emulsion

—emulsive, adj.

/i mul"sheuhn/, n.

1. Physical Chem. any colloidal suspension of a liquid in another liquid.

2. such a suspension used in cosmetics.

3. Pharm. a liquid preparation consisting of two completely immiscible liquids, one of which, as minute globules coated by a gum or other mucilaginous substance, is dispersed throughout the other: used as a means of making a medicine palatable.

4. Photog. a composition sensitive to some or all of the actinic rays of light, consisting of one or more of the silver halides suspended in gelatin, applied in a thin layer to one surface of a film or the like.

[1605-15; < NL emulsion- (s. of emulsio), equiv. to L emuls(us) milked out (e- E- + mulsus, ptp. of mulgere to milk) + -ion- -ION]

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Mixture of two or more liquids in which one is dispersed in the other as microscopic or ultramicroscopic droplets (see colloid).

Emulsions are stabilized by agents (emulsifiers) that (e.g., in the case of soap or detergent molecules) form films at the droplets' surface or (e.g., in the case of colloidal carbon, bentonite clay, proteins, or carbohydrate polymers) impart mechanical stability. Less-stable emulsions eventually separate spontaneously into two liquid layers; more-stable ones can be destroyed by inactivating the emulsifier, by freezing, or by heating. Polymerization reactions are often carried out in emulsions. Many familiar and industrial products are oil-in-water (o/w) or water-in-oil (w/o) emulsions: milk (o/w), butter (w/o), latex paints (o/w), floor and glass waxes (o/w), and many cosmetic and personal-care preparations and medications (either type).

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▪ chemistry

 in physical chemistry, mixture of two or more liquids in which one is present as droplets, of microscopic or ultramicroscopic size, distributed throughout the other. Emulsions are formed from the component liquids either spontaneously or, more often, by mechanical means, such as agitation, provided that the liquids that are mixed have no (or a very limited) mutual solubility. Emulsions are stabilized by agents that form films at the surface of the droplets (e.g., soap molecules) or that impart to them a mechanical stability (e.g., colloidal carbon or bentonite). Unstable emulsions eventually separate into two liquid layers. Stable emulsions can be destroyed by inactivating or destroying the emulsifying agent—e.g., by adding appropriate third substances or also by freezing or heating. Some familiar emulsions are milk (a dispersion of fat droplets in an aqueous solution) and butter (a dispersion of droplets of an aqueous solution in fat).

      Emulsions are important in many fields—e.g., in the dyeing and tanning industries, in the manufacture of synthetic rubber and plastics, in the preparation of cosmetics such as shampoos, and of salves and therapeutic products.

      The term emulsion is often applied to mixed systems that should better be characterized as solutions, suspensions, or gels. For example, the so-called photographic emulsion is actually a gelatin gel in which tiny crystals (e.g., of silver bromide) are dispersed.

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