Although it often has been said that “oil and water don't mix,” food science professionals know better. Not only can oil and water be mixed, but the so-called emulsion can be stabilized by incorporating a type of ingredient known as a surfactant, or surface active agent.
Emulsions are formed when one immiscible phase is dispersed in another through mechanical action such as homogenization. Oil-in-water (O/W) emulsions are formed when oil is dispersed in an aqueous phase (e.g., mayonnaise) while water becomes the dispersed phase in water-in-oil (W/O) emulsions (e.g., margarine). Surfactants help keep the immiscible phases in an emulsion together because they are amphiphilic and thus have properties that are compatible with both the hydrophilic and lipophilic portion of the mixture. Non-polar ends of an emulsifier align themselves within the lipid phase, while the polar ends align in the water phase. As a result, an electrically charged surface forms between the two immiscible phases, or at the interface, causing particles to repel one another rather than coalesce.