I read
The emulsifying agent forms an interfacial film between suspended particles and the medium. The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.
What specific property of each type of emulsifying agent makes it suitable for the particular application for O/W or W/O emulsion?
An emulsifier is a surfactant. A surfactant is characterized by the fact that its molecule has one or more hydrophilic parts and one or more hydrophobic parts. The ratio between the molar mass of the hydrophilic parts and the molar mass of the hydrophobic parts in the molecule is expressed by its HLB value (Hydrophilic-lipophilic balance).
Higher HLB means higher water solubility, and lower HLB means lower water solubility.
The emulsifier should have a certain solubility in the outer phase of the emulsion to be distributed in the emulsion.
An emulsifier for forming o/w-emulsions should have higher water solubility, that means higher HLB, and an emulsifier for forming w/o-emulsions should have lower water solubility (= higher oil solubility), that means lower HLB. The corresponding range of HLB is given in the Wikipedia artivle above.
