Abstract
Previous studies demonstrated that a variety of surfactants induce paraffin oil-in-water emulsions to flocculate when their concentration exceeds a few weight percent. 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. The study has now been extended to monodisperse nonionics (alcohol ethoxylate) and also a pure cationic (alkyltrimethylammonium bromide) surfactant using 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. A few implications of micelle exclusion to emulsion stability and the limitations of the simple model are discussed.
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