A theory of aqueous emulsions I. Negative interfacial tension at the oil/water interface

It was proposed that negative interfacial tension due to high film pressure is responsible for the formation of micro emulsions. It now appears that the initial, negative interfacial tension γφ in mixed films of soap and long-chain alcohols is the result not so much of a high initial film pressure πḠ as of a large depression of the interfacial tension (γo/w)a between the water and the oil phase with its adsorbed alcohol monolayer in accordance with the equation γφ  (γo/w)a  π. This depression is brought about by the spontaneous distribution of alcohol between the interface and the oil phase. It is pointed out that this distribution is dependent upon the initial chemical potential of the particular alcohol in the given oil and that it may vary within wide limits. The fraction of the alcohol that remains in the oil phase is available to depress the oil/water interfacial tension while the remainder of it forms a mixed film with emulsifier adsorbed from the water phase. It is submitted that the interaction of coulombic, hydrogen bonding and van der Waals forces among the heads and tails of the tenants of this film develops an initial pressure gradient across the flat interface which generates the initial film pressure πḠ. Three stages of pressure development are postulated, the maximum pressure corresponding to an intermediate concentration of alcohol. Hypothetical plots of (γo/w)a and πḠ as ordinates versus concentration of alcohol as abscissa provide a graphical characterization of the process of microemulsification.