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.