Apparent equilibration time required for a surfactant-oil-water system to emulsify into the morphology imposed by the formulation

This paper deals with surfactant-oil-water (SOW) system whose formulation corresponds to a Winsor type I phase behavior at equilibrium. When the surfactant is initially dissolved in the oil phase and the system is immediately stirred, the usual emulsion morphology is the anomalous W/O type, which is not the expected one for a Winsor type I formulation. However, if the SOW system is left to rest and to partially equilibrate during some time, it could result in a normal O/W emulsion upon stirring, as if it were fully equilibrated for several days. The minimum rest time required to produce the normal emulsion morphology is called the apparent equilibration time t(APE). It is shown that t(APE) strongly depends on formulation and decreases as formulation approaches the boundary between Winsor I and Winsor III phase behavior. In some instances t(APE) is essentially zero, as if equilibration had taken place instantly. The apparent equilibration time t(APE) is found to depend on the surfactant molecular weight and on the oil viscosity.