Emulsions are part of a more general class of two-phase systems of matter, called colloids (see Colloids). Although the terms colloid and emulsion are sometimes used interchangeably, in emulsion both the dispersed and the continuous phase are liquid. Emulsions are also subclass of Suspensions.
An emulsion is a mixture of two immiscible (unblendable) substances. One substance (the dispersed phase) is dispersed in the other (the continuous phase).
Emulsions have a cloudy appearance, because the many phase interfaces (the boundary between two phases) scatter light (see Light: scattering).
Emulsions are unstable and thus do not form spontaneously. Energy input through stirring, etc., or spray processes are needed to form it. Over time, emulsions tend to revert to the stable state of oil separated from water. Surface active substances (surfactants, see Surface tension) can increase the kinetic stability of emulsions greatly so that, once formed, the emulsion does not change significantly over years of storage.
Emulsification is the process by which emulsions are prepared.
Emulsions are frequently used in drugs.
In daily life
A large field of application is food and cosmetic industry. Examples of emulsions include oil in water and butter. In butter and margarine (are also suspensions), a continuous lipid phase surrounds droplets of water (water-in-oil emulsion).
There are three types of emulsion instabilities:
· flocculation, where the particles form clumps;
· creaming, where the particles concentrate towards the surface (by buoyancy or by e.g. centrifugation ) of the mixture while staying separated;
· breaking, where the particles coalesce (recombination to form bigger ones) due to lack of shaking and form a layer of liquid.
An emulsifier, also known as an emulgent or surfactant, is a substance which stabilizes an emulsion. An example of food emulsifiers is egg yolk (where the main emulsifying chemical is the phospholipid lecithin). Proteins and low-molecular weight emulsifiers are common as well.
Detergents, another class of surfactant, chemically interact with both oil and water, thus stabilizing the interface between oil or water droplets in suspension. This principle is exploited in soap to remove grease for the purpose of cleaning. A wide variety of emulsifiers are used to prepare emulsions such as creams and lotions.
Whether an emulsion turns into a water-in-oil emulsion or an oil-in-water emulsion depends on the volume fraction of both phases and on the type of emulsifier. Generally, the so-called Bancroft rule applies: emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well. For example, proteins dissolve better in water than in oil and so tend to form oil-in-water emulsions (that is they promote the dispersion of oil droplets throughout a continuous phase of water).