Gels

GELS, or jellies, are homogeneous colloidal jellies that do not have a visible microscopic structure, contain large amounts of water or another liquid, and yet exhibit the properties of solids. Depending on the liquid bound by the gel, it receives a corresponding name: hydrogel if it contains water, alcogel in the case of containing alcohol, and so on. Formation. A gel is usually obtained from a colloidal solution, or sol (see). Colloids differ in their relationship to the liquid in which their particles are suspended. Some of them have a more or less significant affinity for it and are called lyophilic (in the case of hydrosols, hydrophilic), while those that do not possess such affinity are called lyophobic (or, accordingly, hydrophobic; see Colloids). Under the influence of various factors, sols easily transition into a solid state. In this process, coagulation (see) occurs in lyophobic colloids: colloidal particles combine into larger aggregates, the degree of their dispersion decreases, and the process ends with the precipitation of the colloid, separating it from the liquid in the form of a colloidal precipitate—a "coagulate." In lyophilic colloids, a colloidal solution can turn into a homogeneous, dense jelly without the separation of its phases, in which the liquid is bound by colloidal particles. Through such gelatinization, a gel is formed. It can, however, also arise in the opposite way—not as a result of the thickening of a colloidal solution, but through the absorption of water (or another liquid) by a solid anhydrous colloid (see swelling below). Examples of colloids that produce gels include agar-agar, gelatin, and silicic acid. Gelatin solidifies into a dense gel at a content of just 2–3%, and agar-agar even at one-tenth of that concentration. Structure. When studying a whole series of natural (protoplasm) and artificial gels, Bütschli observed a cellular or foamy structure in them. The colloidal substance formed the walls of the cells in which, like honey in a honeycomb, minute droplets of liquid were enclosed. Thus

Most gels are microscopically quite homogeneous, so in order to make their structure visible, Bütschli compacted them with alcohol, chromic acid, etc., and also displaced the water with a liquid having a different refractive index. After such treatment, a foamy, alveolar structure clearly appeared in almost all gels. Thus, the arrangement of the colloid and solvent phases turned out to be the reverse of that observed in a colloidal solution. Bütschli assumed that the original hydrogel had the same structure before the water in it was replaced by alcohol and that this preformed structure remains invisible only due to the almost complete identity of the refractive indices of both phases of the hydrogel. The results of these observations led Bütschli to create the theory of the cellular or foamy structure of protoplasm (see). However, thorough ultramicroscopic observations, especially the studies of Bachmann, showed that individual colloidal particles, upon the formation of gels, retain ultramicroscopic dimensions lying beyond the limit of visibility of the microscope. The coarse microscopic structures that Bütschli observed cannot, therefore, exist in the original jelly, but are merely artificial formations, the result of the processing of the gels. It should be borne in mind that the same reagents with the help of which Bütschli tried to make the structure of gels visible are also used in the fixation of a living cell for its microscopic examination (see Histological technique). Such treatment often, instead of "fixing" existing structures, creates new ones, changing the state of cellular colloids. Many cellular structures described by histologists are undoubtedly such artificial formations - artifacts (see). In reality, gels possess only an ultramicroscopic structure. According to Zsigmondy, they consist, like sols, of individual colloidal ultraparticles, to which Nägeli gave the name micelles (see). The properties of a solid body discovered in gels (the ability to maintain their shape, more or less noticeable cohesion of their particles, elasticity, etc.) are apparently due to the cohesion of individual swollen micelles, forming, as it were, a very fine stroma permeating the main mass of liquid in all directions. Diffusion in gels. Such a structure of gels makes it possible to understand many of their properties which would otherwise remain inexplicable, and first of all - the rate of diffusion of substances dissolved in the gel. The rate of diffusion decreases rapidly with an increase in the viscosity (see) of the solution, becoming negligibly small as it approaches the solid state. Meanwhile, in gels, a salt solution exhibits a very significant rate of diffusion. NaCl, for example, diffuses in a weakly concentrated gelatin or agar gel at almost the same rate as in pure water. This is possible only in the presence of a continuous mass of water, held only by the cohesion of colloidal micelles. Like a sponge soaked with water, a gel combines mechanical strength, resembling the strength of solid bodies, with the presence of free masses of liquid. However, studies of recent years have shown the erroneousness of the initial idea about the complete equality of the rate of diffusion in a hydrogel and in pure water. The slowing down of the rate of diffusion in more concentrated gels becomes very significant, and moreover, very unequal for different crystalloids. Colloids do not diffuse in gels. Chemical reactions in gels. Closely connected with the character of diffusion are also chemical reactions occurring in gels. Gels constitute an apparent exception to the old rule that chemical reactions are possible only in liquids ("corpora non agunt, nisi soluta"). Thanks to the free mobility of crystalloid molecules and ions in gels, chemical reactions between them can proceed in exactly the same way as in solutions. An example of the technical application of this property is the so-called dry galvanic cells, in which the liquid is replaced by corresponding dense jellies. This same combination of mechanical properties of a solid body and chemical properties of a liquid determines the biological significance of gels, making them an indispensable material for the construction of living organisms. For the life of both an individual cell and a multicellular organism, diverse, often very complex structures are necessary. Only gels can form such structures (like, for example, the cell membrane), while at the same time not being an insurmountable barrier for the flow of chemical processes lying at the basis of life. Special conditions created during the diffusion of dissolved substances in gels can lead to peculiar chemical reactions. Thus, during the diffusion of substances that produce insoluble precipitates upon their interaction, the latter can be deposited in the form of successive concentric layers, which have received the name "Liesegang rings". Swelling. Based on the ability of gels to bind various amounts of liquid is the very important phenomenon of swelling. Swelling is called the absorption of liquid by a gel (in the case of a hydrogel - water), whereby its volume increases, and cohesion and hardness decrease accordingly; the gel remains microscopically homogeneous in this process. In exactly the same way, a dry, anhydrous colloid can swell, bind water, turning into a gel. The ability to swell in different gels is very unequal. They can be divided in this respect into limitedly and unlimitedly swelling ones. The latter bind all the added liquid, swelling indefinitely until - with a sufficient decrease in the concentration of the colloid - they turn into sols. Much more often, limited swelling is observed, stopping after reaching a certain maximum of swelling. However, there are transitions between both groups. Even for one and the same gel, the character of swelling depends on external conditions: thus, gelatin and agar become unlimitedly swelling with an increase in temperature. During swelling, a very significant pressure develops, especially large during the absorption of the first portions of water and rapidly decreasing as it approaches the maximum of swelling. Thus, for example, during the swelling of starch, the pressure reaches 2,500 atm at the beginning of the process. Swelling pressure was known and found application even in antiquity: to split rocks, dry wood was inserted into cracks, which tore the stone apart upon soaking. The composition of the liquid has a very great influence on the process of swelling. The addition of acids and alkalis, which change the active reaction of the solution, has a particularly strong effect. Ions of salts also play a large role, especially their negatively charged ions - anions. According to the degree of their influence on swelling (as well as on other colloidal and colloidal-biological processes), anions can be arranged in series expressing the gradation in the strength of their action - the so-called Hofmeister series (see). In large concentrations, salts usually suppress swelling. Non-electrolytes have little effect on the binding of water by gels. The swelling of biocolloids plays a large role in the organism. Along with osmotic forces (see Osmotic pressure), it determines the binding and distribution of water in tissues. According to data by Schade, connective tissue binds particularly large amounts of water. Its individual parts behave differently in this process. For example, weak alkalization sharply increases the swelling of the ground substance, while acidification causes the swelling of collagen fibers. Thus, a small change in the reaction causes the movement and redistribution of water between the elements of the tissue. Elastic and inelastic gels. The gels considered above increase in volume upon the absorption of water (swelling), shrink upon the loss of water (deswelling), remaining microscopically homogeneous all the time. Another group of gels behaves differently, an example of which can be the silica jelly studied in detail by Van Bemmelen. Upon drying, it shrinks for some time. However, with further loss of water, the volume ceases to decrease, and voids appear between the colloid particles, making the previously glassy gel opaque. Upon reverse absorption of water, it fills the voids of the jelly without increasing its volume. Accordingly, gels are divided into elastic and inelastic. Only in the former does the absorption of water correspond to the definition of swelling given above. The character of water binding is fundamentally different in both cases. In elastic, swelling gels, it possesses a specificity characteristic of chemical interactions and bringing it closer to dissolution. Thus, celloidin swells in alcohol and ether, rubber in benzene, but both of them do not swell in water; gelatin swells in water, but not in alcohol, etc. On the contrary, any liquids that wet it penetrate equally into the pores of an inelastic, non-swelling gel, obeying capillary forces. An elastic gel can in many cases be converted into an inelastic one both by changing the colloid itself and by replacing the water with another liquid that the colloid does not bind. During the histological processing of living tissue, the main task of fixation is the conversion of swelling biocolloids into inelastic gels, which are no longer capable of changing their shape and volume in the solutions acting upon them.

In some cases, even with elastic gels, and especially with non-elastic gels, the changes associated with the absorption and release of water can occur extremely slowly; they approach equilibrium only very slowly or do not reach it at all. Thus, if one successively places a silica gel into vessels containing water vapor of increasing tension, and then performs the same transition in reverse order, the curves representing the dependence of water content on vapor tension will not coincide in both cases. At the same humidity of the air, the gel in the process of water absorption, or "hydration," will contain less water than in the reverse process of "dehydration." It retains, as it were, an imprint of its previous state—being richer or poorer in water. Such a prolonged aftereffect of previously existing conditions is called hysteresis (see). Closely related to it is the phenomenon of syneresis (see). It consists in the ability of many freshly prepared gels, which initially have a completely dry surface, to contract upon standing, releasing significant amounts of water on the surface. A well-known example of this phenomenon is the contraction of a blood clot, which releases clear serum in the process. Reversible liquefaction and gelatinization. With unlimited swelling, a gel turns into a sol. The same liquefaction of a gel and its reverse gelatinization can occur without it absorbing new quantities of water. It occurs, for example, under the influence of changes in temperature: many gels liquefy upon heating, gradually gelatinizing again upon cooling. It can also be caused by changes in chemical composition, in particular, the active reaction or the concentration of salts. Finally, temporary, reversible liquefaction is in many cases the result of mechanical agitation. This latter phenomenon, which has attracted special attention in recent years, has been named thixotropy (see); it is very well expressed in living protoplasm. The reversible liquefaction and gelatinization of protoplasm or individual cellular structures apparently plays an essential role in many vital processes. It takes place during cell division (karyokinetic figures), during amoeboid movement (the thickening of the outer layer of protoplasm into ectoplasm and the liquefaction of the inner endoplasm), and possibly also during narcosis, etc. Since viscosity changes particularly strongly during this, the observation of the speed of movement of particles located in the protoplasm (for example, Brownian motion) represents the best method for studying such transformations of gel into sol.

Figure 1. A trace remains on the tape in the form of a burn during the intervals of time when the sun was shining. When the sun was not shining, there is no burn. Thus, the number of hours of sunshine per day is calculated from the tape. - The Velichko heliograph (see Figure 2) consists of a brass cylinder containing photographic light-sensitive paper (ferroprussiate) inside, which is turned with the sensitive layer inward; the paper is divided by vertical lines into hourly intervals. The cylinder has three narrow, short slits facing East, South, and West. A sunbeam, passing through the corresponding slit, leaves a trace on the light-sensitive paper, which becomes visible after the paper is developed. The number of hours of sunshine per day is calculated from this trace.