Multiple emulsions1 are complex systems in which the drops of the dispersed phase themselves contain even smaller dispersed droplets that normally consist of a liquid that is miscible, and in most cases identical, with the continuous phase. They are, therefore, emulsions of emulsions. In cosmetics, these systems can prevent degradation of an active ingredient and release it at a controlled rate. This article reviews the different techniques for preparing multiple emulsions. These techniques are more complicated than for simple oil-in-water (o/w) or water-in-oil (w/o) emulsions, but may be worth the extra effort for formulators wishing to protect and deliver sensitive actives.
In multiple emulsions, the internal and external phases are alike and an intermediate phase separates the two like phases. The intermediate phase is immiscible with the two like phases. For example, in water-in-oil-in-water (w/o/w) multiple emulsions, a w/o emulsion is dispersed in a water-continuous phase. An emulsifier is present to stabilize the emulsions and various ionic and nonionic surfactants are available for this purpose. Lipophilic (oil-soluble, low HLB) surfactants are used to stabilize w/o emulsions, whereas hydrophilic (water-soluble, high HLB) surfactants are used to stabilize o/w systems.