Emulsion

An emulsion can be described as a collection of tiny droplets of one liquid (e.g., an oil) dispersed in another liquid (e.g., water) in which it, the first liquid, is insoluble. Emulsions are formed by mixing these two liquids with a third substance, known as an emulsifier, which creates a uniform, stable dispersion of the first liquid in the second.

Emulsifiers belong to the class of chemicals known as surfactants, or surface active agents, which are able to reduce the surface tension of liquids. This ability is important because surface tension, (one of the physical properties which determines how liquids behave) must be overcome for the two liquids to effectively intermingle. Surfactants are able to create this effect by virtue of the dual nature of their molecular structure. One part of the molecule is soluble in water, the other in oil; therefore, when an emulsifying surfactant is added to a mixture of oil and water it acts as a "bridge" between the two immiscible materials. This bridging effect reduces the forces between the liquid molecules and allows them to be broken into, and maintained as, separate microscopic droplets.

In an emulsion millions of these tiny surfactant bridges surround the dispersed droplets, shielding them from the other liquid in which they are mixed. The dispersed drops are called the internal phase, the liquid that surrounds the drops is known as the external phase. Depending on the desired characteristics of the finished emulsion, either water or oil may comprise the external phase. The emulsion is further characterized by the type of charge carried by its emulsifiers: it can be anionic (containing a negative charge), cationic (containing a positive charge), or nonionic (containing no charge).