A familiar, naturally occurring emulsion is whole milk, the inner phase of which consists of globules of butterfat; the external phase is a watery solution of casein, sugar, and other substances. Another natural emulsion is egg yolk, which consists of egg oil in an aqueous solution containing, among other substances, albumen and lecithin; the latter is a lipoid (fatlike) substance that is one of nature's most efficient emulsifying agents. Albumen, also, is a good emulsifier.
Milk and egg yolk are oil in water emulsions. A second type of emulsion is that of water in oil, in which the phases are reversed. An example of this type of emulsion is butter, which has aqueous constituents dispersed in tiny globules throughout the butterfat. Most manufactured emulsions are made by combining the oily ingredient with a colloidal solution, such as casein or albumen; these are not only good emulsifiers, but also confer desirable properties when the emulsion is used as an adhesive.
Emulsions are generally opaque or milky because of the refraction and dispersion of light by the minute droplets, but when dry they become transparent, or nearly so.
In addition to their use in paper and leather manufacture, emulsions are used in edge gilding, gold tooling, and other bookbinding operations. (233 , 235 )
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