Emulsion

An emulsion is a mixture of two immiscible substances. One substance (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions include butter and margarine, mayonnaise, the photo-sensitive side of film stock, and cutting fluid for metalworking. In butter and margarine, a continuous lipid phase surrounds droplets of water (water-in-oil emulsion).

Emulsifier

An emulsifier (also known as a surfactant) is a substance which stabilizes an emulsion. Lecithin (found in egg yolk) is an example of a food emulsifier, but proteins and low-molecular weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called Pickering stabilisation . Both mayonnaise and Hollandaise sauce are oil-in-water emusions stabilized with egg yolk lecithin. Detergents are another class of surfactant, and will bind to both oil and water, thus holding microscopic oil droplets in suspension. This principle is exploited in soap to remove grease from plates, etc..

Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends of the volume fraction of both phases and on the type of emulsifier. Generally, the Bancroft rule applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well. E.g. proteins tend to form oil-in-water emulsions.

Emulsions tend to have a cloudy appearance, because the many phase interfaces scatter light that passes through the emulsion.

Emulsions can suffer from a number of instabilities. Homemade oil and vinegar salad dressing is an unstable emulsion that will quickly separate unless shaken continuously. This phenomenon is called coalescence, and is caused by recombination of small droplets combine to bigger ones. Fluid emulsions can also suffer from creaming, the migration of emulsion droplets to the top of an oil-in-water emulsion under the influence of gravity.

Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colliod and emulision are sometimes used interchangably, emulsion tends to imply that both the dispersed and the continuous phase are liquid.

Emulsion paint

An emulsion paint (often abbreviated to emulsion) is a water-based paint commonly used for painting indoor surfaces. Emulsion paints are also known as latex paints. It is so called because the polymer is formed through an emulsion polymerization whereby the monomers were emsulified in a water continuous phase. The polymer itself is not soluble in water and hence the paint is water resistant after it has dried. Residual surfactants in the paint as well as hydrolytic effects with some polymers cause the paint to still be susceptible to softening and, over time, degradation by water.