Page "Emulsion" Paragraph 8

Whether an emulsion of oil and water turns into a " water-in-oil " emulsion or an " oil-in-water " emulsion depends on the volume fraction of both phases and the type of emulsifier ( surfactant ) ( see Emulsifier, below ) present.

In general, the Bancroft rule applies.

Emulsifiers and emulsifying particles tend to promote dispersion of the phase in which they do not dissolve very well.

For example, proteins dissolve better in water than in oil, and so tend to form oil-in-water emulsions ( that is, they promote the dispersion of oil droplets throughout a continuous phase of water ).