Although the same general principles of physical chemistry apply to all two-phase systems, the term "colloidal condition" is commonly used only in connection with a particular type of dispersions, in which the dispersion medium is a liquid and the dispersed material is either a solid or a liquid.
Thorough and careful studies have shown that when a solid or a liquid is introduced into another liquid, and becomes dispersed or distributed through it, the mixture may be either a true solution, a colloidal solution, or a mechanical suspension. The characteristic differences between these three conditions may be tabulated as follows: although the significance of some of the phrases used will not be apparent until the phenomena in question have been considered in some detail.
| True Solutions. | Colloidal Solutions. | Suspensions. |
|---|---|---|
| (a) Particles of the disperse phase are: | ||
| In molecular subdivision | In colloidal subdivision | In mechanical subdivision |
| Invisible | Visible under "ultrascope" | Visible under microscope or to naked eye |
| Less than 1µµ in diameter[6] | 1µµ to 1µ in diameter | Greater than 1µ in diameter |
| In molecular motion | In Brownian movement | Do not pass through filters or parchment |
| Pass through filters and parchment membranes | Pass through filters but not through parchment | In gravitational movement |
| (b) The system exhibits: | ||
| High osmotic pressur | Low osmotic pressure | No osmotic pressure |
| Transparency | "Tyndall phenomenon" | Is generally opaque |
| No gel-formation | Forms gels | No gel-formation |
[6] 1µ is one-thousandth of a millimeter; 1µµ is one-thousandth of a µ, or one millionth of a millimeter.
It is recognized by all students of these matters that it is not possible to draw a sharp dividing line between these three types of conditions, and that they shade into each other, in many cases; but in general it may be said that a colloidal solution is one in which the dispersed particles are usually between 5µµ and 200µµ in diameter, are difficultly or not at all diffusible through the membrane of a simple dialyzer, cannot be filtered out of solution, do not settle out under the action of gravitation, and are visible only under the "ultramicroscope"; and one which has certain peculiar optical, osmotic, and other physical and chemical properties. Since colloidal particles are very minute in size, they possess very large relative surface areas as compared with their total mass or volume, very high surface tension, and a relatively high surface energy as compared with their total, or molecular, energy. These properties bring into play, in a substance which is in the colloidal condition, in a remarkable degree, all the phenomena which are associated with surface boundaries between solids and liquids, liquids and gases, etc.
The properties arising out of the colloidal condition are of such tremendous importance in connection with the vital phenomena exhibited by cell protoplasm that it is necessary to give some detailed consideration to them here. Many large volumes dealing with this condition of matter have been written, and it is very difficult to condense even the most important facts concerning it into a few pages, but an attempt has been made to present in this brief summary the most essential facts and principles involved in the colloidal phenomena.
NOMENCLATURE AND CLASSIFICATION
Colloidal mixtures may exist in two different forms: one, in which the mixture is fluid and mobile, like a true solution, is known as a "sol"; and the other, which is a semi-solid, or jelly-like, form, is known as a "gel." Sols may be easily converted (or "set") into gels, by changes of temperature or of the electrolyte content, or by changes in the concentration of the mixture, etc., and in most cases gels can be converted again into sols. In some cases, however, gel-formation is irreversible, the gels are permanent and cannot be changed back again into sols by any known change in environmental conditions.
Depending upon whether the liquid dispersion medium is water, alcohol, ether, etc., sols are known as "hydrosols," "alcosols," "ethersols," etc.; and gels as "hydrogels," "alcogels," etc.