At the same time, a given liquid may form a true emulsoid when introduced into one other liquid and a true solution when introduced into another. Thus, soaps form emulsoids with water (true hydrosols); but dissolve in alcohol to true solutions, in which they affect the osmotic pressure, the boiling point of the liquid, etc., in exactly the same way that the dissolving of other crystalloids in water affects the properties of true aqueous solutions. Again, ordinary "tannin," when dissolved in water, produces a sol, which froths easily, is non-diffusible, etc.; but when dissolved in glacial acetic acid, it produces a true solution.
The concentration of the disperse phase may be much greater in the case of emulsoids than it can be in suspensoids. This is probably because the dispersed particles do not carry so large an electric charge and are not in such violent motion.
GEL-FORMATION
The one property which most sharply distinguishes sols from true solutions is their ability to "set" into a jelly-like, or gelatinous semi-solid, mass, known as a "gel," without any change in chemical composition, or proportions, of the two components of the system. In the gel, the two components are still present in the same proportions as in the original sol; but the mixture becomes semi-solid instead of fluid in character. Thus, an agar-agar sol containing 98 per cent of water sets into a stiff gel; while many other gels which contain 90 to 95 per cent of water can be cut into chunks with a knife and no water will ooze from them. The water is not in chemical union with the solid matter in the form of definite chemical hydration, however, as the same gel is formed with all possible variations in the water content.
Gels may be either rigid, as in the case of those of silicic acid, etc., or elastic, as are those of gelatin, egg-albumin, agar-agar, etc. The latter are the common type of gels among organic colloids. They can be easily changed in shape, or form, without any change in total volume.
In gel-formation, the two phases of the system take a different relationship to each other. The disperse, or solid, phase becomes associated into a membrane-like, or film, structure, surrounding the liquid phase in a cell-like arrangement. That is, the whole mass takes on a structure similar to a honeycomb except that the cells are roughly dodecahedral in shape, instead of the hexagonal cylinders in which the bees arrange their comb cells, in which the original disperse phase constitutes the cell-walls and the original liquid, or continuous phase, represents the cell-contents. The cells of an elastic gel resemble closely the cells of a plant tissue in many of their physical properties. They are roughly twelve-sided in shape, as this is the form into which elastic spherical bodies are shaped when they are compressed into the least possible space.
Imbibition and Swelling of Gels.—When substances which are natural gels, such as gelatin, agar-agar, various gums, etc., are submerged in water, they imbibe considerable quantities of the liquid and the cells become distended so that the mass of the material swells up very considerably. This swelling will take place even against enormous pressures. For example, it has been found that the dry gel from sea-weeds will swell to 330 per cent of its dry volume, if immersed in water under ordinary atmospheric pressure; but that it will increase by 16 per cent of its own volume when moistened, if under a pressure of 42 atmospheres.
During the swelling of gels by imbibition of water, the total volume of the system (i.e., that of the original dry gel plus that of the water absorbed) becomes less. For example, a mixture of gelatin and water will, after the gelatin has swelled to its utmost limit, occupy 2 per cent less space than the total volume of the original gelatin and water. It has been computed that a pressure equivalent to that of 400 atmospheres would be necessary to compress the water to an extent representing this shrinkage in volume.
On the other hand, gels when exposed to the air lose water by evaporation, shrink in volume, and finally become hard inelastic solids, as in the case of the familiar forms of glue, gelatin, agar-agar, gum arabic, etc.
The difference in the relation of gels and that of non-colloidal solids to water may be illustrated by the different action of peas, beans, etc., and of a common brick, when immersed in water. Each of these substances, under these conditions, absorbs, or "imbibes," water; but the peas and beans swell to more than twice their original size and become soft and elastic, while the brick undergoes no change in size, elasticity, or ductility. In all cases of colloidal swelling, the swollen body possesses much less cohesion, and greater ductility, than it had before swelling. The essential difference in the two types of imbibition is that in the case of the non-swelling substances the cohesion, or internal attraction of the molecules of the material, is too great to permit them to be forced apart by the water; while in colloidal swelling, the particles are forced apart to such an extent as to make the tissue soft and elastic. It is possible, of course, to make this separation go still further, until there is an actual segregation of the molecules, when a true solution is produced; for example, gum arabic when first treated with water swells into a stiff gel, then into a soft gel, and finally completely dissolves into a true solution.