Emulsions are stabilized
by adding an emulsifier or emulsifying agents. These agents have both a hydrophilic
and a lipophilic part in their chemical structure. All emulsifying agents concentrate
at and are adsorbed onto the oil:water interface to provide a protective barrier
around the dispersed droplets. In addition to this protective barrier, emulsifiers
stabilize the emulsion by reducing the interfacial tension of the system. Some
agents enhance stability by imparting a charge on the droplet surface thus reducing
the physical contact between the droplets and decreasing the potential for coalescence.
Some commonly used emulsifying agents include tragacanth, sodium lauryl sulfate,
sodium dioctyl sulfosuccinate, and polymers known as the Spans® and Tweens®.
Emulsifying agents can be classified according to: 1) chemical structure; or
2) mechanism of action. Classes according to chemical structure are synthetic,
natural, finely dispersed solids, and auxiliary agents. Classes according to
mechanism of action are monomolecular, multimolecular, and solid particle films.
Regardless of their classification, all emulsifying agents must be chemically
stable in the system, inert and chemically non-reactive with other emulsion
components, and nontoxic and nonirritant. They should also be reasonably odorless
and not cost prohibitive.