Stabilisation of protein-containing water-in-oil emulsions

Silicones are highly hydrophobic entities, which can make the formulation of stable water emulsions challenging. Silicone oil and elastomer interfaces are also known to act as protein denaturants. Thus, formulation of sllicone/protein emulsions in which proteins are present as passive or active ingredients might be considered to be of questionable value. However, there are many reasons to consider the formation of such emulsions, including the ability to deliver proteins for industrial or medical applications. Silicone emulsions are already widely used in personal care applications, for example in hair and skin care, where protein/silicone contact will eventually arise. Reports exist of the spontaneous emulsification of silicone oils in contact with proteins, for example in the eye, following retinal repair. Little work, however, has been done on formulating silicone emulsions that contain proteins. Generally, anionic or non-ionic surfactants are used to stabilize oil-in-water and water-in-oil emulsions, respectively. Water-in-silicone oil emulsions containing proteins can readily be prepared using silicone surfactants that bear only a small number of hydrophilic groups. Two classes of silicone surfactants are presented: those such as commercial silicone-polyethylene oxide copolymers that cannot form a covalent bond between protein and silicone, and compounds like (EtO)3Si(CH 2 ) 3 O(Me2SiO)n(CH 2 ) 3 -Si(OEt) 3 , which can. The protein may be a passive participant in the emulsion, or may act as a vital constituent of the oil/water interface. The proteins in these emulsions lie at the oil/water interface regardless of whether they act as cosurfactants. At the interface, and unlike the situation with unstabilized silicone oil emulsions, the proteins have generally been shown using enzyme assays to be as natured as the controls that have not been exposed to silicones. Guidelines for formulating protein-containing water-in-silicone oil emulsions are discussed.