Emulsifiers are added to water-based lubricants, so that oil based additives can be mixed into the water; these include soap, alcohol esters, and ethylene oxides.
Variants may include eggs as an emulsifier and omitting or reducing the bulk ingredient, which makes for a result similar to the Provençal aïoli, Catalan allioli, and so on.
He is known for the Bancroft rule: a predominantly hydrophilic emulsifier stabilizes an oil-in-water emulsion, whereas a predominantly hydrophobic emulsifier stabilizes a water-in oil emulsion.