An emulsifier molecule consists of a hydrophilic and a lipophilic part; because of this ambivalence, in an environment made up of water and oil the emulsifier tries to find a compromise between its tendencies by placing itself at the interface between the water and the oil with the hydrophilic part orientated towardsthe water and the lipophilic part towards the oil.
In this way a drop of oil is surrounded by many molecules with the outer part soluble in water, so the oil itself becomes more compatible with thewater. An emulsifier system capable of stabilizing an emulsion of a certain oil does not necessarily also stabilize the other oil components of the fatty phase of the emulsion.
Different oily substances require different emulsions or emulsion systems. The first attempt to predict which emulsion system would be needed to emulsify a given fatty substance was made with the introduction of the HLB number.