Surface tension

The preparation of aerosols can be as simple as sneezing (which produces an aerosol). Laboratory and commerical methods make use of several techniques. Chemical precipitation sometimes results in a colloid. A precipitate (e.g. silver iodide) already formed may be dispersed by the addition of a peptizing agent (e.g. potassium iodide). Clays may be peptized by alkalis, the OH⁻ ion being the active agent. Emulsions are normally prepared by shaking the two components together, although some kind of emulsifying agent has to be used in order to stabilize the product. This emulsifier may be a soap (a long chain fatty acid), a surfactant (Section 23.11), or a lyophilic sol that forms a protective film around the dispersed phase. In milk, which is an emulsion of fats in water, the emulsifying agent is casein, a protein containing phosphate groups. That casein is not completely successful in stabilizing milk is apparent from the formation of cream on the surface : the dispersed particles coalesce into oily droplets which float to the surface. Aerosols are formed when a spray of liquid is torn apart by a jet of gas. The dispersal is aided if a charge is applied to the liquid, for then the electrostatic repulsions blast the jet apart into droplets. This procedure may also be used to produce emulsions, for the charged liquid phase may be squirted into another liquid. Colloids are often purified by dialysis. As in the discussion of Donnan effect membrane (e.g. cellulose) is selected which is permeable to solvent and ions, but not to the colloidal particles. Dialysis is very slow, and is normally accelerated by applying an electric field and making use of the charge carried by many colloids; the technique is then called electrodialysis.