Colloids

Colloids, Colloidal Chemistry. Colloids (from Greek kolla, meaning glue or gelatin), a name given by Graham to a group of substances of which typical representatives are gelatin or gum arabic. Colloidal chemistry is the youngest chemical discipline. Its beginning can be considered 1861, when Graham's research on colloids and crystalloids was published. The basis for dividing all substances into colloids and crystalloids was their different behavior in the dissolved state. Crystalloids (for example, NaCl) form stable solutions and have definite solubility, i.e., in the presence of an excess of crystalloid, the solution at a given temperature has a constant concentration independent of the preparation method. From a more concentrated, "supersaturated" solution, the excess substance precipitates in the form of crystals having a strictly defined, characteristic shape and structure. Solutions of colloids, however, are characterized by instability and metastability. Depending on minor differences in preparation method, they contain either a larger or smaller concentration of dissolved substance, and this concentration can undergo very sharp changes under the influence of sometimes completely insignificant amounts of foreign impurities. From solution, they precipitate in the form of amorphous, structureless bodies, often in the form of a gelatinous jelly that binds large amounts of water. The starting point for dividing substances into colloids and crystalloids for Graham was careful measurements of the diffusion rate of dissolved substances. It turned out to be very different for various substances. However, while some substances diffused at a more or less significant speed, the diffusion rate of others proved to be negligibly small, practically equal to zero. Many simple mineral compounds belonged to the first group, as well as other substances that precipitate from solution in the crystalline state and therefore received the name crystalloids. A typical representative of the second group is glue (kolla), which gave rise to the name of all colloids. An even more important characteristic for dividing colloids and crystalloids than free diffusion was osmosis through colloidal membranes, natural or artificial. The basic experiment of Graham consisted in that he separated the solutions being studied from pure water with parchment membrane. Crystalloids diffused freely through the parchment membrane, whereas for colloids it was completely impermeable. This phenomenon was used as a general method for separating colloids from crystalloids (see Dialysis). The differences mentioned seemed so significant, the boundary between colloids and crystalloids so sharp, that Graham considered them as "two different worlds of matter." Subsequent research largely smoothed this boundary. In this respect, the research of Krafft and Paal played an important role, showing that depending on the nature of the solvent, the same substance can exhibit either colloidal or crystalloid properties. Krafft found that sodium stearate, which forms a colloidal solution in water, when dissolved in alcohol has the properties of a crystalloid. Conversely, such a typical crystalloid as NaCl, in a benzene solution according to Paal exhibits all the properties of a colloid. Thus, there are no colloidal substances, there is only the colloidal state of matter. The name colloid should include not only the colloidal dissolved substance, but also its solvent; together they form a colloidal system. Such colloidal systems find their natural place among other dispersed systems (see). On the one hand, they border on more coarsely dispersed systems - suspensions and emulsions, the particles of which have microscopic dimensions; on the other hand, they are connected by gradual transitions with true solutions of crystalloids, which with all modern research methods appear completely homogeneous. Subsequent research to an even greater extent smoothed the sharp boundary that separated colloids along with other heterogeneous systems from crystalloids. Thus, various "semi-colloids" (for example, breakdown products of true colloids, such as dextrins and peptones) became known, representing sequential transitions from typical colloids to true solutions. Many colloids were also obtained in the crystalline state. Thus, crystals of egg albumin, plant albumin of seeds, etc., are well known. On the other hand, methods have now been developed that allow preparing colloidal solutions of typical crystalloids. Graham himself, who spoke of colloids and crystalloids as "two different worlds of matter," in other cases admitted that the same substance can exist both in the crystalloid and in the colloidal state and that a colloidal particle can be "built by combining many smaller crystalloid molecules." Classification of Colloids - Colloids can exist in both liquid and solid states. In the first case, they form colloidal solutions, or sols, in the second - jellies, or gels. However, while in crystalloids there is a sharp boundary between solid and liquid aggregate states (see), in colloids they can be connected by gradual and imperceptible transitions (for example, during the gradual solidification of a gelatinous jelly). In both cases, colloids form dispersed systems in which the dispersion medium is some liquid. Depending on the composition of the latter, they receive different designations. They speak of hydrosols and hydrogels if such a liquid is water; the names alkasol, ethersol, etc. indicate that the dispersion medium is alcohol, ether, etc. Colloids whose dispersion medium is some molten body are called pyrosols, colloids that exist only at low temperatures - cryosols. A very essential characteristic of colloids is the size of colloidal particles. They are characterized by the submicroscopic size of particles - approximately from 1 to 100 mμ. Thus, in terms of the size of their particles, colloids occupy an intermediate position between true solutions (molecularly or ionally dispersed systems) on the one hand and suspensions and emulsions on the other. Using an analogy with suspensions and emulsions, Ostwald and Hober (Wo. Ostwald, Hober) divided sols into suspensive and emulsive based on the aggregate state of the dispersed phase. Accordingly, Weimarn included them in the general system of dispersoids as suspensoids and emulsoids. A much more significant characteristic affecting many properties of colloidal solutions is the degree of affinity between colloidal particles and the liquid surrounding them. According to the degree of affinity between the dispersed phase of a hydrosol and water, Perrin introduced a division into hydrophobic and hydrophilic colloids. The former are weakly connected with the solvent and, easily separating from it under the influence of sometimes very minor effects, form a water-poor precipitate. Conversely, the latter are characterized by much greater stability and, passing into a solid state, form jellies that continue to retain large amounts of water. Freundlich extended this classification to colloidal systems having a different dispersion medium instead of water. All colloids, based on the absence or presence of affinity between them and their solvent, he divides into lyophobic and lyophilic. Between them there are various transitions corresponding to different degrees of lyophilicity. Precipitated lyophobic colloids usually cannot be returned to solution by simply removing the coagulating agent or adding solvent. This in the terminology of Zsigmondy - irreversible colloids. They in turn break down into two subgroups. One includes, for example, pure metal sols. The colloidal substance cannot reach in them any significant concentration and, once separated from the solution (in the form of a powdery precipitate), needs for return to solution the application of ordinary dispersion methods. An example of the second subgroup can be colloidal solutions of various oxides that give sufficiently concentrated sols: silicic or stannic acid, ferric hydroxide, etc. For a short time, their freshly formed jelly-like precipitates can be returned to solution again. However, prolonged drying soon makes the precipitate as irreversible as in the previous case: neither washing the coagulant nor adding solvent can after this restore the original sol. Completely different are the behavior of reversible colloids. Even when completely dried, upon contact with the solvent they bind it, swell and finally, like soluble crystalloids, spontaneously pass into solution. Here too, a distinction should be made between such colloids as agar-agar, gelatin, starch, etc., which have limited swelling. The binding of the solvent in them is limited at ordinary temperature to certain limits and only upon heating continues until transformation into a sol.

On the contrary, typical reversible colloids, such as albumin, gelatin, gum arabic, and others, by their ability to spontaneously pass into a dissolved state, even more closely approach true solutions of crystalloids, differing from them, however, in the absence of constant solubility. The classification of colloids according to these various characteristics in many cases gives coinciding results. Hydrophobic, or lyophobic colloids are at the same time irreversible; hydrophilic, or lyophilic ones more often belong to the reversible category. Due to the binding of a large amount of water, hydrophilic particles at the same time acquire an emulsional character, whereas hydrophobic ones can retain the properties of a solid body and give a suspensoidal sol. However, a colloid does not always possess all the characteristics typical of the representatives of one group or another. In particular, the division of colloids according to the state of aggregation of the dispersed phase may not coincide with the classification according to the more essential characteristic for them - affinity for the solvent. Thus, there are emulsoids that do not possess the properties of lyophilic colloids. The most successful division of colloids into lyophilic and lyophobic (or hydrophilic and hydrophobic), based on the most important difference between the two groups of colloids, must be recognized. Methods of preparing colloidal solutions. The most diverse substances, both organic and inorganic, by means of special techniques can be obtained in a colloidal state. The most important task in preparing colloidal solutions is to achieve the necessary degree of dispersion, creating particles of the proper size. Corresponding to their intermediate position between true solutions and coarse-heterogeneous systems, particles of colloidal size can be obtained from the former by condensing their ions and molecules into larger aggregates, as well as by dispersing the latter. Accordingly, Svedberg, who especially thoroughly developed and systematized the methods of preparing sols, divided them into condensation and dispersion methods. In the presence of an excess of their own solvent, crystalloids break down into individual molecules. They must be insoluble in the dispersion medium in order to form larger aggregates. Therefore, the basis of condensation methods lies in chemical reactions that convert soluble compounds into insoluble ones. Most often these are reduction reactions. They are used, for example, for obtaining hydrosols of noble metals. Many reducing agents (such as hydrazine, hydroquinone, pyrogallic acid) act in the cold, others (ethyl alcohol, formaldehyde, tannin, etc.) - upon heating. To prepare a gold sol, an extremely dilute solution of some gold salt is treated with a reducing agent, for example AuCl₃ or HAuCl₄ (in a concentration of 0.1 g or less per 1 liter of water). Similarly, silver hydrosols are prepared from AgNO₃, and sols of other metals from the corresponding salts. Colloidal silver metal was also obtained by using gaseous hydrogen, passed through the solution, as a reducing agent. Various other chemical reactions (oxidation, double decomposition) can lead to the same result - the obtaining of insoluble substances in colloidal distribution. Thus, colloidal sulfur is obtained by decomposing sodium thiosulfate (Na₂S₂O₃) with concentrated sulfuric acid. Such a sulfur sol was studied in great detail by Oden (Sven Oden). The object of numerous investigations was also colloidal trisulfur arsenic (As₂S₃), which is obtained when hydrogen sulfide acts on arsenious acid (the excess hydrogen sulfide is then displaced by passing a current of hydrogen through the solution): As₂O₃ + 3H₂S = As₂S₃ + 3H₂O. Hydrolysis reaction is also often used, in which one of the substances entering into the decomposition is water. By this method, ferric hydroxide (FeCl₃ + 3H₂O = Fe(OH)₃ + 3HCl) and many other sols are obtained. However different the chemical reactions used, they all reduce to the formation of substances insoluble in the given dispersion medium. According to Weimann, the mechanism of condensation represents a special case of crystallization from a supersaturated solution. Only this crystallization must begin simultaneously in a very large number of places, and the growth of particles must be stopped before they reach microscopic sizes. Since at too high a concentration of colloidal substances they quickly precipitate completely, very weak concentrations of reacting substances are usually used for preparing sols. Finally, it should not be forgotten that in all the considered processes of sol formation, electrolytes participate either as initial substances or as reaction products. As will be clarified below, they cannot be considered as foreign, accidental impurities. On the contrary, electrolytes take the most active part in the construction of colloidal particles, in determining their chemical and electrical properties, and in maintaining the stability of colloids. The complete removal of all electrolytes usually leads to the destruction of the sol, to its transition into a more coarsely dispersed state. Electrolytes play the role of dispersants maintaining the necessary degree of dispersion of colloidal particles. Another group consists of dispersion methods. The task of mechanical crushing of substances - admittedly not to very high degrees of dispersion - was recently (1920) successfully solved by the device of the so-called 'colloidal mill'. It consists of a rapidly rotating shaft with blades inside the liquid, which during rotation pass close to fixedly secured projections (without touching them). The body churned up in the liquid is crushed by the impacts of the blades against the water to colloidal sizes. The electrical method is more often used. Bredig first applied it for preparing sols of noble metals. By immersing electrodes of the metal to be dispersed in water, he passed a voltaic arc between them. At the same time, clouds of sprayed particles, both colloidal and larger sizes, rise from the cathode. Due to the strong heating of the solution, it must be cooled at the same time. Apparently, as Bredig himself believed, thermal processes play the main role here: evaporation of the metal in the voltaic arc with subsequent condensation of its vapors in water. Thus, in its mechanism, this method actually approaches condensation methods. Svedberg greatly improved the method of electrical spark dispersion, mainly by the use of oscillatory discharge. By means of it he succeeded in preparing a large number of different sols, in particular organosols (for example, etherosols) of alkali metals, the preparation of which presented great difficulties. The method of peptization also belongs to the dispersion methods (see below). Biocolloids. The described methods allow the preparation of various artificial, or synthetic colloids. A large and very important group of natural colloids behaves completely differently. It includes various biocolloids - organic substances of such complex composition that even their individual molecules or ions have characteristic colloidal particle sizes and consequently - colloidal properties. Sol solutions of biocolloids are therefore prepared in exactly the same way as ordinary solutions of crystalloids - by treating them with a suitable solvent. Most often water serves as such a solvent. Gums, starch, gum arabic, agar, tannin, gelatin, albumin dissolve in cold or hot water, forming hydrosols. In other cases, special solvents have to be used: an ammonia solution of copper oxide ('Schweitzer's reagent') for cellulose, acetone, acetic acid or a mixture of alcohol with ether for nitrocellulose, benzene or carbon disulfide for rubber, etc. Methods of purifying colloids. In most cases, the usual methods of purifying chemical substances are not applicable to colloids. Only a few colloids (namely some biocolloids) can be separated from each other and isolated, due to their unequal solubility in various solvents, by fractional precipitation or crystallization. Much more often special colloidal methods have to be applied. They are based on the inability of colloidal particles to pass through colloidal membranes permeable to crystalloids. If such a membrane, inside which the solution to be purified is located, is washed outside with pure distilled water, the crystalloids contained in the colloidal solution will diffuse through the membrane into the latter. By repeatedly changing the water, one can by means of dialysis (see) gradually extract from the colloidal solution practically all diffusible impurities. Another method of purifying colloids is ultrafiltration. The solution is filtered under high or very high pressure through a colloidal membrane used as a filter.

The separation of the dispersion medium with impurities dissolved in it from colloidal particles can in this case also be significantly accelerated by using electroosmosis instead of mechanical pressure to push the liquid through an ultrafilter; this method is called electro-ultrafiltration. Optical properties. In transmitted light, colloidal solutions often appear completely transparent and homogeneous, similar to true solutions. However, their heterogeneity becomes clearly apparent in reflected light: when viewed from the side of the incident light, colloidal solutions appear turbid, opalescent. The optical heterogeneity of colloidal solutions becomes even more apparent if a bright beam of light (a sunbeam or a beam from an electric arc lamp, concentrated by a converging lens) is directed onto them and the liquid is observed from the side: the entire path of the beam in the colloidal solution glows with a uniform opalescent light. Faraday was the first to use this method to detect the smallest particles in turbid media. After the name of Tyndall, who studied the described phenomenon in detail, this glowing cone is usually called the Tyndall cone (see Tyndall's phenomenon). All colloidal solutions exhibit such opalescence, which is one of their most important differences, a sign of their optical heterogeneity. The color of colloidal solutions also in many cases depends on the scattering of light by their particles. There is a regular dependence, studied mainly by Rayleigh, between the size of the dispersed particles and the color of the light they scatter. This color can be superimposed on the inherent color of the colloids, depending on their absorption of a certain part of the spectrum. An example of this phenomenon can be the colloidal solutions of mastic, which are yellow or brown in transmitted light and bluish in reflected light. This type of coloration, noticeable in some cases with non-conductors, manifests most brightly and reaches special intensity with colloidal metals. It depends on the optical properties of the metal, on the size of its particles and mainly on their combination into larger aggregates. The degree of dispersion has a particularly strong influence, with which the color changes correctly. Colloidal solutions of gold, for example, pass through a whole range of diverse colors as the size of its particles changes. Zsigmondy succeeded in preparing a complete series of its solutions with a uniformly changing degree of dispersion over enormous ranges. Coarsely dispersed gold gives the solution a blue or violet color, while highly dispersed gold gives it a pure and bright red color (the origin of the color of ruby 'gold glass' is the same). With further reduction in particle size and approach to molecular dispersion, a brown or yellow color is achieved, characteristic of true solutions of gold salts. No less diverse are the colors of various colloidal solutions of silver (red, brown, violet, green, black). To give an idea of their intensity, it is sufficient to note that the brown color of colloidal silver is clearly visible in a layer 1 cm thick when containing 1 part of silver to 5 million parts of water. Size of colloidal particles. The scattering of light by the smallest particles forms the basis of the ultramicroscope, in which a strong concentrated beam of light illuminates the solution being studied from the side and passes through it without entering the microscope's objective. In the field of view of the microscope, the Tyndall cone is thus observed. At the focus where the rays converge, the maximum intensity of illumination makes the smallest submicrons visible. In the immersion ultramicroscope, it proved possible to observe the smallest submicroscopic particles, only a few thousandths in size. Submicrons appear in the ultramicroscope as glowing points, giving no idea of their shape or true size. To determine the latter, the number of individual particles in a certain, extremely small volume of liquid is counted. Knowing the total amount of dispersed substance and its specific gravity, it is easy to find the mass of one particle and its diameter (assuming for simplicity that it has approximately a spherical shape). In addition to this optical method, there are mechanical methods that allow determining the size of colloidal particles. For this purpose, the already mentioned method of ultrafiltration is used. Ultrafilters are by no means always impermeable to all colloidal substances. Bechhold was the first to show that by using a series of ultrafilters with different, sequentially changing pore sizes, fractional ultrafiltration can be performed: separating one colloidal substance from another. An ultrafilter that retains a given colloidal substance also does not pass those that have larger particles. By calibrating a series of ultrafilters (for example, using colloidal solutions with known particle size), the size of particles in the colloidal solution under study can be determined based on its ability to pass through certain ultrafilters. Furthermore, the size of particles can be judged by their rate of sedimentation. According to Stokes' formula, the rate of sedimentation of a spherical body (of sufficiently small size) in a liquid is proportional to the square of its diameter. Therefore, the size of a particle can be determined by its rate of sedimentation (provided that the specific gravity of the sedimenting body and the liquid, as well as the viscosity of the latter, are also known). This method is not directly applicable to colloidal-sized particles, as their sedimentation rate is too negligible. However, by using a very large centrifugal force instead of gravity, the sedimentation of colloids can be accelerated and made measurable. This method is called 'ultracentrifugation'. The listed methods of direct counting of the number of colloidal particles, their mechanical separation by filtration or centrifugation do not essentially differ from similar methods applied to coarser microscopic suspensions. However, along with this, methods are also used to measure the size of colloidal particles that are used to measure molecular weight in solutions of crystalloids. The ultramicroscope made the particles of colloidal solutions ('submicrons') visible and thus deepened the difference between them and optically unresolved solutions of crystalloids. At the same time, it allowed the extension of molecular-kinetic concepts to colloidal solutions and even to coarser suspensions and revealed a complete analogy between the behavior of various dispersed particles and molecules. This most important generalization was the result of the study of Brownian motion (see). As shown by the research of Einstein, Smoluchowski, Perrin, and others, it represents true molecular motion, the faster the closer the diameter of the particles approaches molecular dimensions. The study of Brownian motion of colloidal particles showed that their kinetic energy does not depend on their size and equals the kinetic energy of molecules in true solutions (at the same temperature). Therefore, the osmotic pressure of colloids is proportional to the concentration of colloidal particles. Knowing the total weight of the dissolved colloids and their density, their size can be determined from the number of particles. However, measuring the osmotic pressure of colloids presents considerable difficulties and cannot always be performed with sufficient accuracy. In contrast to kinetic energy, the rate of diffusion decreases with increasing size of colloidal particles and provides a further way to determine the latter. Shape and structure of colloidal particles. In calculating the diameter of a colloidal particle, it was usually assumed to have a spherical shape. It was assumed that in contrast to crystalline bodies, colloidal particles are amorphous and, under the influence of surface forces, take on a spherical shape corresponding to the minimum of free surface. Nageli was the first to express the view that a colloidal particle, or micelle, represents the smallest ultramicroscopic crystal. The crystalline properties, in particular double refraction, exhibited by many organic substances and vital structures, Nageli explained by the fact that these substances are built from the smallest, microscope-invisible particles (in modern terminology 'submicrons'), crystalline micelles. These micelles play the same role in colloidal systems as molecules do in true solutions. Unlike molecular solutions, colloidal systems represent, in Nageli's expression, 'micellar solutions'. By combining with each other, micelles can maintain a strict, correct orientation and grow into real crystals or into organic fibers having certain crystalline properties. With rapid combination, they more often fuse into chaotic, irregular, often tree-like branched complexes that form, for example, the basis of gels. Despite their primary crystalline structure, they appear externally amorphous in this case. Nageli's views, which did not initially receive recognition, were later revived by Weimarn, Ambronn, Scherrer, and others.

By various methods, the crystalline nature of very many, though still far from all, colloidal submicrons has been proven. Amorphousness is no longer considered a characteristic feature of the colloidal state, and the micelle represents the basic concept in the modern understanding of the structure of colloids. Without dwelling here in more detail on the crystalline structure of micelles, it should be pointed out that in very many cases it can indeed be detected. The most reliable method for studying the structure of crystals at present is the X-ray method. Crystals are characterized by the regular arrangement of atoms or ions, immovably fixed at equal distances from one another. The regular geometric form of crystals is the external expression of this spatial crystal lattice of atoms. It causes the diffraction of X-rays falling on the crystal, similar to how a diffraction spectrum of visible light is obtained with the help of coarser artificial diffraction gratings. Contrary to the old concept of the amorphousness of colloids, by means of this method (developed mainly by Scherrer), the crystalline structure of the particles of very many colloids (for example, colloidal gold, silver, and many others) has been established with certainty. Along with this, some colloids really consist of amorphous particles. A crystal lattice must be accompanied by an external crystal form. It can be clearly detected in those cases when it sharply deviates from spherical: namely, when one axis of the crystal is strongly developed or, conversely, very shortened compared to the other two. In the first case, the colloidal particle has a rod-like, in the second a plate-like form. If under the influence of some external force they arrange themselves with their longitudinal axes parallel to each other, then their form can be determined by the phenomena of light polarization given by such a solution. Such a parallel orientation of crystalline particles is obtained, for example, in a flowing liquid due to the friction that arises during movement.--Changes in the degree of dispersion that often occur in colloidal solutions lead to another characteristic feature of the structure of colloidal particles. As a crystal grows, its form remains unchanged; similarly, when droplets of an emulsion merge, they again form spherical droplets. On the contrary, when the degree of dispersion of a colloidal solution decreases, occurring by the joining of its particles, the latter come into contact only at a few of their points, giving a loose, flocculent combination. Therefore, from the primary colloidal particles, the form and structure of which were considered above, one should distinguish secondary particles, formed by the flocculent combination of two or more primary particles. For secondary colloidal particles, Zsigmondy proposed the name polyons, for primary ones - monons, or protons (the latter name cannot be retained, as it is used to denote the unit of positive electricity-the atomic nucleus of hydrogen; see Hydrogen ions). The combination of primary particles into secondary ones is often accompanied by a sharp change in color. A well-known example of such a change in color is given by the hydrosol of gold. Colloidal processes. If in the doctrine of structure an ever more complete analogy was established between colloidal systems and solutions of crystalloids, a profound difference remained in the nature of the forces acting in them and the processes occurring in them. Between closely approaching molecules, as is known, significant forces of attraction arise, rapidly decreasing with distance. Mutually balancing each other in the middle of molecular aggregates, they manifest themselves on their surface in the form of surface tension. Instead of osmotic pressure, which represents the main form of mechanical energy in true solutions, in the colloidal distribution of matter, this surface energy, directly dependent on the magnitude of the boundary surfaces, and consequently on the degree of dispersion of the colloid, on the size of its particles, acquires particularly great importance. The boundary difference of potential, the electric charge on the surface of colloidal particles, also acquires no less importance. The energy of a colloidal system (energy predominantly surface) thus turns out to be a function of the degree of dispersion of the colloid. Consequently, various energetic changes in colloidal systems (especially changes in electric charge) have as their direct result rapid changes in their degree of dispersion, the joining of small colloidal particles into larger aggregates or, conversely, the disintegration of the latter (peptization). In these characteristic colloidal processes, in the ease of changing the degree of dispersion, lies their main difference from the stable molecular distribution of so-called true solutions. The concept of the predominant influence of surface, capillary (and electrocapillary) forces in their most extreme form was developed by Freundlich, treating all colloidal chemistry as 'capillary chemistry.' On the purely chemical processes occurring in colloidal systems, the concept of their dependence on capillary forces, of the inapplicability to them of general chemical laws, was extended. Instead of the combination of reacting substances in simple equivalent ratios, for colloids, adsorption compounds, quantitatively expressed by Freundlich's adsorption isotherm, were considered characteristic. In this respect, influences that change their hydrophilicity, the affinity between the colloid and the solvent, have a particularly great effect on the properties of colloids. The study of the peculiarities of the colloido-chemical action of electrolytes is mainly associated with the name of Hofmeister (see Hofmeister's series).-A completely opposite position was taken by another group of researchers, among whom Pauli should be mentioned first. According to these researchers, upon eliminating numerous sources of error that in many cases obscure the picture, general chemical laws are fully applicable to colloidal systems, in particular to the most practically important of them-protein solutions. In regard to their chemistry, there is no fundamental opposition between them and crystalloids, just as there is none in regard to other properties. This viewpoint was carried through with particular consistency by Loeb. The theoretical basis for a completely new interpretation of colloidal processes served Loeb with Donnan's principle, establishing a special form of equilibrium of ions on both sides of a membrane impermeable to one of them (see Donnan equilibrium). A whole series of colloidal properties and colloidal processes (osmotic pressure, swelling, viscosity, their dependence on electrolytes, etc.) can be directly derived from the inability of colloidal ions to penetrate through colloidal membranes and gels. Colloidal properties are manifested in the presence of a barrier that retains a given (colloidal) ion but is permeable to the other ions present. Only under such conditions does the solution behave colloidally. In this sense, Loeb speaks not of a 'colloidal state' but of 'colloidal behavior' of protein solutions. Electric charge. The application of electric forces to colloidal solutions shows that colloidal particles carry positive or negative charges and therefore move in an electric field (see Cataphoresis). Electrokinetic phenomena make it possible to study the properties of this charge and determine its magnitude. The cause of the charge cannot be considered finally clarified; apparently it is not in all cases the same. Often the charge depends on the chemical nature of the colloidal particle. Substances differing in acidic character, for example tannin, mastic, silicic acid, acquire a negative charge in pure water; basic substances, such as metal hydroxides (iron, aluminum, etc.), acquire a positive charge. Obviously, despite the apparently complete insolubility of these substances, a small amount of hydrogen or hydroxyl ions passes into solution, leaving on the colloidal particle a charge of the opposite sign. In most cases, the greatest importance is attached to the adsorption (see) of electrolytes present in the solution on the surface of the colloidal particle: the more strongly adsorbed ion imparts its charge sign to it. In this respect, the greatest activity is shown on the one hand by multivalent cations of heavy metals, on the other hand by certain multivalent anions. Finally, mention should be made of Coehn's rule, according to which if a colloidal system consists of two non-conductors, then the substance with the greater dielectric constant acquires a positive charge (see Dielectrics). Since water has a very high dielectric constant, greater than that of most colloids, the latter (in the absence of the first two causes of charge formation) acquire a negative charge in pure water. Due to the electrical neutrality of the solution as a whole, the charge of the colloidal particle is balanced by the charge of opposite sign of the adjacent layer of liquid, and both opposite charges form an electric double layer (see). Chemical composition of the colloidal particle.

The electric charge, which determines many properties of colloids, in turn depends on the chemical composition of both the colloidal micelle and the surrounding ('intermicellar') liquid. However, the conventional designation of colloids does not yet provide a sufficient representation of its chemical composition. For example, when speaking of arsenic sulfide sol or ferric hydroxide, the substances mentioned indeed constitute the main, quantitatively predominant part of the micelle. However, the latter contains along with them a small impurity of electrolytes, the composition and concentration of which depend on the method of preparation (or further treatment) of the colloid. These electrolytes, often adsorbed in negligible amounts on the surface of the colloidal particle, constitute its active part, which determines a whole series of its most important properties. Zsigmondy proposed that in the designation of colloids, the formula of the main mass of the colloidal substance (established by ordinary chemical analysis of its precipitate) be enclosed in a square frame, placing outside this frame the active, ionogenic part of the micelle. Thus, in the methods of preparation of arsenic sulfide sol described above, the active part is the impurity of hydrogen sulfide in it, the partial dissociation of which (into HS' and H') gives the colloid a negative charge. Without fixing the quantitative ratio between As₂S₃ and H₂S (which can vary within very wide limits), the corresponding colloid is given the formula: |As₂S₃| HS' + H'. In a similar way, the micelle of ferric hydroxide has the composition |Fe(OH)₃| Fe''' + 3Cl'. As the given formulas show, by micelle is understood not only the main mass of the colloidal particle together with the ions adsorbed by it, but also the ions of opposite sign, forming the outer layer of the double layer. For the charged colloidal particle alone without the oppositely charged ions adjacent to it, French authors use the term 'granule.' The granule represents the colloidal ion. The oppositely charged crystalloid ions in solution, proposed by Pauli to be called 'counter-ions' (Gegenionen). Crystalloid impurities associated with colloidal particles must be in adsorption equilibrium with the concentration of the same substances in the surrounding liquid. Therefore, however small this concentration in carefully dialyzed sols, it still cannot be zero. Thus, the intermicellar liquid contains at least in very low concentration the same electrolytes that constitute the active part of the micelle; it is never pure WATER. Factors of stability of colloids. Microscopic suspensions, for example the suspension of erythrocytes in blood, settle with considerable speed. But as the size of the particles decreases, their rate of fall rapidly decreases. For particles of colloidal size it is negligibly small, and the solution can for a long time maintain a more or less uniform distribution. This is also facilitated by Brownian motion, which mixes the submicroscopic particles, just as molecular motion mixes the molecules of a true solution. However, a whole series of influences can cause extremely rapid, almost instantaneous precipitation of colloids from solution. Their action amounts to causing agglutination of colloidal particles, their combination into larger aggregates. The inevitable result of such enlargement of suspended particles is their rapid settling. Therefore, all factors that prevent the combination of colloidal particles maintain the stability of the colloidal solution. Such a stabilizing factor is above all the electric charge. The forces of electrostatic repulsion prevent the combination of like-charged particles. A series of studies has shown that the boundary potential of colloidal particles must fall below a certain limit—the so-called critical potential—in order to make coagulation of colloids possible. When the charge decreases below this critical value, particles in Brownian motion, upon collision, can combine with each other. However, at first apparently only a small percentage of collisions (the strongest or central impacts) lead to combination. With a further decrease in boundary potential, this percentage (and with it the rate of coagulation) rapidly increases, approaching a constant limit. The latter is reached when every collision of colloidal particles results in their combination. Due to such a stabilizing influence of the electric charge, changes in its sign or magnitude have a decisive effect on many colloidal processes. As indicated above, the electrolyte adsorbed by the colloidal particle constitutes the active part of the micelle, giving it an electric charge and determining its stability. If by prolonged dialysis the colloid is freed from the stabilizing electrolyte, it becomes extremely unstable and often spontaneously coagulates. It is even easier to cause coagulation by adding an electrolyte from which the colloid adsorbs an oppositely charged ion, neutralizing its own electric charge. The precipitate formed can be redissolved if acted upon by an electrolyte, one of the ions of which is strongly adsorbed and again charges the colloidal particles. Such an action can sometimes be produced even by the very electrolyte that caused the precipitation. The first portions of it neutralize the charge of the colloidal particle and therefore act coagulatingly; subsequent portions cause the appearance of a new charge (of opposite sign) and consequently dissolve it. Such dissolution of a colloidal precipitate by treating it with a stabilizing electrolyte is called 'peptization.' Peptization is one of the most important dispersion methods for preparing colloidal solutions. While for the stabilization of hydrophobic (or lyophobic) colloids the electric charge is of decisive importance, in hydrophilic colloids, in addition to the influence of charge, there is another, no less important factor. This factor is the hydrophilicity of the colloid itself, the affinity between the colloid and the solvent, i.e., the same factor that determines the stability of true solutions. For the precipitation of hydrophilic colloids, to which most biocolloids belong, it is necessary to eliminate both factors of stability—hydrophilicity and charge. The hydrophilicity of protein solutions can be eliminated either by reversibly removing water (for example by the action of alcohol) or as a result of irreversible chemical change (see Denaturation). In both cases, precipitation by electrolytes is then carried out in the same way as in the case of hydrophobic colloids. The influence of ions on the hydrophilicity of colloids is particularly strongly manifested in the so-called Hofmeister series (see). On the stabilizing influence of some colloids on solutions of other colloids—see Protective action. Biological significance of colloids. It must be said that while the basic principles of the doctrine of the structure of colloidal systems are now firmly established, the mechanism of the most important colloidal processes remains still very controversial. The relationship between colloid chemistry and general chemistry, the role of adsorption and chemical processes, the significance of capillary forces and the Donnan principle—all these questions continue to be the subject not only of experimental research but also of heated theoretical disputes. The rapid development of colloid chemistry, which in a short time has become an extensive independent scientific discipline, is explained mainly by the interest it represents for the biological sciences. The living organism consists of colloidal substances, and the study of the colloidal substrate of life constitutes the necessary basis for understanding life phenomena. The investigation of the physiological action of ions (see), as well as of most other physiological agents, shows that it completely coincides with the effect of the same influences on biocolloids. This determines the enormous interest that colloid chemistry acquires for understanding the processes occurring in the living organism. Numerous most complex biological problems can be studied on simple colloidal models, and it is not surprising that a whole series of biologists not only used in their work the results obtained from colloid chemistry, but also took an active part in the development of this science.