The word “colloids” currently refers to ultramicroheterogeneous systems, that is, systems where ultramicroscopic particles representing a separate phase are suspended in a so-called dispersion medium. By colloids, Gregg meant substances that, being distributed in a particular liquid and displaying all the external signs of ordinary solutions, upon more detailed study differ sharply from the latter.
He believed that this difference lies in the very nature of the substance; therefore, he divided the entire material world into two classes: colloids and crystalloids. Later studies showed, however, that there is no insurmountable barrier between crystalloids and colloids and that, in principle, any substance that gives a true solution in a given liquid can form a system with all the properties of colloids in another liquid.
Therefore, now it is more correct to talk not about colloids, but about the colloidal state or colloidal systems. Example: table salt (NaCl) gives an exclusively true solution in an aqueous medium; on the contrary, being distributed in liquids such as ether or benzene, which are not solvents for it, under certain conditions it can give systems that, in all their properties and characteristics may be called colloidal systems.
Colloids and their features
Features of colloidal systems that distinguish them mainly from ordinary or true solutions.
Brownian motion and associated osmotic pressure and diffusion. While in true solutions the particles suspended in a given liquid are molecules (sugar solution in water) and sometimes ions (electrolyte solutions), the particles of colloidal systems are immeasurably larger: they consist of thousands and sometimes hundreds of thousands of molecules , connected into one dense complex called the core.
Some colloidists believe that in nature there may be substances with such large molecules that these latter play the role of colloidal particles. This issue is still considered controversial. At the same time, we must not forget that the essence of the matter is not the size of the molecule, but that this molecule can play the role of an independent phase, that is, that we have reason to recognize such a system of molecular degree of dispersion as a microheterogeneous system.
Molecules of a solution (or gas), as is known, are in continuous thermal chaotic motion. The particles of the colloidal system are also subject to the same movement, although on a different, extremely reduced scale. For historical reasons, this movement of colloidal particles bears the special name of Brownian movement, but we must not forget that its physical essence is completely identical to molecular movement.
The difference here is not qualitative, but quantitative. The same must be said about those properties that are a direct expression of Brownian motion, namely, osmotic pressure and diffusion. Osmotic pressure, just like gas pressure, is a function of the number of particles per unit volume or, as they say, a function of partial concentration. This concentration in colloidal systems is extremely small compared to truly dissolved systems, and therefore the osmotic pressure in them is negligible. It is so small that only very recently it was possible to find methods for its quantitative determination.
The same must be said about diffusion. Gregham believed that diffusion is absent in colloidal systems, and considered this absence of diffusion to be a striking qualitative characteristic of colloids. This turned out to be incorrect and could not have been otherwise, because the fact of the existence of Brownian or, what is the same, molecular motion of particles logically required the existence and diffusion. But this diffusion process, in accordance with the scale of Brownian motion, turned out to be so slow that it was possible to state it, let alone quantify it, only recently, using all modern technical capabilities.
COLLOIDS, COLLOID CHEMISTRY. Colloids (from the Greek ko 11a-glue, gelatin), the name given by Graham to a group of substances, typical representatives of which are gelatin or gum arabic. Colloid chemistry is the youngest chemical. discipline. Its beginning can be considered in 1861, when Graham's research was published. K. and crystalloids. The basis for dividing all substances into crystalloids and crystalloids was their unequal behavior in the dissolved state. Crystalloids (eg Nad) form stable solutions and have a certain solubility, i.e., in the presence of an excess of crystalloid, the solution at a given temperature has a constant concentration, independent of the method of preparation. 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 K. are characterized by instability and metastability. Depending on minor differences in the method of preparation, they contain either more or less the concentration of the dissolved substance, and this concentration can be subject to very sharp changes under the influence of sometimes completely insignificant amounts of foreign impurities. They precipitate from the solution in the form of structureless, amorphous bodies, often in the form of a gelatinous jelly that binds large amounts of water. The starting point for division into colloids. and crystalloids served for Graham to carefully measure the rate of diffusion of dissolved substances. It turned out to be very different for different substances. However, while some substances diffused at a significant rate, the rate of diffusion of others turned out to be negligible, almost equal to zero. The first group included many simple mineral compounds, as well as other substances that precipitated from solution in a crystalline state and were therefore called crystalloids. A typical representative of the second group is glue (koPa), which gave rise to the name of all K. An even more important feature for the separation of colloids and crystalloids than free diffusion was osmosis through colloidal membranes, natural or artificial. Graham's main experience was that he separated the solutions under study from pure water with a parchment membrane. Crystalloids diffused freely through the parchment membrane, while it was completely impermeable to colloids. This phenomenon has been used as a general method for separating colloids from crystalloids (see Dialysis). These differences seemed so significant, the line between colloids and crystalloids so sharp, that Graham considered them I as “two different worlds of matter.” Further research has smoothed out this line to a large extent. The studies of Krafft and Paal played an important role in this regard, showing that, depending on the nature of the solvent, the same substance can exhibit either colloidal or crystalloid properties. Kraft found that sodium stearate, which forms a colloidal solution in water, has the properties of a crystalloid when dissolved in alcohol. On the contrary, such a typical crystalloid as NaCl in a benzene solution exhibits, according to Paal, all the properties of a colloid. That. There are no colloidal substances, there is only a colloidal state of matter. The name of a colloid must include not only the colloidal solute, but also its solvent; together they form a colloidal system. Such colloidal systems find a natural place in a number of other dispersed systems(cm.). On the one hand, they border on more coarsely dispersed systems—suspensions and emulsions, the particles of which have a microscopic size; on the other hand, they are connected by gradual transitions with true solutions of crystalloids, which, with all modern research methods, seem to be completely homogeneous. Subsequent studies smoothed out even more the sharp line that separated K., along with other heterogeneous systems, from crystalloids. Thus, various “semi-colloids” have become known (for example, decomposition products of real colloids, such as dextrins and peptones), representing successive transitions from typical colloids to true solutions. Many K. were also obtained in crystalline form. condition. Thus, crystals of Hb, egg albumin, and plant seed albumin are well known. On the other hand, methods have now been developed that make it possible to prepare colloidal solutions of typical crystalloids. Graham himself, who spoke of crystalloids and crystalloids as “two different worlds of matter,” in other cases admitted that the same substance can exist in both the crystalloid and colloidal states and that a colloidal particle can be “built by combining many smaller crystalloid molecules." Classification K. - Colloids can be in both liquid and solid states. In the first case, they form colloidal solutions, or sols, in the second, jellies, or gels. However, while crystalloids are between solid and liquid state of aggregation(see) there is a sharp line; in K. they can be connected by gradual and insensitive transitions (for example, during the gradual hardening of gelatin jelly). In both cases, K. form dispersed systems, in which the dispersion medium is some kind of liquid. Depending on the composition of the latter, they receive different designations. They talk about hydrosols and hydrogels if the liquid is water; the names alcosol, etherosol, etc. indicate that the dispersion medium is alcohol, ether, etc. Colloids, the dispersion medium of which is some kind of molten body, are called pyrosols, colloids that exist only at low temperatures are called cryosols . A very significant feature of K. is the size of colloidal particles. They are characterized by a submicroscopic particle size - from approximately 1 to 100 t/l. That. in terms of the size of their particles, chlorides occupy an intermediate position between true solutions (molecular or ion-dispersed systems) on the one hand and suspensions and emulsions on the other. Using an analogy with suspensions and emulsions, Ostwald and Geber (Wo. Ostwald, Hober), based on the aggregative state of the dispersed phase, divided sols into suspension and emulsion. Accordingly, Weimarn included them in the general system of dispersoids as suspensionoids and emulsoids. A much more significant feature that affects many properties of colloidal solutions is the amount of affinity between colloidal particles and the liquid surrounding them. Based on the degree of affinity between the dispersed phase of the hydrosol and water, Perrin introduced a division into hydrophobic and hydrophilic K. The former are weakly bound to the solvent and, easily separated from it under the influence of sometimes very minor influences, form a water-poor sediment. On the contrary, the latter are distinguished by much greater resistance and, turning into a solid state, form jellies that continue to retain a large amount of water. Freundlich extended this classification to colloidal systems that have a different dispersion medium instead of water. Based on the absence or presence of affinity between them and their solvent, he divides all K. into lyophobic and lyophilic. There are various transitions between them, corresponding to different degrees of lyophilicity. Precipitated lyophobic coagulants usually cannot be returned to solution by simply removing the coagulating agent or adding a solvent. These are, in Zsigmondy's terminology, irreversible Ks. They, in turn, fall into two subgroups. One includes, for example. pure metal sols. The colloidal substance cannot reach any significant concentration in them and, once separated from the solution (in the form of a powdery sediment), requires the use of conventional dispersion methods to return to the solution. An example of the second subgroup is colloidal solutions of various oxides, which give fairly concentrated sols: silicon or tin, iron hydroxide, etc. Within a short time, their freshly isolated gelatinous precipitates can be returned to the solution again. However, prolonged drying soon makes the precipitate as irreversible as in the previous case: neither washing the coagulator nor adding a solvent can then restore the original sol. Reversible crystalloids behave completely differently. Even when completely dried, when they come into contact with a solvent, they bind it, swell, and finally, like soluble crystalloids, spontaneously go into solution. And here it is necessary to distinguish between such substances as agar-agar, gelatin, starch, etc., which have limited swelling. The binding of the solvent is limited to certain limits at ordinary temperatures and only when heated does it continue until it turns into a sol. On the contrary, typical reversible crystalloids, such as albumin, Hb, gum arabic, etc., in their ability to spontaneously pass into a dissolved state, are even closer to true solutions of crystalloids, however, differing from them in the absence of constant solubility. Classification of K. according to these various characteristics in many cases gives the same results. Hydrophobic or lyophobic K. are at the same time irreversible; hydrophilic or lyophilic are more often reversible. Due to the binding of a large amount of water, hydrophilic particles at the same time acquire an emulsion character, while hydrophobic ones can retain the properties of a solid and give a suspension sol. However, K. does not always have the entire set of characteristics characteristic of typical representatives of one or another group. In particular, the division of chemicals according to the aggregate state of the dispersed phase may not coincide with the classification according to a more significant feature for them - affinity for the solvent. Thus, emulsoids are known that do not have 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 both groups of colloids, must be considered. Methods for preparing colloidal solutions. A wide variety of substances, both organic and inorganic, can be obtained in a colloidal state using special techniques. The most important task in the preparation of colloidal solutions is to achieve the required degree of dispersion and create particles of the proper size. According to their intermediate position between true solutions and roughly heterogeneous systems, particles of colloidal sizes can be obtained both from the former by condensing their ions and molecules into larger aggregates, and by dispersing the latter. Accordingly, Svedberg, who developed and systematized the methods of preparing sols in particular detail, divided them into condensation and dispersion. In the presence of an excess of their solvent, crystalloids break down into individual molecules. They must be insoluble in the dispersion medium in order to form larger aggregates. Therefore, condensation methods are based on chemistry. reactions that convert soluble compounds into insoluble ones. Most often these are recovery reactions. They are used, for example, to obtain hydrosols of noble metals. Many reducing agents (such as hydrazine, hydroquinone, pyrogallol) act in the cold, others (ethyl alcohol, formaldehyde, tannin, etc.) act when heated. To prepare a gold sol, an extremely dilute solution of some gold salt is treated with a reducing agent, for example. AIS1 3 or NAiS1 4 (at a concentration of 0.1 G or even less per 1 liter of water). In a similar way, silver hydrosols are prepared from AgN0 3 and from the corresponding salts and sols of other metals. It was also possible to obtain colloidal metallic silver using transmission as a reducing agent. wash through the solution with hydrogen gas. Various other chem. reactions (oxidation, double exchange decomposition) can lead to the same result—the production of insoluble substances in a colloidal distribution. Thus, colloidal sulfur is obtained by decomposition of sodium sulfate (Na 2 S 2 0 3) with concentrated sulfuric acid. This sulfur sol was studied in great detail by Sven Oden. The object of numerous studies has also been colloidal arsenic trisulfide (As 2 S 3), which is obtained by the action of hydrogen sulfide on arsenous acid (excess hydrogen sulfide is then displaced by passing a hydrogen current through the solution): As 2 0 3 +3H 2 S =As 2 S 3 + 3H 2 0. The hydrolysis reaction is also often used, in which one of the substances that enters into metabolic decomposition is water. This method produces iron hydroxide (FeCl 3 + +3H 2 0 = Fe(OH) 3 +3HCl) and many other sols. No matter how different the chemical reactions used are, they all boil down to the formation of substances insoluble in a given dispersion medium. According to Weymarn, the condensation mechanism 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 stop before they reach microscopic size. sizes. Since if the concentration of colloidal substances is too high, they quickly completely fall out, very weak concentrations of the reacting substances are usually used to prepare sols. Finally, one should not lose sight of the fact that in all the considered processes of sol formation, electrolytes take part as starting substances or reaction products. As will be clarified below, they cannot be considered as extraneous, accidental impurities. On the contrary, electrolytes take an active part in the construction of colloidal particles and in determining their chemical properties. and electrical properties, in maintaining K resistance. Complete removal of all electrolytes usually leads to the destruction of the sol, to its transition to a more coarsely dispersed state. Electrolytes play the role of dispersants, maintaining the required degree of dispersion of colloidal particles. Another group consists of dispersion methods. The problem of mechanical fragmentation of substances - although not to very high degrees of dispersion - was recently (1920) successfully solved by means of the so-called device. "colloidal mill". It represents a rapidly rotating shaft with blades inside the liquid, which, when rotating, pass near fixedly reinforced protrusions (without touching them). The body, agitated in the liquid, is crushed to colloidal sizes by the impacts of the blades on the water. The electrical method is most often used. It was first used by Bredig for the preparation of sols of noble metals. By immersing electrodes of the sprayed metal in water, he passed a voltaic arc between them. At the same time, clouds of sputtered particles of both colloidal and larger sizes rise from the cathode. Due to the strong heating of the solution, it must be cooled. Apparently, as Bredig himself believed, the main role is played here by thermal processes: evaporation of the metal in a voltaic arc followed by condensation of its vapor in water. That. in its mechanism, this method actually approaches condensation. Svedberg greatly improved the method of electric spark atomization, Ch. arr. by using an oscillatory discharge. Using it, he managed to prepare a large number of different sols, in particular organosols (for example, etherosols) of alkali metals, the preparation of which was very difficult. Dispersion methods also include the peptization method (see below). Biocolloids. The described methods make it possible to prepare various artificial or synthetic compounds. An extensive and very important group of natural compounds behaves completely differently. This group includes various biocolloids—organic substances of such complex composition that even their single molecules or ions have dimensions characteristic of colloidal particles and, as a result, This is colloidal properties. Biocolloid sols are therefore prepared in exactly the same way as conventional crystalloid solutions, by treating them with a suitable solvent. Most often, this solvent is water. Gums, starch, gum arabic, agar, tannin, gelatin, albumin dissolve in cold or hot water, forming hydrosols. In other cases, it is necessary to use special solvents: an ammonia solution of copper oxide (“Schweitzer’s reagent”) for cellulose, acetone, acetic acid or a mixture of alcohol and ether for nitrocellulose, benzene or carbon disulfide for rubber, etc. Cleaning methods K In most cases, conventional chemical cleaning methods are not applicable to K. substances. Only a few colloids (namely, certain biocolloids) can be separated from each other and isolated, due to their unequal solubility in certain solvents, by fractional precipitation or crystallization. Much more often it is necessary to use special colloidal methods. They are based on the inability of colloidal particles to penetrate colloidal membranes permeable to crystalloids. If such a membrane, with a solution to be purified inside, is washed from the outside with clean distilled water, then the crystalloids contained in the colloidal solution will diffuse into the latter through the membrane. By changing the water several times, you can dialysis(see) gradually remove from the colloidal solution almost all diffusion-capable impurities. Another method of cleaning K. is ultrafiltration. The solution is filtered under b. or 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 also be significantly accelerated in this case by using a mechanical filter to push the liquid through an ultrafilter instead of a mechanical one. pressure by electroosmosis; This method is called electroultrafiltration. Optical properties. In transmitted light, colloidal solutions often appear completely transparent and homogeneous, like true solutions. However, their heterogeneity is clearly revealed in reflected light: when viewed from the side of the light incident on them, colloidal solutions appear cloudy and opalescent. The optical heterogeneity of colloidal solutions becomes even clearer if you direct a bright beam of light at them (a sunbeam or an electric arc lamp beam concentrated by a collecting lens) and observe the liquid from the side: the entire path of the beam in the colloidal solution glows with an even opalescent light. Faraday first used this technique to detect tiny particles in turbid environments. By the name of Tyndall (Tup-dall), who studied the described phenomenon in detail, this luminous cone is usually called the Tyndall cone (see. Tyndall phenomenon). All colloidal solutions give 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 natural relationship studied by Chap. arr. Rayleigh, between the size of dispersed particles and the color of the light scattered by them. This color can be superimposed on the cell’s own color, which depends on its absorption of a certain part of the spectrum. An example of such a phenomenon is colloidal solutions of mastic, yellow or brown in transmitted light, bluish in reflected light. This kind of coloring, noticeable in some cases in non-conductors, is most clearly manifested and reaches particular intensity in colloidal metals. It depends on the optical properties of the metal, on the size of its particles and ch. arr. from their combination into larger aggregates. A particularly strong influence is exerted by the degree of dispersion, with a change in which the color changes correctly. Colloidal solutions of gold, e.g. As the size of its particles changes, a whole range of different colors pass through. Zsigmondy was able to prepare a particularly complete series of its solutions with a degree of dispersion uniformly varying within enormous limits. Coarsely dispersed gold gives the solution a blue or violet color, while highly dispersed gold gives it a pure and bright red color (this is the same origin of the color of gold “ruby glass”). With a further reduction in particle size and approaching molecular dispersion, a brown or yellow color is achieved, characteristic of true solutions of gold salts. Various colloidal solutions of silver have no less varied colors (red, brown, violet, green, black). To give an idea of their intensity, it is enough to point out that the brown color of colloidal silver is clearly visible in a layer 1 cm with a content of 1 part silver per 5 million parts of water. Size of colloidal particles. An ultramicroscope is based on the scattering of light by tiny particles, in which a strong concentrated beam of light illuminates the solution under study from the side and passes through it without entering the microscope lens. In the field of view of the microscope, so is observed. Tyndall's cone. At the focal point where the rays converge, the maximum illumination intensity makes the smallest submicrons visible. In an immersion ultramicroscope, using this method it turned out to be possible to observe the smallest submicroscope particles, only a few in size. shopping center Submicrons appear in an ultramicroscope as luminous points that give no idea of their shape or their true size. To determine the latter, the number of individual particles located 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 an approximately spherical shape). In addition to this optical method, there are mechanical techniques that make it possible to determine the size of colloidal particles. The ultrafiltration method mentioned above is used for this. Ultrafilters are not always impermeable to all colloidal substances. Bechhold was the first to show that by using a series of ultrafilters with different, successively changing pore sizes, it is possible to carry out fractional ultrafiltration: to separate some cells from others. The ultrafilter, which retains a given particle, does not allow through all those that have larger particles. Having calibrated a series of ultrafilters (for example, using colloidal solutions with known particle sizes), it is possible to determine the size of particles in the colloidal solution under study based on the ability of the latter to pass through certain ultrafilters. Further, the size of the particles can be judged by the speed of their fall. According to the Stokes formula, the speed at which a spherical body (of fairly small size) falls in a liquid is proportional to the square of its diameter. Therefore, the size of a particle can be determined by the speed of its fall (provided that the specific gravity of the falling body and liquid, as well as the viscosity of the latter, are also known). This method is not directly applicable to particles of colloidal sizes, since their settling rate is too negligible. However, using instead of force cha- | tin has a very significant centrowhite. force, it is possible to accelerate the sedimentation of K. and make it accessible to measurement. This method is called "ultracentrifugation". The listed methods of directly counting the number of colloidal particles and their mechanical separation by filtration or centrifugation do not differ essentially from similar methods applied to coarser microscopes. suspensions. However, along with this, the methods used to measure the molecular weight in solutions of crystalloids are also used to measure the size of colloidal particles. The ultramicroscope made particles of colloidal solutions (“submicrons”) visible and thus seemed to deepen the difference between them and optically insoluble crystalloid solutions. However, at the same time, he made it possible to extend 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 studying Brownian motion (cm.). As studies by Einstein, Smoluchowski, Perrin and others have shown (Einstein, Smoluchowski, Perrin), it represents real molecular movement, the faster the closer the diameter of the particles approaches the molecular size. A study of the Brownian motion of colloidal particles showed that their kinetic energy does not depend on their size and is equal to the kinetic energy of molecules in true solutions (at the same t°). Therefore, the osmotic pressure of K. is proportional to the concentration of colloidal particles. Knowing the total weight of dissolved carbon and its density, it is possible to determine their size by the number of particles. However, measuring K.'s osmotic pressure presents significant difficulties and cannot always be made with sufficient accuracy. In contrast to kinetic energy, the rate of diffusion decreases as the size of colloidal particles increases and represents a further path to determining the latter. Shape and structure of colloidal particles. When calculating the diameter of a colloidal particle, it was usually assigned a spherical shape. It was accepted that, in contrast to crystalline bodies, crystalline particles are amorphous and, under the influence of surface forces, take on a spherical shape corresponding to the minimum free surface. For the first time, Nageli expressed the view that a colloidal particle, or mycelium, is the smallest ultra-microscopic particle. crystal. Nägeli explained the crystalline properties, in particular birefringence, found in many organic substances and life structures by the fact that these substances are built from tiny particles invisible under a microscope (in modern terminology “submicrons”), crystalline mycels. These micelles play the same role in colloidal systems as molecules do in true solutions. In contrast to molecular solutions, colloidal systems are, according to Nägeli’s expression, “micellar solutions.” By connecting with each other, micelles can maintain strict, correct orientation and grow into real crystals or into organic fibers that have certain crystalline properties. With rapid connection, they often grow together into chaotic, irregular, often tree-like branched complexes, making up, for example. gel base. Despite the primary crystalline structure, in this case they turn out to be externally amorphous. Nägeli's views, which did not initially receive recognition, were then resurrected by Weimarn, Ambronn, Scherrer. The crystalline nature of many, although still not all, colloidal submicrons has been proven by a variety of methods. Amorphism is not considered a more characteristic feature of the colloidal state, and mycelium represents the basic concept in the modern understanding of the structure of colloids. Without dwelling here in more detail on the crystal structure of mycelium, it should be pointed out that in very many cases it can actually be detected. The most reliable method for studying the structure of crystals is currently X-ray diffraction. Crystals are characterized by a regular arrangement of atoms or ions, fixedly fixed at equal distances from each other. The regular geometric shape of crystals is the outer expression of this spatial crystal lattice of atoms. It causes diffraction of X-rays incident on the crystal. rays, just as the diffraction spectrum of visible light is obtained using coarser artificial diffraction re-shots. Contrary to the old idea of the amorphous nature of crystals, using this method (developed by the chief scientist Scherrer), the crystalline structure of particles of very many crystals (for example, colloidal gold, silver, and many others) was established with certainty. Along with this, some K. actually consist of amorphous particles. The crystal lattice must be accompanied by an external crystalline form. It can be clearly detected in those cases when it sharply deviates from the spherical: namely, when one of the crystal axes is strongly developed or, on the contrary, very shortened in comparison with the other two. In the first case, the colloidal particle has a rod-shaped form, in the second case, it has a lamellar shape. If, under the influence of some external force, they are located with their longitudinal axes parallel to each other, then their shape can be determined by the phenomena of light polarization produced by such a solution. A similar parallel orientation of crystalline particles is obtained, for example. in a flowing liquid due to the friction that occurs during movement. Changes in the degree of dispersion that often occur in colloidal solutions lead to different characteristics of the structure of colloidal particles. As a crystal grows, its shape remains unchanged; in the same way, when emulsion droplets merge, they again form the same spherical drops. On the contrary, when the degree of dispersion of a colloidal solution decreases, which occurs by combining its particles, the latter come into contact with only a few of their points and give a loose, flocculent compound. Therefore, from primary colloids of modern particles, the shape and structure of which were discussed above, one should distinguish secondary particles formed by the flocculent combination of two or more primary ones. For secondary colloidal particles, Zsigmondy proposed the name polions, for primary ones - monomons, or protons (the latter name cannot be retained, because it serves to designate a unit of positive electricity - the atomic nucleus N; 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 color change is provided by gold hydrosol. Colloid processes. If in the doctrine of structure an increasingly complete analogy was established between colloidal systems and crystalloid solutions, then a deep difference remained in the nature of the forces acting in them and the processes occurring in them. As is known, significant attractive forces arise between closely adjacent molecules, which quickly decrease with distance. Mutually equilibrating in the middle of molecular aggregates, they appear on their surface in the form of surface tension. Instead of osmotic pressure, which represents the main type of mechanical energy in true solutions, with the colloidal distribution of a substance this surface energy becomes especially important, directly depending on the size of the boundary surfaces, and therefore on the degree of dispersion of the colloid, on the size of its particles. No less important is the boundary potential difference, the electric charge on the surface of colloidal particles. The energy of a colloidal system (primarily surface energy) therefore turns out to be a function of the degree of dispersion of the colloid. As a result, various energy changes in colloidal systems (especially changes in electrical charge) have as their immediate result rapid changes in their dispersity, the combination of small colloidal particles into larger aggregates or, on the contrary, the disintegration of the latter (peptization). In these characteristic colloidal processes, the ease of changing the degree of dispersion lies their main difference from the stable molecular distribution of the so-called. true solutions. The idea of the predominant influence of surface, capillary (and electrocapillary) forces was developed in its most extreme form by Freundlich, who interpreted all colloidal chemistry as “capillary chemistry.” The idea of their dependence on capillary forces and the inapplicability of general chemical laws to them was extended to purely chemical processes occurring in colloidal systems. Instead of combining reactants in simple equivalent ratios, adsorption compounds, quantitatively expressed by the Freundlich adsorption isotherm, were considered characteristic of colloids. The properties of colloids are especially strongly influenced by influences that change their hydrophilicity and the affinity between the colloid and the solvent. The study of the features of the colloidal chemical action of electrolytes is associated mainly with the name of Hofmeister (see. Chamberlain ranks).-A completely opposite position was taken by another group of researchers, among whom Pauli should be mentioned first of all. According to these researchers, when eliminating numerous sources of errors, which in many cases obscure the picture, general chemistry is quite applicable to colloidal systems, in particular to the practically most important of them, protein solutions. laws. In terms of chemistry, there is no fundamental opposition between them and crystalloids, just as there is none in terms of other properties. Loeb was especially consistent with this point of view. The theoretical basis for a completely new interpretation of colloidal processes was Loeb's Donnan principle, which establishes a special form of equilibrium of ions on both sides of a membrane impermeable to one of them (see. Donnan's equilibrium). A number 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 colloidal membranes and gels. Colloidal properties are found when there is a barrier that retains a given (colloidal) ion, but is permeable to other ions present. Only under such conditions does the solution behave like a colloid. In this sense, Loeb is not even talking about the “colloidal state”, but about the “colloidal behavior” of protein solutions. Electric charge. The application of electrical forces to colloidal solutions shows that colloidal particles carry positive or negative charges and therefore move in an electric field (see Fig. 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 definitively clarified; Apparently it is not the same in all cases. Often the charge depends on the chemical. nature of the colloidal particle. Substances that are acidic in nature, e.g. tannin, mastic, flint, acquire a negative charge in clean water; basic substances such as metal hydroxides (iron, aluminum, etc.) are positive. Obviously, despite the apparently complete insolubility of these substances, a small amount of hydrogen or hydroxyl ions goes into solution, leaving a charge of the opposite sign on the colloidal particle. In most cases, what matters most is adsorption(see) electrolytes present in solution on the surface of a colloidal particle: the more strongly adsorbed ion imparts its charge sign to it. The greatest activity in this regard is shown, on the one hand, by multivalent cations of heavy metals, and on the other, by certain multivalent anions. Finally, we should mention Coehn’s rule, according to which if a colloidal system consists of two nonconductors, then a substance with a large dielectric constant acquires a positive charge (see Fig. Dielectrics). Since water has a very high dielectric constant, greater than most of them, the latter (in the absence of the first two causes of charge) 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 opposite charge of the adjacent liquid layer, and both opposite charges form an electric double layer(cm.). Chemical composition of a colloidal particle. The electric charge, which determines many of the properties of a cell, depends in turn on the chemical. the composition of both the colloidal mycelium itself and the surrounding (“intermicellar”) liquid. However, the usual designation of K. does not yet give a sufficient idea of its chemistry. composition. Eg. when they talk about arsenic sulfide ash or iron hydroxide, then these substances really constitute the main, quantitatively predominant part of the mycelium. However, the latter contains, along with them, a small admixture of electrolytes, the composition and concentration of which depend on the method of preparation (or further processing) of the colloid. These electrolytes, often adsorbed in minute quantities on the surface of the colloidal particle, represent its active part, which determines a number of its most important properties. Zsigmondy proposed, when designating K., to encircle with a square frame the formula of the bulk of the colloidal substance (established by the usual chemical analysis of its sediment), placing the active, ionic part of the mycelium outside this frame. Thus, with the methods described above for preparing arsenic sulfide sol, the active part is an admixture of hydrogen sulfide, the partial dissociation of which (into HS" and H") imparts a negative charge to K. Without fixing the quantitative relationship between As 2 S 3 and H 2 S (which can vary within very wide limits), for the corresponding K. the formula is obtained: |As 2 S:i | HS" + H". Similarly, iron hydroxide mycela has the composition jFe(OH) 3 l Fe""" + ZSG. As the above formulas show, a micelle is understood not only as the bulk of a colloidal particle along with the ions adsorbed by it, but also ions of the opposite sign that form the outer lining of the double layer. For only one charged colloidal particle without oppositely adjacent ones charged ions in French, the authors use the name “granule.” The granule is represented as a giant colloidal ion. Pauli proposed to call the opposite crystalloid ions in solution “counterions” (Gegenionen). equilibrium with the concentration of the same substances in the surrounding liquid. Therefore, no matter how small this concentration is in thoroughly dialyzed sols, it still cannot be equal to zero. Thus, the intermicellar liquid contains at least in a very low concentration the same electrolytes that make up the active one. part of the mycelium; it is never pure WATER. Factors of stability K. Microscope. suspensions, for example a suspension of red blood cells in the blood settles at a significant rate. But as the size of the particles decreases, the speed of their fall decreases rapidly. For particles of colloidal sizes it is negligible, and the solution can retain b. or m. uniform distribution. This is also facilitated by Brownian motion, which mixes submicroscopic particles in the same way that molecular motion mixes the molecules of a true solution. However, a number of influences can cause extremely rapid, almost instantaneous loss of calcium from solution. Their effect boils down to the fact that they cause agglutination of colloidal particles, combining them 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 primarily an electric charge. Electrostatic repulsion forces prevent like-charged particles from joining together. A number of studies have shown that the boundary potential of colloidal particles should fall below a known limit. n. critical potential to make coagulation K possible. When the charge decreases below this critical value, particles in Brownian motion can combine with each other upon collision. However, at first, apparently only a small percentage of collisions (the most powerful or central impacts) lead to connection. With a further decrease in the boundary potential, this percentage (and with it the coagulation rate) quickly increases, approaching a constant limit. The latter is achieved when each collision of colloidal particles ends in their connection. Due to this stabilizing influence of the electric charge, changes in its sign or magnitude have a decisive influence on many colloidal processes. As mentioned above, the electrolyte adsorbed by the colloidal particle represents the active part of the micelle, imparting electrical energy to it. charge and the durability that determines it. If, through prolonged dialysis, K. 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 K. adsorbs an oppositely charged ion, neutralizing its own electrical energy. charge. The precipitate that has formed can be transferred back into solution if it is exposed to an electrolyte, one of the ions of which is strongly adsorbed and again charges the colloidal particles. A similar effect can often be produced even by the same electrolyte that caused the precipitation. The first portions of it neutralize the charge of the colloidal particle and therefore have a coagulating effect; subsequent ones cause the appearance of a new charge (of the opposite sign) and as a result dissolve. This dissolution of the colloidal precipitate by treating it with a stabilizing electrolyte is called “peptization.” Peptization is one of the most important dispersion methods for the preparation of colloidal solutions. - While for the stabilization of hydrophobic (or lyophobic) K. the electric charge is of decisive importance, for hydrophilic K. another, no less important factor is added to the influence of the charge. This factor is the very hydrophilicity of the solution, the affinity between the solution and the solvent, i.e. the same factor that determines the stability of true solutions. To precipitate hydrophilic colloids, to which most biocolloids belong, it is necessary to eliminate both stability factors—hydrophilicity and charge. The hydrophilicity of protein solutions can be eliminated both by reversible removal of water (for example, by the action of alcohol) and as a result of irreversible chemical reactions. changes (see Denaturation). In both cases, deposition with electrolytes is then carried out in the same way as in the case of hydrophobic compounds. The influence of ions on the hydrophilicity of compounds is especially pronounced in the so-called. Chamberlain's ranks(cm.). For the stabilizing effect of certain colloids on solutions of other colloids, see Protective action. Biological significance of eK. It must be said that while the basic principles of the study of the structure of colloidal systems are now firmly established, the mechanism of the most important colloidal processes remains highly controversial. The relationship between colloidal chemistry and general chemistry, the role of adsorption and chemistry, the significance of capillary forces and the Donne principle - all these issues continue to be the subject of not only experimental research, but also fierce theoretical debate. The rapid development of colloidal chemistry, which in a short time turned into an independent extensive scientific discipline, is explained in Chap. image. the interest it represents for biol. Sci. A living organism consists of colloidal substances, and the study of the colloidal substrate of life constitutes a necessary basis for understanding life phenomena. Research Physiol. actions ions(see), as well as most other fiziol. agents, shows that it completely coincides with the influence of the same influences on biocolloids. This determines the enormous interest that chemistry acquires for understanding the processes occurring in a living organism. Numerous complex biol. problems can be studied using simple colloidal models, and it is not surprising that a number of biologists not only used the results obtained by colloidal chemistry in their work, but took an active part in the development of this science. Lit.: Alexander D., Colloid Chemistry, Leningrad, 1926; Andreev II., Introduction to colloidal chemistry, Moscow, 1924; B e i l and V., Colloidal state in medicine and physiology, M.-L., 1925; Handovsky H., Basic concepts of colloidal chemistry, Berlin, 1925; G a t h e k E., Introduction to the physics and chemistry of colloids, M.-L., 1927; Duclos J., Colloids, M., 1924; Joël E. (.Toyo1E.), Clinical colloidal chemistry, Berlin, 1923; Kurbatov V., Chemistry of colloids and jellies, L., 1925; Mikh ae-l and with L., Workshop on physical chemistry, L., 1925; Naumov V., Colloid Chemistry, Leningrad, 1926; Ostwald V., A brief practical guide to colloid chemistry, L., 1925; Pauli V., Proteins and colloids, M.-L., 1928; Peskov N., Colloids, Ivanovo-Voznesensk, 1925; P e sh l V., Introduction to colloidal chemistry, Odessa, 1912; Przheborov-s k and y Ya., Introduction to physical and colloid chemistry, M.-L., 1928; Svedberg T., Formation of colloids, Leningrad, 1927; S h a d e G., Physical chemistry in internal medicine, Leningrad, 193 0 (German publisher-Dresden-Lpz., 1923); Alexander J., Colloid chemistry, v. I-Theory a. methods, v. II-Biology a. medicine, N.Y., 1926-28; Bechhold H., Die Colloid in Biologie und Medizin, Dresden-Lpz., 1929; Freundlich H., Kapillarchemie, Dresden, 1923; aka, Kolloidchemie u. Biologie, Dresden-Lpz., 1924 (Russian publishing house - Leningrad, 1925); about n e, Grundzuge der Kolloidlehre, Lpz., 1924; aka, Fortschritte der Kolloidchemie, Dresden-Lpz., 1927; H e i 1 b r u n n L., The colloid chemistry of protoplasm, Berlin, 1928; Kolloidforschungen in Einzeldarstellungen, hrsg. v. R. Zsigmondy, Lpz., since 1926; Lepeschkin W., Kolloidchemie des Protoplasmas, V., 1924; L i e s e-gang R., Biologische Kolloidchemie, Dresden-Lpz., 1928; Loeb J., Protein and the theory of colloidal behavior, N. Y., 1922; Ostwald Wo., Grundriss der Kolloidchemie, Dresden-Lpz., 1909; Pauli W.u. V a 1 k 6 E., Elektrochemie der Kolloide, V., 1929; Svedberg Th., Methoden zurHerstellungkolloider LOsungen, Dresden, 1909; Zsigmondy R., Kolloidchemie, T. 1-2, Lpz., 1925-27 (lit.). Periodical publication. - Kolloid-Zeitschrift, Dresden-Lpz., from 1906 (until 1913 - under the name Zeitschrift f. Chemie u. Industrie der Kolloide; from 1910 it gives the supplement - Kolloidchemische Beihefte). D. Rubinstein.Colloidal systems are widespread in nature and have played an important role in human life since the appearance of humans.
Studying the properties of mixtures of water - silver chloride, water - sulfur, water - Prussian blue, etc., the Italian scientist F. Selmi (1845) established that under certain conditions they form systems that are homogeneous in appearance, similar to solutions. However, these systems, unlike aqueous solutions of sodium chloride, copper sulfate and other substances readily soluble in water, do not form spontaneously. F. Selmi proposed to call such systems pseudosolutions.
T. Graham (1861), studying such systems, found that some substances (potassium hydroxide, potassium sulfate, magnesium sulfate, sucrose, etc.) have a high diffusion rate and the ability to pass through plant and animal membranes, while others (proteins, dextrin,
gelatin, caramel, etc.) are characterized by a low diffusion rate and lack of ability to pass through membranes.
The first group of substances crystallizes quite easily, while the second, after removing the solvent, forms glue-like masses. The first one T. Graham called crystalloids, and the second - colloids(from the Greek "κολλά" - glue, "λεδεσ" - view). Crystalloids form true solutions, while colloids form sols (colloidal solutions).
In 1899, the Russian scientist I.G. Borshchov suggested that many substances capable of forming colloidal solutions have a crystalline structure, and therefore we should not talk about special colloidal substances, but about the colloidal state.
At the beginning of the last century, professor of the St. Petersburg Mining Institute P. P. Weymarn experimentally proved that the division into colloids and crystalloids is very arbitrary. Typical crystalloids NaCl, KΙ, etc. can form colloidal solutions in suitable solvents, for example, a colloidal solution of NaCl in benzene.
Finally, it was proven that the same substance in the same solvent, depending on a number of conditions, can manifest itself as both a colloid and a crystalloid. It was proposed to call such substances semicolloids. Colloidal solutions (colloidal systems) are a special case of dispersed systems.
A disperse system is a system consisting of a dispersed phase - a collection of crushed particles and a continuous dispersion medium in which these particles are suspended.
To characterize the fragmentation of the dispersed phase, use degree of dispersion 8, which is measured by the reciprocal of the average particle diameter c1
The solutions discussed above are systems in which the solute breaks down into individual molecules and ions. There is no boundary (interface) between the solute and the solvent, and the solution is a single-phase system, since the concept of surface is not applicable to individual atoms, molecules and ions. In a liquid medium there may be aggregates of substances consisting of a large number of molecules and ions. Particles having a diameter of the order of 1 micron (10 -6 m) exhibit the usual properties of a given substance. In the case of a solid, these particles are crystals, and in the case of a liquid, they are small droplets. Particles of this size contain millions of structural units. When formed in solution as a result of chemical reactions, they quickly settle to the bottom of the vessel.
Substances acquire special properties if the particles have a size of 10 -9 -10 -7 m (1 - 100 nm). Systems consisting of particles of this size
measure is called colloidal dispersed. The total surface area of a system consisting of particles of this size reaches an unusually large value. For example, 1 g of a substance with a particle size K) -8 m will have a surface of the order of several hundred square meters.
Based on the degree of dispersion, two groups of systems are distinguished: coarsely dispersed and colloidal-dispersed.
Systems with particle sizes smaller than 10 -9 m are sometimes incorrectly called ionic-molecular dispersed systems. These systems lack the main characteristic feature of dispersed systems - heterogeneity. Therefore, such systems are homogeneous and are called true solutions.
Depending on the state of aggregation of the dispersed phase and the dispersion medium, eight types of colloidal systems are distinguished (Table 23.2).
It should be noted that colloidal systems formed by gases do not exist under normal conditions for the reason that gases mix with each other indefinitely.
Table 23.2
Classification of colloidal systems according to the state of aggregation of phases
|
Aggregate state |
System type |
Aggregate state of the dispersed phase |
Conditional designation |
Name |
|
Aerosol |
||||
|
Liquid |
||||
|
Solid |
||||
|
Liquid |
||||
|
Liquid |
Emulsoid |
|||
|
Solid |
Suspensoid |
|||
|
Solid |
Solidozol |
Solid foam |
||
|
Liquid |
||||
|
emulsoid |
||||
|
Solid |
Untitled |
Methods for obtaining and purifying colloidal systems. To obtain colloidal solutions it is necessary: 1) to achieve a colloidal degree of dispersion; 2) select a dispersion medium in which the substance of the dispersed phase is insoluble; 3) select the third component - a stabilizer, which imparts stability to the colloidal system.
Metals, poorly soluble oxides, hydroxides, acids, and salts can form colloidal solutions in water. Substances that prevent the aggregation (combination) of colloidal particles into larger ones and their precipitation are used as stabilizers.
According to the method of achieving the colloidal degree of dispersion, methods are distinguished (Fig. 23.22):
- - dispersive (from the Latin "sPare^ge" - to grind) - obtaining dispersed phase particles by crushing larger particles;
- - condensation (from Latin - to enlarge) - obtaining dispersed phase particles by combining atoms, molecules, ions.
Rice. 23.22.
Colloidal solutions obtained by one of the methods considered contain impurities of dissolved low-molecular substances and coarse particles, the presence of which can negatively affect the properties of the sols, reducing their stability. To purify colloidal solutions from impurities, filtration, dialysis, electrodialysis and ultrafiltration are used.
Filtration is based on the ability of colloidal particles to pass through the pores of conventional filters. In this case, larger particles are retained. Filtration is used to purify colloidal solutions from impurities of coarse particles.
Dialysis is the removal of truly dissolved low molecular weight compounds from colloidal solutions using membranes. In this case, the property of membranes to allow molecules and ions of normal sizes to pass through is used. All dialyzers are built on a general principle: the dialyzed fluid is in an internal vessel, in which it is separated from the solvent by a membrane (Fig. 23.23). The rate of dialysis increases with an increase in the membrane surface, its porosity and pore size, with an increase in temperature, mixing intensity, rate of change of external liquid and with a decrease in membrane thickness.
To increase the rate of dialysis of low molecular weight electrolytes, a constant electric field is created in the dialyzer. The dialysis rate can be increased if the dialyzed solution is forced through a membrane (ultrafilter). This method of purifying systems containing particles of colloidal sizes from solutions of low molecular weight substances is called ultrafiltration.
Rice. 23.23.
- 1 - dialyzed fluid: 2 - solvent; 3 - dialysis membrane;
- 4 - mixer
Introduction
Pure substances are very rare in nature. Colloidal systems occupy an intermediate position between coarse systems and true solutions. They are widespread in nature.
The global role of colloids in natural science lies in the fact that they are the main components of such biological formations as living organisms. Our entire body consists of colloidal systems. There is a whole science - colloidal chemistry. The question immediately arose before me: why does nature prefer the colloidal state?
In this regard, the following goals and objectives arise:
Purpose of the work: to find out what colloidal systems are and what properties they have.
Objectives: 1. Conduct experimental experiments to study the properties of colloidal solutions.
2. Answer the question: why does nature prefer the colloidal state.
Types of colloidal solutions
The term "colloid" was introduced in 1861 by the English chemist Thomas Graham. In his experiments, he noticed that solutions of gelatin, starch and other glue-like substances were very different in a number of properties from solutions of inorganic salts and acids. The name comes from the Greek prefix “kolo” - glue. It is correct to speak not about colloidal substances, but about colloidal systems. This term was introduced by the Russian scientist P.P. Weimarn in 1908. A variety of colloidal systems can be seen in the pictures.
Particles of colloidal sizes can have different internal structures. There are several main types of colloidal systems:
- 1) smoke is a stable dispersed system consisting of small solid particles suspended in gases. Smoke is an aerosol with solid particle sizes ranging from 10?7 to 10?5 m. Unlike dust, a more coarsely dispersed system, smoke particles practically do not settle under the influence of gravity
- 2) aerosol - a dispersed system consisting of small particles suspended in a gaseous environment, usually in the air. Aerosols, the dispersed phase of which consists of liquid droplets, are called mists, and in the case of solid particles, if they do not precipitate, they speak of fumes (freely dispersed aerosols) or dust (coarsely dispersed aerosols).
- 3) emulsion - a dispersed system consisting of microscopic droplets of liquid (dispersed phase) distributed in another liquid. The most common representative of this type of colloidal system is milk.
- 4) foam - dispersed systems with a gas dispersed phase and a liquid or solid dispersion medium.
- 5) gel - systems consisting of high-molecular and low-molecular substances. Due to the presence of a three-dimensional polymer framework (mesh), gels have some of the mechanical properties of solids (lack of fluidity, ability to retain shape, strength and ability to deform (plasticity and elasticity).
- 6) suspension is a coarsely dispersed system with a solid dispersed phase and a liquid dispersion medium.
Particles of colloidal sizes can have different internal structures, which significantly affects both the methods for preparing colloidal solutions and their properties. There are the following three types of internal structure of primary particles of colloidal sizes.
Type I - suspensionoids (or irreversible colloids, lyophobic colloids). This is the name for colloidal solutions of metals, their oxides, hydroxides, sulfides and other salts. The primary particles of the dispersed phase of colloidal solutions of these substances in their internal structure do not differ from the structure of the corresponding compact substance and have a molecular or ionic crystal lattice. Suspensoids are typical heterogeneous highly dispersed systems, the properties of which are determined by a very highly developed interphase surface. They differ from suspensions in their higher dispersion. They were called suspensions because, like suspensions, they cannot exist for a long time in the absence of a dispersion stabilizer. They are called irreversible because the precipitates remaining during the evaporation of such colloidal solutions do not form a sol again upon contact with the dispersion medium. They were called lyophobic (Greek “lios” - liquid, “phobio” - hate) on the assumption that the special properties of colloidal solutions of this type are due to the very weak interaction of the dispersed phase and the dispersion medium. The concentration of lyophobic sols is low, usually less than 0.1%. The viscosity of such sols differs slightly from the viscosity of the dispersion medium.
Type II - associative, or micellar, colloids. They are also called semicolloids. Colloidal particles of this type arise with a sufficient concentration of amphiphilic molecules of low molecular weight substances through their association into aggregates of molecules - micelles - spherical or lamellar in shape (Fig. 10.4)
Molecular, true solution - Micellar colloidal solution (sol).
Micelles are clusters of regularly arranged molecules held together primarily by dispersion forces.
The formation of micelles is typical for aqueous solutions of detergents (for example, soaps - alkaline salts of higher fatty acids) and some organic dyes with large molecules. In other media, for example in ethyl alcohol, these substances dissolve to form molecular solutions.
Type III - molecular colloids. They are also called reversible or lyophilic (from the Greek “filio” - love) colloids. These include natural and synthetic high-molecular substances with a molecular weight from ten thousand to several million. The molecules of these substances have the size of colloidal particles, therefore such molecules are called macromolecules.
Dilute solutions of high-molecular compounds are true, homogeneous solutions, which, when diluted to the limit, obey the general laws of dilute solutions. Solutions of high molecular weight compounds can also be prepared with a high content by weight - up to ten percent or more. However, the molar concentration of such solutions is low due to the high molecular weight of the solute. Thus, a 10% solution of a substance with a molecular weight of 100,000 is only approximately 0.0011 M solution
The dissolution of macromolecular colloids passes through the swelling stage, which is a characteristic qualitative feature of substances of this type. When swelling, solvent molecules penetrate the solid polymer and push the macromolecules apart. The latter, due to their large size, slowly diffuse into the solution, which is externally manifested in an increase in the volume of the polymer. Swelling can be unlimited, when its end result is the transition of the polymer into solution, and limited, if swelling does not reach the dissolution of the polymer. Polymers with a special, “three-dimensional” structure, characterized by the fact that the atoms of the entire substance are connected by valence bonds, usually swell to a limited extent. Chemical modification of polymers by “cross-linking” their macromolecules in order to reduce the swelling of the polymer is an important stage in the production of many materials (tanning rawhide, vulcanizing rubber when turning it into rubber).
The term “colloids”, which means “glue-like” (from the Greek “colla” - glue, “eidos” - kind), arose in 1861 Γ..; when T. Graham used dialysis to separate substances (Fig. 10.5).
The dialysis method is based on the unequal ability of the components of solutions to diffusion through thin films - membranes (made of cellophane, parchment, nitrocellulose, cellulose acetate). This method is widely used for the purification of colloidal solutions and solutions of high molecular weight compounds. Substances that do not penetrate membranes during dialysis were called colloids. Any substance under suitable conditions can be obtained in a colloidal state (P.P. Weymarn, 1906).
In the 30-40s of the 20th century, the chemical nature of the primary particles of reversible (lyophilic) colloids, which turned out to be macromolecules, was clarified. In connection with this, a new chemical discipline was separated from colloidal chemistry - the physical chemistry of high-molecular compounds. However, due to historical reasons, the common molecular kinetic properties of lyophilic and lyophobic colloids, the frequent formation of heterogeneous structures in molecular colloids, as well as the existence of numerous compositions of high molecular weight compounds and highly dispersed systems.
Receipt
Lyophobic sols, as dispersed systems in general, in accordance with their intermediate position between the world of molecules and large bodies, can be obtained in two ways: by methods of dispersion, i.e., grinding of large bodies, and by methods of condensation of molecular or ionic dissolved substances. Grinding by crushing, grinding, and abrasion produces relatively coarse powders (< 60 мкм). Более тонкого измельчения достигают с помо-щью специальных аппаратов, получивших название коллоидных мельниц, или применяя ультразвук.
The condensation method consists of obtaining insoluble compounds through exchange reactions, hydrolysis, reduction, and oxidation. Carrying out these reactions in highly dilute solutions and in the presence of a slight excess of one of the components, not precipitation, but colloidal solutions are obtained. Condensation methods also include the production of lyosols by replacing the solvent. For example, a colloidal solution of rosin can be obtained by pouring its alcohol solution into water, in which rosin is insoluble.
As was found out earlier, the higher the dispersion, the greater the surface tension, the greater the tendency to spontaneously reduce dispersity. Therefore, to obtain stable, i.e. long-lasting, suspensions; emulsions and colloidal solutions, it is necessary not only to achieve a given dispersion, but also to create conditions for its stabilization. In view of this, stable disperse systems consist of at least three components: a dispersion medium, a dispersed phase and a third component - a stabilizer
disperse system.
The stabilizer can be of both ionic and molecular, often high-molecular, nature. Ionic stabilization of sols of lyophobic colloids is associated with the presence of low concentrations of electrolytes, creating ionic boundary layers between the dispersed phase and the dispersion medium.
High-molecular compounds (proteins, polyvinyl alcohol and others) added to stabilize dispersed systems are called protective colloids. Adsorbed at the phase interface, they form mesh and gel-like structures in the surface layer, creating a structural-mechanical barrier that prevents the unification of particles of the dispersed phase. Structural-mechanical stabilization is crucial for the stabilization of suspensions, pastes, foams, and concentrated emulsions.
To obtain solutions of molecular colloids, it is enough to bring the dry substance into contact with a suitable solvent. Non-polar macromolecules dissolve in hydrocarbons (for example, rubbers - in benzene), and polar macromolecules - in polar solvents (for example, some proteins - in water and aqueous solutions of salts). Substances of this type are called reversible colloids because after evaporating their solutions and adding a new portion of the solvent, the dry residue goes back into solution. The name lyophilic colloids arose from the assumption (as it turned out - erroneous) that a strong interaction with the environment determines their difference from lyophobic colloids.
Solutions of high molecular weight compounds have significant viscosity, which increases rapidly with increasing solution concentration. An increase in the concentration of macromolecular solutions, the addition of substances that reduce the solubility of the polymer, and often a decrease in temperature lead to gelation, i.e., the transformation of a highly viscous but fluid solution into a solid-like jelly that retains its shape. Solutions of polymers with highly elongated macromolecules gel at low solution concentrations. Thus, gelatin and agar-agar form jellies and gels in 0.2-0.1% solutions. Dried jellies are able to swell again (a significant difference from gels).
Jelly formation is an important stage in the production of fibrous materials from polymer solutions. The properties of solutions of high-molecular compounds with increasing concentrations differ more and more from the properties of solutions of low-molecular compounds. This occurs as a result of the interaction of individual macromolecules with each other, leading to the formation of supramolecular structures that have a great influence on the quality of products (fibers, silk masses) made of polymers.
High-molecular compounds, like any other substances, under suitable conditions can be obtained in a highly dispersed colloidal state. Such dispersions of polymers in liquids that do not dissolve them, most of them in water, are called latexes. Particles of the dispersed phase of latexes have close κ spherical
shape and size of the order of 10-100 nm.
COAGULATION
The potential energy of interaction (E mv) between colloidal particles is the algebraic sum of the potential energy of electrostatic repulsion (Eot) and the potential energy of dispersive attraction (E pr) between them:
E mv = E pr + E from
If E from > E pr (in absolute value), then repulsion prevails over attraction and the dispersed system is stable. If E is from< Е пр, то происхо-дит слипание сталкивающихся при броуновском движении коллоидных частиц в более крупные агрегаты и седиментация последних. Коллоидный раствор ко-агулируетп, т. е. разделяется на коагулят (осадок) и дисперсионную среду.
This is the essence of the theory of electrical stabilization and coagulation of dispersed systems by Deryagin, Landau, Verwey and Overbeck (DLVO theory).
Fig.1. Potential energy of interaction between two equally charged particles: 1- electrical repulsion (E from); 2 - dispersion attraction (E P p); 3 - resultant interaction energy (E mv); 4 - the same, but with a steeper drop in curve 1; g - distance between particles; E max is the potential barrier to the interaction of dispersed particles.
Ha fig. Figure 1 shows the dependences of the values of E on and E pr on the distance between colloidal particles. As can be seen, the resulting interaction energy (curve 3 in Fig. 10.17) leads to κ attraction (E mv< 0) на очень малых и отталкиванию (Е мв >0) at large distances between particles. Of decisive importance for the stability of dispersed systems is the value of the potential barrier of repulsion (E max), which, in turn, depends on the course of the E from and E pr curves. At large values of this barrier, the colloidal system is stable. The adhesion of colloidal particles is possible only when they are sufficiently close. This requires overcoming the potential barrier of repulsion. At some small positive values of E max (curve 3), only a few colloidal particles with a sufficiently high kinetic energy can overcome it. This corresponds to the stage of slow coagulation, when only a small part of the collisions of colloidal particles leads to their adhesion. With slow coagulation, over time there is a slight decrease in the total number of colloidal particles as a result of the formation of aggregates of 2-3 primary particles, but the coagulum does not fall out. Such coagulation, not accompanied by a visible change in the colloidal solution, is called latent coagulation. With a further decrease potential barrier, the coagulation rate, characterized by a change in the number of particles per unit time, increases. Finally, if the potential barrier moves from the area of repulsion to the area of attraction (curve 4 in Fig. 1), rapid coagulation occurs; the collision of colloidal particles leads to their adhesion. ; a precipitate is formed in the colloidal solution - a coagulum occurs obvious coagulation.
The potential barrier of repulsion (Emax) arises as a result of the summation of the forces of repulsion and attraction acting between colloidal particles. Therefore, all factors influencing the course of curves 1 and 2 (Fig. 1) lead to a change in both the value of E max; there and the position of the maximum (i.e., the distance corresponding E max).
A significant decrease in Emax occurs as a result of a change in the potential energy of electrostatic repulsion (i.e., the course of curve 1) caused by the addition of electrolytes to the colloidal solution. With an increase in the concentration of any electrolyte, a restructuring of the electrical double layer surrounding the colloidal particles occurs: an increasing part of the counterions is forced out from the diffuse into the adsorption part of the electrical double layer. The thickness of the diffuse part of the electrical double layer (layer 4 in Fig. 10.14), and with it the entire electrical double layer (layer 2 in Fig. 10.14) decreases. Therefore, the potential energy curve of electrostatic repulsion decreases more steeply than that shown in Fig. 10.17 curve 1. As a result of this, the potential barrier of repulsion (E max) decreases and shifts towards a smaller distance between colloidal particles. When the electric double layer is compressed to the thickness of the adsorption layer (layer 3 in Fig. 10.14), then the entire interaction curve of dispersed particles appears in the area of attraction (curve 4 in Fig. 10.17), and rapid coagulation occurs. This measurement of the stability of a colloidal solution occurs when any electrolyte is added.
The coagulating effect of electrolytes is characterized by coagulation threshold, i.e., the lowest concentration of electrolyte that causes coagulation. Depending on the nature of the electrolyte and colloidal solution, the coagulation threshold varies from 10-5 to 0.1 mol per liter of sol. The most significant influence on the coagulation threshold is charge coagulating ion electrolyte, i.e. an ion whose charge is opposite in sign to the charge of the colloidal particle.
Gels
Dispersed systems can be freely dispersed(Fig. 10.2) and cohesively dispersed(Fig. 10.3, a-f) depending on the absence or presence of interaction between particles of the dispersed phase. Freely dispersed systems include aerosols, lyosols, diluted suspensions and emulsions. They are fluid. In these systems, particles of the dispersed phase have no contacts, participate in random thermal motion, and move freely under the influence of gravity. Cohesively dispersed systems are solid; they arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network. This structure limits the fluidity of the dispersed system and gives it the ability to retain its shape. Such structured colloidal systems are called gels. The transition of a sol to a gel, which occurs as a result of a decrease in the stability of the sol, is called gelation(or gelatinization). The highly elongated and film-leaf shape of dispersed particles increases the likelihood of contacts between them and favors the formation of gels at low concentrations of the dispersed phase. Powders, concentrated emulsions and suspensions (pastes), foams are examples of cohesive disperse systems. The soil formed as a result of contact and compaction of dispersed particles of soil minerals and humus (organic) substances is also a coherently dispersed system.
