Colloid, << KOL oyd, >> is a material composed of tiny particles of one substance that are dispersed (distributed), but not dissolved, in another substance. The mixture of the two substances is a colloidal system. A colloidal system composed of solid or liquid particles dispersed in a gas is called an aerosol (see Aerosol ). A system made up of solid or liquid particles in water is sometimes called a sol or a hydrosol. The word colloid is often used alone to mean colloidal system. The remainder of this article uses colloid in this way.
Colloids include such familiar products as milk, soap solutions, paint, and ink. Other common products, such as pottery and paper, are made from colloids. Blood and most other fluids in living things are colloids.
The solid particles of a colloid may be crystals, groups of molecules, or large, single molecules. At least one dimension of a typical colloidal particle measures between a few nanometers and a few thousand nanometers. One nanometer equals one billionth of a meter, or 1/25,400,000 inch. Some individual particles are too small to see, even with an optical microscope. These particles scatter light, however, producing bright dots that are visible in an instrument called an ultramicroscope (see Ultramicroscope ).
Liquid colloids can be divided into three groups: (1) lyophobic, (2) lyophilic, and (3) association.
In lyophobic colloids, the particles have little attraction for the liquid in which they are dispersed. As a result, the particles tend to coagulate (clump). Mixing a chemical called a dispersant with a lyophobic colloid can decrease this tendency, however. For example, dispersants minimize the coagulation of pigments that give certain inks their color.
In lyophilic colloids, there is an attraction between the particles and the liquid, so the particles have little tendency to coagulate. Many animal and plant fluids, such as blood and gums, are lyophilic colloids.
In association colloids, the colloidal particles are large molecules that are part lyophilic and part lyophobic. These molecules form clusters called micelles that turn their lyophobic parts away from the liquid molecules and expose only the lyophilic parts. Soaps and detergents are association colloids. Their micelles surround and hold oily pieces of dirt.
