Analysis of dynamic surfactant mass transfer and its relationship to the transient stabilization of coalescing liquid-liquid dispersions

In this work, experiments describing the behavior of the separation of a model liquid-liquid dispersion with various concentrations of a synthetic surfactant are presented which indicate that there is a dynamic stabilization of initially unstable emulsions that occurs when the initial surfactant concentration approaches the concentration that provides stable emulsions. A simple model is presented to suggest the mechanism for the dynamic stabilization of these emulsion systems that considers the redistribution of surfactant into the continuous phase after a coalescence event at the emulsion-bulk dispersed phase interface and the dynamic mass transport of surfactant in the continuous phase of the emulsion. The results indicate that coalescence at the interface between the emulsion layer and the bulk dispersed phase creates a local region in the vicinity of this interface where the concentration of the surfactant is much higher than the bulk surfactant concentration which could lead to a locally, dynamically stabilized emulsion at this interface. The extent to which the local excess surfactant concentration reduces the coalescence rate depends strongly on the rate of coalescence at the dense packed layer-bulk dispersed phase interface relative to the rate of surfactant diffusion through the dense packed layer and, of course, on the surfactant adsorption constant, the maximum adsorbed surfactant concentration, and the surface to volume ratio of the dispersed phase. Furthermore, the results indicate that coalescence can also act to significantly increase the local concentration of an initially very dilute surfactant in the vicinity of the interface between the emulsion layer and the bulk dispersed phase interface.