Flocculation of Emulsions by Free Surfactant. II. Purified Systems

Abstract

Previous studies demonstrated that a variety of surfactants destabilize paraffin oil-in-water emulsions when their concentration exceeds about 1–5 Wt%. We suggested that this process was driven by micelle exclusion and presented a simple model that was consistent with experimental observations, e.g., effect of droplet size. However, because commercial multi-component surfactants and a complex oil were used, questions arose about the role of impurities. The study has now been extended to monodisperse nonionic (alcohol ethoxy-late) and cationic (alkyltrimethylammonium bromide) surfactants and highly purified alkanes. The pure nonionics display the same behavior as their commercial counterparts, i.e., accelerated creaming above a critical surfactant concentration. The process is exclusively reversible flocculation and is not accompanied by a change in droplet size. Cationics also display a destabilizing effect: however, the sensitivity of oil-in-water emulsions to surfactant concentration is very dependent on total ionic strength. At low ionic strength, the emulsions are insensitive to cationic surfactant concentration. This behavior is consistent with the effect of the electrical double layer. A few implications of micelle exclusion to emulsion stability and its dependence on surfactant structure are discussed.