Formulation of oil-in-water emulsions begins with the simplest of equations: Emulsion = Emulsifier + oil/wax phase + water phase. Yet as with so many simple concepts, it may not be easy to get the desired result, or to keep it stable. A common problem with emulsions arises when the formulation produces unwelcome “soaping” in manufacture or in use.
In simple terms, soaping is the incorporation of air into an emulsion via exposure to air and agitation. When you rub lotion vigorously onto skin, for example, the thin film of tiny bubbles appears as a white film. In time, the emulsion destabilizes due to evaporation, and the foam breaks up. Even though the initial soaping effect does disappear, it is usually undesirable in lotions and creams.