Colloid

Colloid, suspension of tiny particles of one substance, called the dispersed phase, in another phase, called the continuous phase, or dispersion medium.

Both the suspended, or dispersed, phase and the dispersion medium may be solid, liquid, or gaseous, although the dispersal of one gas in another is not known as colloidal dispersion. An aerosol is a colloidal dispersion of either a solid in a gas (such as cigarette smoke) or a liquid in a gas (such as insecticide spray). An emulsion is a colloidal dispersion of liquid particles in another liquid; mayonnaise, for example, is a suspension of tiny globules of oil in water. A sol is a colloidal suspension of solid particles in a liquid; paints, for example, are a suspension of minute solid pigment particles in an oily vehicle. A gel is a sol in which the suspended particles are organized in a loose, but definite three-dimensional arrangement, giving some rigidity and elasticity to the mixture, as in jelly.

The particles of a true colloidal dispersion are so small that the incessant bombardment of the molecules of the medium is sufficient to keep the particles in suspension; the random motions of the particles under the influence of this molecular bombardment is called Brownian motion. If, however, the force of gravity is greatly increased in a high-speed centrifuge, the suspension can be broken and the particles made to settle.

Colloidal dispersions in liquids are produced industrially by intensive grinding of a solid in a colloid mill or by intensive mixing and whipping of two liquids together in an emulsifier; wetting of the suspended phase is aided by the addition of a wetting agent known as a stabilizer, a thickener, or an emulsifying agent.

The movement of colloidal particles through a fluid under the influence of an electric field is known as electrophoresis. One method of electrophoresis, devised in 1937 by the Swedish biochemist Arne Tiselius, is used to study proteins and to diagnose diseases that cause abnormalities of blood serum.

Because of their size, colloidal particles cannot pass through the extremely fine openings in a semi-permeable membrane, such as parchment, by osmosis. Although a colloidal dispersion cannot be purified by filtration, it can be dialysed by placing it in a semi-permeable bag with pure water on the outside. Dissolved impurities then gradually diffuse through the bag, while the colloidal particles remain imprisoned within it. If the process of dialysis is carried to completion, the suspension will often break down, or settle, because the stability of colloidal systems frequently depends on the electrical charges on the individual particles, and these are in turn generally dependent on the presence of dissolved electrolytes.

Although individual colloidal particles are too small to be seen with an ordinary microscope, they can be made visible by means of an ultramicroscope, or dark-field microscope. If a colloidal dispersion is placed under a microscope and a beam of light is directed through from one side, the path of the beam becomes visible by scattering from the colloidal particles. This same phenomenon makes the path of a beam of light visible in a darkened room, but under the microscope separate flashes of light are observed. The particles are seen to be in random motion as the result of Brownian motion, and their speed is exactly that calculated for molecules the size of the colloidal particles.