Water-in-oil-in-water (w/o/w) emulsions have a wide range of uses (see Future of W/O/W Emulsions), but numerous factors can affect their stability. Among these factors are the method of preparation, the type of entrapped material, the type of emulsifier, the type of oil, and the effects of electrolyte, phase volumes and concentration. Of particular interest in this article is the type of oil.
The oils most frequently used to form multiple emulsions are hydrocarbons, triglycerides and esters. Hydrocarbons are compounds with a carbon backbone and attached hydrogen atoms. Among the hydrocarbons the most widely used is mineral oil, which also is called liquid paraffin; mineral oil is used in finished commercial preparations and for research in multiple emulsions. Hexadecane, dodecane, octane and cyclohexane have principal applications in research, especially in theoretical studies to explain the structure and mechanisms of multiple emulsions. Squalane, which is a hydrogenated form of squalene, is used occasionally in the industry for preparation of multiple emulsions. Squalane is also called cosbiol and perhydrosqualene.
Triglycerides are compounds in which glycerol is esterified with three fatty acids. The main triglycerides used are the oils of peanut, olive, sesame, almond, maize, castor and soybean. They are used mainly for in vivo experiments because of their tolerability.
Esters are compounds in which an organic group replaces a hydrogen atom in an oxygen acid. Esters of long-chain fatty acids including isopropyl myristate or oleate also have been used to obtain stable multiple emulsions.
The different molecular structures of these types of oils means they have different effects on the behavior of the emulsion system and the stability of the oil membrane against leakage of the entrapped material. If a rupture occurs in an oil layer, the compartments disappear instantly and the inner aqueous phase in the compartments is mixed with the aqueous suspending fluid. Therefore, the polarity, density, viscosity and other physicochemical properties of oils influence the behavior of emulsions.