Contact angles in oil-in-water emulsions stabilized by ionic surfactants

Abstract Contact angles in aqueous films separating emulsion droplets have been measured for various ionic surfactants as a function of type and concentration of added electrolyte, and of temperature. As in foam films in air, appreciable contact angles develop only with specific electrolytes and above a critical electrolyte concentration, which increases with temperature. The relative effectiveness of different counter-ions depends on the nature of the surfactant head group. When the contact angle is significant, emulsion droplets deform when they flocculate. This alters the structure and probably also the mechanical properties of creams formation emultions by sedimentation. It was shown, on the basis of their equilibrium volume fraction of oil, that the structure of these creams is sensitive to the type and concentration of the counter-ion, to temperature, and to the manner in which the creams are formed, i.e., by sedimentation under normal gravity forces, or by centrifugation followed by relaxation. The presence of a contact angle dramatically alters the rate of clearing of dilute emulsions composed of very small droplets. This rapid flocculation is highly counter-ion specific and reversible by increasing the temperature.