A chemical used in preparation and maintenance of an oil- or synthetic-base drilling fluid that forms a water-in-oil emulsion (invert emulsion). An oil-mud emulsifier lowers the interfacial tension between oil and water, which allows stable emulsions with small drops to be formed. Historically, oil-mud emulsifiers have been classified as primary and secondary. Secondary emulsifiers are generally not used alone to make a stable oil mud. Emulsifiers can be calcium fatty-acid soaps made from various fatty acids and lime, or derivatives such as amides, amines, amidoamines and imidazolines made by reactions of fatty acids and various ethanolamine compounds. These emulsifiers surround water droplets, like an encapsulating film, with the fatty acid component extending into the oil phase. Emulsifier molecules that cannot fit around drops form clusters (micelles) in the oil phase or adsorb onto solids. Oil-mud emulsion drops each behave like a small osmotic cell. The emulsifier around the drops acts like a semipermeable membrane through which water can move but ions cannot pass. Thus, oil muds have the special capability (which water muds do not have) to control water transfer to and from the drops simply by adjusting salinity within the water phase of the oil mud.