Reversibility of Gel-formation.—In some cases, the change of a sol to a gel is an easily reversible one. Glue, gelatin, various fruit jellies, etc., "melt" to a fluid sol at slightly increased temperatures and "set" again to a gel on cooling, and the change can be repeated an indefinite number of times. On the other hand, many gels cannot be reconverted into sols; that is, the "gelation" process is irreversible. For example, egg-albumin which has been coagulated by heat cannot be reconverted into a sol; casein of milk when once "clotted" by acid cannot again be converted into its former condition, etc. Irreversible gelation is usually spoken of as "coagulation." Some coagulated gels, by proper treatment with various electrolytes, etc., can be converted into sols, the process being known as "peptization"; but in such "peptized" hydrosols, the material usually exists in a different form than originally, having undergone some chemical change during the peptization, and the coagulation and peptization cannot be repeated, that is, the process is not a definitely reversible one.
Importance of Gel-formation.—From the physiological point of view, gel-formation is undoubtedly the most important aspect of colloidal phenomena. In the first place, the ability to absorb and hold as much as 80 to 90 per cent of water in a semi-solid structure is of immense physiological importance. In no other condition can so large a proportion of water, with its consequent effect upon chemical reactivity, be held in a structural, or semi-solid, mass. But a vastly more significant feature of the conditions supplied by the gel lies in the fact that the non-water phase, or phases, of the system are spread out in a thin film, or membrane, thus giving it enormous surface as compared with its total volume. This effect is easily apparent if one thinks of the enormous surface which is exposed when a tiny portion of colloidal soap is blown out into a "soap-bubble" several inches in diameter. This condition brings into play all the phenomena resulting from surface boundaries between solids and liquids, liquids and liquids, liquids and gases, etc., from surface tension, surface energy, etc. Among these effects may be cited those of adsorption, increased chemical reactivity due to enlarged areas of contact, permeability and diffusion, etc., the importance of which in the vital phenomena of cell-protoplasm will be discussed in detail in the following chapter.
GENERAL PROPERTIES OF COLLOIDAL SOLUTIONS
Non-diffusibility.—The most characteristic property of all sols is the failure of the suspended particles to pass through a parchment, or any similar dialyzing membrane.
Visibility under the "Ultramicroscope."—The particles of a sol, in contrast with the molecules of a true solution, are visible as bright scintillating points under the ultramicroscope. This is a modification of the type of dark-field illumination of the ordinary microscope, as applied to microscopic studies, in which the solution to be studied is contained in a small tube or box of clear glass which is mounted on the stage of an ordinary microscope and instead of being illuminated from below by transmitted light is illuminated by focusing upon it the image of the sun, or of some other brilliant source of light such as an electric arc, by passing the rays from the source of light through a series of condensing lenses which are adjusted at the proper distance and angles to bring the image of the illuminating body within the tube containing the substance which is to be examined and in the line of vision of the microscope. Obviously, this results in intense illumination of any particles in the solution which come within this brilliant image of the sun, or arc, and therefore renders visible particles which are of less diameter than the wave-length of ordinary light (450µµ to 760µµ for the visible spectrum) and, hence, are not visible by the ordinary means of illumination in the direct line of vision. It will be apparent that what is seen in the field of the ultramicroscope is not the particles themselves, but rather the image of the sun (or other illuminating body) falling upon the particles which come within the image, just as one does not see the paper but only the image of the sun when the rays from the sun are brought to a focus upon a sheet of paper through any ordinary convex lens, or "burning glass." Hence, the ultramicroscope gives no idea of the shape, color, or size of the particles upon which the image falls; but it does permit the counting of the number of particles within a given area, and a study of their movements, from which it is possible, by mathematical computations, to calculate the relative size of the particles themselves. Repeated studies have shown that particles of the sizes between 5µµ and 250µµ in diameter, which are visible under the ultramicroscope, are sufficiently small to bring about the surface phenomena which are known as properties of colloidal solutions. Further, the ultramicroscope permits the observation of the growth, or disintegration, under various chemical reagents, of the individual colloidal particles, which appear as scintillating points in the field of the microscope; and the study of changes in relationships during gel-formation, peptization, etc.
The "Tyndall Phenomenon."—Colloidal solutions exhibit this phenomenon; that is, if a bright beam of light be passed through a sol which is contained in a clear glass vessel having parallel vertical sides, and the solution be viewed from the side, it appears turbid and often has a more or less bluish sheen. This effect is due to the small particles in the sol, of polarizing the light which is reflected from them, the blue rays being bent more than are those in the other part of the spectrum. The Tyndall phenomenon is similar in its effect in making the tiny particles of the sol visible to the illumination of the dust particles in the air of a darkened room when a ray or narrow beam of light passes through it. In a true molecular solution, the particles are too small to be visible by this mode of illumination.
Other Optical Properties.—Sols are generally translucent and opalescent; many of them are highly colored, some of the sols of gold, platinum and other heavy metals possessing particularly brilliant colors. In general, metallic suspensoids are red, violet, or some other brilliant color; while inorganic suspensoids are bluish white, and emulsoids generally blue to bluish white.
Formation of Froth, or Foam.—Colloidal solutions, especially those of the natural proteins, fats, glucosides, gums, and the artificial soaps, have a strong tendency to produce froth, or foam, when shaken; this being due to the enormous surface tension resulting from the finely divided condition of the dispersed material.
Low Osmotic Pressure.—All colloidal solutions exhibit a very low osmotic pressure; the freezing point of the dispersion medium is lowered only very slightly and its boiling point is only very slightly raised by the presence of the dispersed particles in it.
Precipitation by Electrolytes.—Sols of all kinds are precipitated, or caused to form gels, by the addition of electrolytes, since these cause a disturbance of the electric charge on the dispersed particles, to which the colloidal condition is due. In the case of most emulsoids and of a few of the suspensoids, this change converts the mass into a stiff gel; but in that of many of the metallic suspensoids, the dispersed particles are gathered together into larger aggregates, which settle out of the liquid in the form of a gelatinous precipitate. In the latter case, the effect is usually spoken of as "precipitation" by electrolytes; while in the former, it is called "coagulation," or "gelation."