An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. The word "emulsion" comes from the Latin word emulgere, which means "to milk out". This is because milk is an emulsion of fat and water.

Emulsions are often used in cosmetics, food, and pharmaceuticals. In cosmetics, emulsions are used in a variety of products, such as lotions, creams, and sunscreens. They are also used in food, such as mayonnaise and salad dressing. In pharmaceuticals, emulsions are used to deliver drugs to the body.

Emulsions are dispersions of liquid droplets in a liquid or semi-solid matrix. If the droplets consist of oil dispersed in an aqueous liquid, they are called oil-in-water (o/w) emulsions, while water-in-oil (w/o) emulsions are water disperse in an oil matrix. Elegant emulsion-based cosmetic formulations can be prepared by incorporating oils using a balanced surfactant system based on non-ionic surfactants, such as polyglycerol esters, fatty alcohol ethoxylates, monoglycerides, sucrose and sorbitan esters, polymers or combinations of emulsifiers and stabilisers. The surfactant system associates primarily with either the oil or aqueous phase but has sufficiently balanced polarity to reside at the interface between the two phases. Thus the surfactant system stabilises the interface and allows two phases to exist as a stable emulsion. This is achieved by stabilising the aqueous (hydrophilic) and oil (lipophilic) components together as an emulsion formulation using the hydrophilic–lipophilic balance (HLB) system.

Source: Cosmetic Formulation Principles and Practice - Heather A.E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva, Kenneth A. Walters

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