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.
Synthetic Emulsifying Agents
Cationic and anionic surfactants are generally limited to use in topical, o/w emulsions. Cationic agents (quarternary ammonium salts) are incompatible with organic anions and are infrequently used as emulsifiers. Soaps are subject to hydrolysis and may be less desirable than the more stable detergents.
Natural Emulsifying Agents
A variety of emulsifiers are natural products derived from plant or animal tissue. Most of the emulsifiers form hydrated lyophilic colloids (called hydrocolloids) that form multimolecular layers around emulsion droplets. Hydrocolloid type emulsifiers have little or no effect on interfacial tension, but exert a protective colloid effect, reducing the potential for coalescence, by:
Hydrocolloid emulsifiers may be classified as:
Naturally occurring plant hydrocolloids have the advantages of being inexpensive, easy to handle, and nontoxic. Their disadvantages are that they require relatively large quantities to be effective as emulsifiers, and they are subject to microbial growth and thus their formulations require a preservative. Vegetable derivatives are generally limited to use as o/w emulsifiers.
The animal derivatives general form w/o emulsions. Lecithin and cholesterol form a monomolecular layer around the emulsion droplet instead of the typically multimolecular layers. Cholesterol is a major constituent of wool alcohols and it gives lanolin the capacity to absorb water and form a w/o emulsion. Lecithin (a phospholipid derived from egg yolk) produces o/w emulsions because of its strong hydrophilic character. Animal derivatives are more likely to cause allergic reactions and are subject to microbial growth and rancidity. Their advantage is in their ability to support formation of w/o emulsions.
Semi-synthetic agents are stronger emulsifiers, are nontoxic, and are less subject to microbial growth. Synthetic hydrocolloids are the strongest emulsifiers, are nontoxic, and do not support microbial growth. However, their cost may be prohibitive. These synthetic agents are generally limited to use as o/w emulsifiers.
Finely Divided or Finely Dispersed Solid Particle Emulsifiers
These agents form a particulate layer around dispersed particles. Most will swell in the dispersion medium to increase viscosity and reduce the interaction between dispersed droplets. Most commonly they support the formation of o/w emulsions, but some may support w/o emulsions. These agents include bentonite, veegum, hectorite, magnesium hydroxide, aluminum hydroxide and magnesium trisilicate.
Auxiliary Emulsifying Agents
A variety of fatty acids (e.g., stearic acid), fatty alcohols (e.g., stearyl or cetyl alcohol), and fatty esters (e.g., glyceryl monostearate) serve to stabilize emulsions through their ability to thicken the emulsion. Because these agents have only weak emulsifying properties, they are always use in combination with other emulsifiers.