colloids

Notes on Colloidal Suspensions

The formal study of colloids began in the latter part of the 19th century with the studies of Thomas Graham. The first colloids studied were gelatins and glues, and so Graham used the Greek work kolla, meaning glue, as the root for his newly coined term. Colloidal particles may be gaseous, liquid, or solid. They may occur in various types of suspensions, e.g., solid/gas (aerosol), solid/solid, liquid/liquid (emulsion), gas/liquid (foam). It may be useful to observe that a suspension is any system in which small solid or liquid particles are more or less evenly dispersed in a second medium, typically a gas or a liquid. Some sources do not regard colloidal suspensions as suspensions at all, instead reserving this latter term for systems involving particles larger than colloidal size, but we will not adhere to this distinction.

In the size range of colloidal particles, the surface area of the particle is so much greater than its volume that some unusual behavior is observed, e.g., the particles do not settle out by gravity (i.e., they neither float nor sink). Many macromolecules are at the lower limit of this size range (a nanometer). The upper limit to colloidal particle size is commonly taken to be the size at which the particles become visible in an optical (i.e., light) microscope (about 1 micrometer). Natural colloidal systems include rubber latex, milk, blood, and egg-white.

Aerosols are suspensions of liquid or solid particles in a gas. The particles are often in the colloidal size range, making many aerosols colloidal suspensions. Fog (water/air) and smoke (C/air) are common examples of natural aerosols. Fine sprays such as those used with perfumes, insecticides, inhalants, anti-per spirants, and paints are man-made aerosols.

An emulsion is a stable mixture of two or more immiscible liquids held in suspension by small amounts of substances called emulsifiers. Proteins or small carbohydrate polymers (which are themselves colloidal in size) often act as emulsifiers by coating the surfaces of the dispersed phase and thus preventing coalescence. Such emulsifying agents are called protective colloids as they protect the dispersed phase from coalescence and subsequent separation. Long-chain alcohols and fatty acids can also act as emulsifiers by "solubilizing" the dispersed phase by virtue of the former's solubility in the dispersing medium (often water). These emulsifying agents are called detergents. Commercial polymerization reactions are often carried out in emulsion form. Floor and glass waxes, many drugs, photographic coatings, and paints are all examples of emulsion systems.

Foams are dispersions of gases in liquids or solids. If the gas globules are of colloidal size, the foam is a colloidal foam. Yeast breads are examples of solid foams. Shaving cream and whipped cream are good examples of liquid foams. Useful foams for automobile seats and mattresses are made from natural and synthetic (e.g., polystyrene, polyurethane) latexes. Metal and concrete foams are also possible.

The Tyndall effect can be used to differentiate a true solution from a colloidal suspension in cases where a beam of light can be passed through the material. In a true solution, the light will pass through relatively unimpeded and so not be visible from side view. In a colloidal suspension, the colloid particles are sufficiently large to significantly scatter the light and so make the beam visible from side view.

In closing, it should be noted that true solutions are distinct from colloids in that the dispersed agents in true solutions (i.e., the solutes) are typically atoms, ions, or fairly small molecules.

Written by: James Aldridge: jaldr@onramp.net