What Is a Colloid Solution?

Also called a colloid suspension, a colloid solution is the result derived from the uniform admixture of one phase, the “dispersed phase,” within another, the “continuous phase.” The continuous phase may be solid, liquid or gas. A colloid solution is not a true solution, since colloidal particles are generally microscopically visible, ranging in size typically from 1-1,000 nanometers in size. These particles vary considerably in form, from plates to rods to spheres. Stabilizing a colloid is termed peptization, whereas de-stabilization is called flocculation.

The broad categorization of a colloid solution may be narrowed down according to the form of dispersed phase and of continuous phase. Liquid dispersed in a gas is called an aerosol, whether a fog or a mist, whereas a gas dispersed in liquid is referred to as a foam, exemplified by shaving cream or whipped cream. If liquid is dispersed in a solid it a "gel," but solid dispersed in a liquid is a "sol" — one example of the former is dessert gelatin, whereas paint is a sol. Milk is an emulsion — a liquid-liquid colloid called a hydrocolloid.

Brownian motion is the most noteworthy mechanical force stabilizing colloidal fluids. The continuous phase, sometimes called the "solvent" phase stirs colloid solution particles by means of individual molecules bombarding the colloidal particles. This Brownian mechanical force successfully stabilizes, simply because the downward gravitational force of small colloidal particles isn’t great enough to overpower them. An additional factor, repellant electrical forces, exhibits short-range stabilizing behavior toward a colloid solution. There are other forces, attractive ones, which seem to modify the nature of colloids, by producing voids; these are under ongoing investigation.

Proof that the action of electrical forces peptizes colloidal particles can be observed by bringing a colloidal solution under the influence of an electric field. The particles migrate in response. Peptization can be increased through addition of an appropriate surfactant or substance that provides ions that attach to colloidal particles. Conversely, flocculation can be achieved using different additives that remove electrostatic charge and that may also add bulk. Flocculation is especially important for solids removal in wastewater treatment plants.

One instrument used to study a colloid solution — a zeta meter — measures the potential difference between the dispersed layer of colloidal particles and the surrounding continuous phase. The lower the potential difference, the more likely the particles are to flocculate; the higher it is, the more stable the colloid is. Another important tool is the nephelometer. It is often used to detect suspended particles in a liquid or gas colloid. Closely related to this is the turbidimeter, used to detect haziness in water samples such as that taken from lakes and streams.