Multiple emulsions 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.
Potential applications for multiple emulsions are well-documented and many of these applications have been patented. The important applications are in cosmetics (see Cosmetics Using W/O/W Multiple Emulsions), pharmaceuticals and foods. For example, in cosmetics they have a fine texture and a smooth touch upon application, and they are aimed for slow and sustained release of active matter from an internal reservoir into the continuous phase (mostly water). They can serve as an internal reservoir to entrap matter from the outer diluted continuous phase into the inner confined space. They can also improve dissolutions or solubilization of insoluble materials. Due to these properties, multiple emulsions find applications related to protecting sensitive and active molecules such as vitamins C and E from the external phase—a process called antioxidation. Patents from the past 15 years indicate that work is now being carried out on the stability and preparations aspects of multiple emulsions, so one can expect many more applications to emerge in the near future.