlow, P., 92, 106, 108 Pelet, I.., 60 Perrin, J., 30, 50, 51, 68, 187, 189, 191, 192, 194, 198, 201, 202, 203, 204, 205, 206, 208, 209 Pfeffer, W., 232, 238, 239, 242, 253 Pfenning, F., 225, 261 Pickering, S. U., 46, 177 Picton, H., 34, 124, 131, 136, 141, 181, l82, 2l6, 219, 22O, 221, 224, 228, 229, 232 Pieroni, A., 93 Pockels, A., 77, 181 Posnjak, G., 104, 144 Posternak, S., 251 Potts, H. E., 177 Prange, A. J., 134 Preuner, G., 224, 225 Procter, H., 173 Pukall, 263 Quincke, G., 52, 62, 63, 64, 107, 117, 124, 126, 181, 182, 191, 263 Quincke, H., 122 Raffo, M., 93, 136 Ramsden, W., 181 Rankin, 106 Raoult, F., 128 Rayleigh, Lord, 77, 85, 181 RegSczy, E. von, 219, 221 Reichel, 263 Reid, E. W., 233, 236, 241 Reissig, J., 134 Reynold, 77 Richter, B. J., 52, 59 Ringer, W. E., 154 Roaf, H. E., 232, 233, 242, 243, 245, 249 Robertson, T. B., 95, 131, 141, 142, 143, 177 Rodewald, H., 122, 123, 127, 143 Rohland, P., 100 Rontgen, W., 116 Rose, G., 121 Rossi, G., 153 Rothe, R., 105 Rothmund, V., 177, 179 Rotinjanz, L., 105 Rucker, 77 Sabanejew, A., 130, 131, 142 Sachs, W., 262 Sackur, O., 153, 169, 173, 174 Sahlbom, N., 15, 16, 154, 182, 229, 230 Samec, 153 Scala, A., 69, 72, 135, 137 Scarpa, O., 153 Schade, H., 36, 79, 94, 107, 108 Scheffer, G., 214 Schenk, R., 177, 184 Schidrowitz, P., 153 Schmidt, C., 62 Schmidt, W., 122, 126, 264 Schneider, 116 Schneider, J., 103 Schoep, A., 266 Schorr, K., 153, 158 Schroeder, P. von, 153, 154, 155, 156, 159, 160, 161, 165, 166, 167, 168, 169, 180, 256 Schiitt, 77 Seddig, M., 193, 198, 199 Siedentopf, H., 27, 29, 58, 87, 88, 106, 193, 194, 200 Simon, J., 153, 156, 173, 177 Smith, A., 105 Smits, A., 129, 130, 131 Smoluchowski, M. von, 206, 208, 209, 216, 217, 218, 219, 261, 262 Spring, W., 238 Stahl, F., 262 Starling, E. H., 232, 233 Stas, 97 Stefan, 214 Steiner, H., 156, 158, 159, 163. 164, 165 AUTHOR INDEX 271 Steinwehr, H. von, 92 Stodel, 153 Stokes, G., 198, 204, 205, 207, 209 Stoltzenberg, H., 112 Strassburger, E., 262 Strutz, A., 143 Suzuki, S., 227 Svedberg, The, 31, 76, 99, *33, *39, 189, 190, 192, 193, 194, 195, 196, 197, 198, 199, 201, 206, 207, 213, 214, 216, 217, 218, 262 T Tammann, G., 107, 130, 131, 141 Teague, O., 224, 225 Teletow, J., 93, 94 Thomson, J. J., 95, 201, 205 Traube, Moritz, 160 Traube-Mengarini, M., 60, 69, 72, 135, 137 V Vanino, L., 134, 139 Vegesack, A. von, n, 140, 154, 155, 176, 233, 235, 237, 238, 239, 240, 241, 242, 243, 249, 258, 260, 261, Victorow, C., 154, 169, 181, 184 Vignon, L., 214, 225 Vogelsang, H., 62 Voightlander, F., 212, 221 Vries, H. de, 213 W Waals, van der, 77, 115 Wagner, R., 153 Washburn, G. H., 31 Weber, C. O., 153 Wedekind, E., 98 Weimarn, P. P. von, 24, 33, 45, 54, 57, 58, 59, 60, 61, 62, 63, 65, 89, 90, 91, 92, 97, 99, 100, 101, 102, 105, 107, 108, no, 139 Weinmayr, J., 180 Wenzel, 93, 96, 97 Whitney, W. R., 131, 135, 220, 221, 263 Wiener, Chr., 189, 190, 192, 196, 206 Wigand, A., 106 Wild, A., 60 Wittich, J. von, 221 Wolff, L. H., 27, 139 Wiillner, A., 128 Zangger, H., 158, 232 Zirkel, F., 62 Zlobicki, L., 181, 182 Zoja, L., 170, 173 Zsigmondy, R., 27, 29, 30, 32, 42, 50, 52, 58, 76, 101, 135, 139, 186, 187, 188, 196, 210, 218, 223 SUBJECT INDEX Acid, arsenious, 103 Acids, and viscosity, 170, 171; and Brownian movement, 200; and os- motic pressure, 245 Adsorption, 95, 179, 234, 266 After-effects, 243 Agar-agar, viscosity of, 159 Age, and viscosity of suspensoids, 151; and viscosity of emulsoids, 154 Albumin, viscosity of, 171; diffusion of 142,219,221,245; osmotic pressure of, 247, 249, 250; filtrability of, 265 Alkalies, and viscosity, 170, 171; and Brownian movement, 200; and os- motic pressure, 245 Alcohol, and viscosity of gelatine, 156 Alcohol-sol, 41 Allocolloids, 103 Allotropism, 105 Analysis, colloid, 3, 12, 16; capillary, IS, 229 Arsenic trisulphide, 220 Arsenious acid, 103 Associated liquids, 3, 103 Avogadro's constant N, 208, 254 B jff-Gelatine, 160 Beer's law, i Behavior of electrified sulphur, 69 Benzopurpurin, 155; osmotic pressure of, 237 Boiling point, 128; of colloids, 130 Boundaries, 21 Brownian movement, 186; characteris- tics of, 1 86; independence of, 189; measurement of, 192; photogra- phy of, 193; rotary motion of, 194; uniformity of, 195; velocity of, 195; Svedberg's law of, 195; and specific surface, 196; and concentration of dispersoid, 196; and viscosity, 197, 198; and temperature, 198; and added substances, 199; of rubber, 200; and electrical charge, 201; and gravity, 201; and Stokes* law, 204; kinetic theory of, 205; and molecular weight, 209 Calcium, colloid, 197 Capillary analysis, 15, 229 Capillary phenomena, 72 Capillary pressure, 91 Caramel, 240 Casein, 142, 265 Castor oil, 178 Catalysis, 94 Cellulose, 176 Chalk-sacs, 187 Chamberland filter, 263 Chemical energy, and specific surface, 93 Chemical heterogeneity, 22 Cinematograph, 193 Classification, of Zsigmondy, 29, 34; of disperse systems, 29, 33 Closed phase, 25 Clotting, 40 Coagulation, 40 Collodion, 266 Colloidality, 32 Colloids, recognition of, i ; diffusion of. 9, 10, 142, 210, 214, 219, 221, 245; suspension and emulsion, 12; lyo- philic and lyophobic, 13, 51, 52; coagulation of, 13, 16; viscosity of, 13; electrical properties of, 14, 15; mutual precipitation of, 16; as dis- 273 274 SUBJECT INDEX Colloids, cont'd perse heterogeneous systems, 23; specific surface of, 28; character- istics of, 39; thermal coefficient of expansion in, 126; molecular weight of, 140, 258; surface tension of, 180; movement in, 186; diffusion coeffi- cients of, 214; osmotic pressure of, 232; filtration of, 263, 265 Colloid analysis, 3; special, 12; outline of methods of, 16 Colloid-chemical nomenclature, 40 Colloid ice, 106 Colloid metals, 32, 197, 207, 255 Colloid solutions (see also colloids and colloid systems), 4; differentiation of, from true, 6, 9; optical proper- ties of, 6; vapor tension of, 128; boiling point of, 130; freezing point of, 131; saturation in, 134. Colloid state, 2, 14; and independence of chemical composition, 2; theory of, 3, 21 ; concept of, 21, 99; uni- versality of, 99 Colloid systems, reversible and irrever- sible, 40; volume and density, relations in, 115, 120; concentra- tion-variable and complex, 136; supersaturation in, 138; viscosity of, 145; dialysis of, 222; osmosis of, 231 Complex dispersoids, 36, 136 Concentration, and disperse systems, 35 ; effects of, 47, 48, 49, 136 Concentration- variable systems, 36, 136 Condensation, 87; theory of, 88 Congo red, 239, 255, 260 Copper ferrocyanide, 240 Cosmic dust, 43 Critical mixtures, 37, 82, 105 Crystal formation, 59, 62 Crystals, liquid, 62 Crystalline constitution of colloids, 56 Crystallinity, concept of, 56; of colloids, 58; theory, 58 Crystallization, 56, 62 Crystalloids, characteristics of, 39, 101 Cube, increase in surface with division of, 27 Degree of dispersion, 4, 26; and diffu- sion velocity, 215 Density and colloids, 1 24 Determination of osmotic pressure of colloids, 258 Dialysis, 10, 222, 223, 224 Dialyzers, 224 Dializability, 226 Diffusion, 9; apparatus, 10, 211; coeffi- cients, 214; of colloids, 210, 211, 217; of serum albumin, 222 Diffusibility, 210, 211 Diminution of surface, 84 Discontinuity of matter, 96 Disintegration tension, 82 Disperse phase (see also disperse sys- tems), 25 Disperse systems, 24, 32; classification of, 29, 33, 42; ionic 31; of gold, 32; concentration- variable, 35; tem- perature-variable, 36; complex, 36; solid + solid, 43; solid + liquid, 43; solid + gas, 43; effect of con- centration on, 45; energetics of, 66; effects of electrolytes on, 170 Dispersion, 4, 26, 31, 33, 77 Dispersion means, 25 Dispersions, 31 Dispersoids (see disperse systems) Droplet formation, 86 Dust, cosmic, 43 Dyes, surface tension of, 182; dialy- sis of, 225 Dynamic surface tension, 67, 185 Egg-albumin (see also albumin), 142, 245 Einstein-Smoluchowski formula, 206 Electrical charge (see also electrolytes) and Brownian movement, 201 Electrical energy and specific surface, 92 Electrical fountain, 68 Electrical heart, 68 Electrolytes, and viscosity of suspen- soids, 151; and gelatine, 156; and colloid diffusion, 219; and osmotic pressure of colloids, 245 SUBJECT INDEX 275 Electrophoresis, 16 Emulsion colloids (see emulsoids) Emulsions, 5 Emulsoids, 12; general properties of, 49, 54, 124; crystallinity of, 64; and suspensoids, 124; viscosity of, 153, 159, 162, 164, 165, 169; filtrability of, 266 Energetics of dispersoids, 66 Energy, surface, 74, 77; and specific sur- face, 92, 93, 97 Expansive surface tension, 68, 69, 70 Ferments, 94 Filter paper, 15; and capillary analysis, 230, 263 Filters, 5; ultra, 12, 263 Filtrability, 263 Filtration, 5, 263 Fluid mixtures, critical, 105 Fluorescence, 8 Foams, in Fog, 43 Formation, of crystals, 59; of droplets, 87 Formula of Gibbs, 184; of Einstein- Smoluchowski, 206; of Svedberg, 195 Freezing point, 128; of colloids, 131 Gas + gas dispersions, 43 Gas + liquid dispersions, 43 Gas + solid dispersions, 43 Gelatine, no, 139, 265; viscosity of 154, 156, 159, 162, 167, 169; surface tension of, 182; osmotic pressure of , 236; thermal history of, 242; and acids and alkalies, 169, 245; swell- ing of, 247, 256; as filter medium, 264 Gelation, 40 Gels, 24, 40; permeability of, 264 Gibbs' theorem, 184 Glycogen, 142 Gold, 32, 217, 220, 255 Gravity and Brownian movement, 201 Gum arabic, 142, 239, 240 Gutta percha, 177, 187, 191, 200, 204 H Heat, 243, 262 Hemoglobin, 241, 257 Heterogeneity, 3, 21, 23; concept of, 21; chemical and physical, 22 Hofmeister series, 168 Homogeneous liquids, 3 Hydrates, 39 Hydrogels, 41 Hydrosols, 41 Hylotropic changes, 3 Hysteresis, 243 Ice colloids, 106 Ice cream, no Increase of surface, 78 Independence of Brownian movement, 189 Instability of mechanical suspensions, 5; of osmotic pressure of colloids, 235 Internal friction (see viscosity) Ionic dispersoids, 31 Ionic series, 168; and osmotic pressure of colloids, 249 Iron hydroxide, 228, 229, 240, 265 Iron nitrate, capillary analysis of, 230 Isocolloids, 4, 8, 102; of water, 107 Isodispersoids, 4, 8, 102, 107 Isomeric compounds, 3 Kinematograph, 193 Kinetic theory and Brownian move- ment, 204 Latex, 200 Law, Beer's, .1; mass, 142; Svedberg's, 195; Stokes', 204; van't Hoff's, 259; Wenzel's, 96 Light, and Tyndall effect 7, 8; and col- loid movement, 262 276 SUBJECT INDEX Light cone of Tyndall, 7, 8 Liquid crystals, 52 Liquid + gas dispersions, 49 Liquid -f- liquid dispersions, 48 Liquid + solid dispersions, 48 Liquids, heterogeneous and homo geneous, 3 Lyophilic colloids, 13, 52 Lyophobic colloids, 13, 52 M Mass law, 142 Mastic, 204 Masticized rubber, 177 Matter, discontinuity of, 96 Measurement of osmotic pressure 'of colloids, 233 Mechanical suspensions, 4; instability of,S Membranes, liquid, 87; osmotic, 232 Metameric compounds, 3 Metastyrol, 3, 104 Methods of dialysis, 223; of studying diffusion, 211 Microns,, 29 Milk, 176, 186 Minerals, 43 Mixtures, critical, 37, 82, 105 Molar surface energy, 3 Molecular dispersions, 4, 31, 54; viscos- ity of, 145 Molecular weight of colloids, 140, 142, 258; and Brownian movement, 209 Moleculo-kinetic theory of osmosis, 261 Movement in colloids, 186 N N. (Avogadro's number) 208, 254 Nicol prism, 8 Night-blue, 140; viscosity of, 158, 163; osmotic pressure of, 243, 247, 249 Nomenclature, colloid-chemical, 40 Normal liquids, 4 Nucleus, 138 O Oil, 5, 178 Optical behavior of colloids, 6, 58 Osmosis (see also osmotic pressure) , 231; kinetic theory of, 261 Osmotic pressure, of benzopurpurin, 237; of night-blue, 243, 247, 249; of gela- tine, 245, 250; of albumin, 247, 249; and molecular weight, 258 Osmotic pressure of colloids, 144, 232, 249; instability of, 235; and pre- vious treatment, 236; and shaking, 236; and stirring, 237; and time, 237; and concentration, 238; and tem- perature, 242; and added sub- stances, 244; and acids and alkalies, 245; theory of, 253 Particles, size of, 30 Peptization, 40 Permeability of niters, 263; of gels, 264 Phase rule, 105 Phases, 22; closed, 25; disperse, 25; physical state of, 42 Phosphorus, 104 Photography of Brownian movement, iQ3 Physical heterogeneity, 21 Platinum, 94, 197, 207 Polydisperse systems, 35, 54, 138 Polydispersoids, 35, 54, 138 Polymeric compounds, 3 Polysuspensoids, 54 Pores in filters, 263 Practical introduction, i Precipitation, 40 Pressure, capillary, 91; osmotic, 232, 233 Prism, Nicol, 8 Protective action, 5 Proteins, no, 139, 142, 265; viscosity of, 154, 156, 159, 162, 167, 169; and acids and alkalies, 170; surface tension of, 182; diffusion of, 222; osmotic pressure of, 236; thermal history of, 242; and acids and alka- lies, i6v), 245; swelling of, 247, 256; as filter medium, 264 Pukall filter, 263 SUBJECT INDEX 277 Radiant energy, 97 Radio-activity, 98 Recognition of colloids, i Reichel niters, 263 Reversible systems, 40 Rosin, 55, 179 Rubber, 177, 187, 191; masticized, 177; Brownian movement in, 200 Rule of Gibbs, 184; of Einstein-Smu- luchowski, 206; of Svedberg, 195 Sacs, diffusion, 10; chalk, 187 Salol, 60 Selenium, 104 Serum-albumin, viscosity of, 171; diffu- sion of, 222 Silicic acid, as filter medium, 264 Silver, 75, 190 Size of particles, 30 Smoke, 43 Smoluchowski-Einstein formula, 206 Soap, 143, 257 Solid -f- gas dispersoids, 43 Solid + liquid dispersoids, 43 Solid -f- solid dispersoids, 43 Sols, 24, 60; alcohol, 41; water, 41; sulphuric acid, 41 Solubility, of salol, 60; of silver chlo- ride, 75 Solutions, true, 4, 54; colloid, 4, 6, 9; molecular-disperse, 4, 6, 9; optical behavior of, 6; supersaturated, 33, 60; vapor tension of, 128; boiling point of, 130; freezing point of, 131; saturation in colloid, 134, 136; supersaturation in colloid, 138 Solvates, 38, 54 Special colloid-chemistry, 115 Specific surface, 26, 72, 91, 92, 93; of colloids, 29; electrical energy and, 92, 93; chemical energy and, 93; and Brownian movement, 196 State, colloid (see also colloid state), 2. 14; theory of, 3, 21 ; concept of, 99; universality of, 99 Stokes' law, 32 Strong colloidality, 32 Styrol, 104 Submicrons, 29 Sulphur, 60, 104, 105 Supersaturated solutions, 33 Surface, discontinuous diminutions in, 84 Surface energy, molar, 3 ; of first order, 66, 74; of second order, 67, 74; and other energies, 71; reciprocal effects of two kinds of, 79, 80 Surface increase, 78 Surface tension, 61; dynamic, 67, 185; static, 67, 185; negative, 68; expan- sive, 68; properties of, 69, 70; and specific surface, 76; of colloids, 118; of gelatine, 182; of dyes, 183 Surfaces, 21, 27; in colloids, 29 Suspension colloids, 12; general prop- erties of, 49, 54, 124; viscosity of 146, 151, 152 Suspensions, mechanical, 4,5; colloid. 49 Suspensoids (see suspension colloids) Svedberg's law of Brownian movement, 195 Swelling of gelatine, 247, 256 ' Systems, 22; disperse, 24, 32; classifi- cation of, 29, 33; submolecularly disperse, 32; supermolecularly dis- perse, 32; poly disperse, 35; con- centration-variable, 35, 136; tem- perature-variable, 36; colloid, 40, 115, 120; reversible and irrever- sible, 40; dispersoid, 43 Temperature, and viscosity, 164; and Brownian movement, 198; and hys- teresis, 242 Tension, surface, 61, 67, 79, 80; dis- integration, 82 Theorem of Gibbs, 184 Theory of condensation, 88; of osmotic pressure of colloids, 253 Thermal coefficient, and expansion in colloids, 136 Thermal history, 242 Thorium hydroxidesol, 240 278 Time, and osmotic pressure of colloids, 237 Tobacco smoke, 43 Transition phenomena, 39 Transition systems, 12, 39 True dispersions, 30 True solutions, 4, 54 Tyndall phenomenon, 7, 8 U Ultrafiltration, 12, 263 Ultramicrons, 29 Universality of colloid state, 99 Van't Hoff's laws, 259 Vapor pressure, 128 Vapor tension, 128 Vectorial constitution of colloids, 56, 58, 64 Viscosimeter, 145 Viscosity (see also viscosity of emul- soids), 13; of colloid systems, 145; of molecular dispersoids, 145; of suspensoids, 146, 150, 151; and electrolytes, 151; mechanical the- ory of, in suspensoids, 152; of emul- SUBJECT INDEX soids, 153, 154, 158, 159, 161, 162, 163, 164, 165, 167, 169, 171, 173, 174, 175, 179; of gelatine, 154, 156, 159; of benzopurpurin, 154; and inoculation, 158; of night- blue, 158, 159; of agar-agar, 159; and degree of dispersion, 175; and type of disperse phase, 179; and Brownian movement, 197, 198; and character of dispersion means, 198 Viscosity of emulsoids (see also vis- cosity) and age, 154; and electro- lytes, 154; and mechanical treat- ment, 158; and concentration, 161; and temperature, 164; and added substances, 165, 169, 171; and non-electrolytes, 173; and electrical charge of dispersion phase, 174. W Water, isocolloids of, 107 Weak colloidality, 32 Wenzel's law, 96 Zoospores, 262 Zsigmondy, classification of, 29 YD 07397 337588 , ( , UNIVERSITY OF CALIFORNIA LIBRARY