CHAPTER XV

THE COLLOIDAL CONDITION

Reference has frequently been made, in preceding chapters, to the fact that proteins, enzymes, lipoids, etc., exist in the protoplasm of plants and animals in the colloidal condition. The properties and uses of these compounds by plants depend so much upon this fact that, before proceeding to the consideration of the actual physical chemistry of protoplasm itself, it will be appropriate and profitable to give some attention to the nature and significance of the colloidal condition of matter and of some of the phenomena which grow out of it.

Every discussion of the colloidal condition in general properly begins with reference to the work of the English physicist, Thomas Graham, who carried on his investigations of the so-called "colloids" through a period of forty years, beginning with 1851. His most important results were published, however, from 1861 to 1864. Graham studied the diffusibility of substances in solution through the parchment membrane of a simple dialyzer. As a result of his earlier investigations, he divided all the chemical compounds which were known to him into two groups, which he called "crystalloids" and "colloids," respectively, the first including those substances which readily diffused through the parchment membrane and the second those which diffused only very slowly or not at all. He at first thought that crystalloids are always inorganic compounds, while colloids are of organic origin. He soon learned, however, that this distinction in behavior is not always related to the organic or inorganic nature of the compound. He further discovered that the same individual chemical element or compound may exist sometimes in crystalloidal, and sometimes in colloidal, form. This latter discovery led to the conclusion that diffusibility depends upon the condition, rather than upon the nature, of the material under observation.

As a result of the long series of investigations which were stimulated by Graham's work, the modern conception is that diffusibility is a condition of matter when in minute subdivision, or in solution, in some liquid, as contrasted with its state, or condition, when existing alone. That is, the state of a substance may be either gaseous, liquid, or solid; and its condition when in solution may be either crystalloidal or colloidal. Substances which are in crystalloidal form, in true solution, exist there in molecular or ionized condition; but, as will be pointed out below, when in the colloidal condition they exist in aggregates which are somewhat larger than molecules, but not large enough to be visible as individual particles under the ordinary microscope, even under the highest magnification which has yet been obtained. Colloidal particles are, however, generally visible under the Zigmondy "ultramicroscope." (See below.)

The use of the word "colloid" as a noun, or as the name for a substance which is in the colloidal condition, is of the same nature as the use of the words "gas," "liquid," and "solid," in such statements as "ice is a solid," "water is a liquid," or "steam is a gas," etc.; i.e., the noun represents a state or condition rather than an actual object or thing. Hence, the expression "enzymes are colloids," means only that enzymes exist in the colloidal condition, and not that enzymes represent a definite type of substances having the group name "colloids."

THE COLLOIDAL CONDITION A DISPERSION PHENOMENON

When one substance is distributed through the mass of another substance, the mixture is said to be a "two-phase system," composed of the dispersed phase, or substance, and the dispersion medium, or continuous phase, through which the other substance is distributed. The following examples illustrate the possibilities of such two-phase systems:

  1. Dispersion medium a gas.
    1. Disperse phase a liquid—mist in the air.
    2. Disperse phase a solid—smoke or dust in air.

    1. Disperse phase a gas—foams.
    2. Disperse phase a liquid—emulsions.
    3. Disperse phase a solid—suspensions.
  3. Dispersion medium a liquid.
    1. Disperse phase a gas—solid foams, pumice stone, etc.
    2. Disperse phase a liquid—liquid inclusions in minerals.
    3. Disperse phase a solid—alloys, colored glass, etc.

  5. Dispersion medium a solid.