Flocculation of weakly charged oil-water emulsions

This study concerns dodecane-water emulsions stabilized primarily by the nonionic surfactant decyl-beta-D-glucoside (C(10)beta Glu). The emulsions are stable with respect to coalescence and Ostwald ripening but unstable with respect to flocculation and creaming. Emulsions stabilized by monolayers of pure C(10)beta Glu flocculate and show rapid creaming. A sharp transition from a flocculated state to a nonflocculated state can be induced by the addition of small mole fractions of either the anionic sodium octadecylsulfate or the cationic octadecyltrimethyIammonium bromide surfactants. The nonflocculated emulsions show slower creaming and the discontinuous change in creaming rate can be used to detect the flocculation transition. We have measured the concentration of ionic surfactant required to induce the flocculation transition as a function of sodium chloride concentration in the continuous aqueous phase. Using a model in which the colloidal interactions between the deformable emulsion drops are taken to consist of electrostatic repulsion, van der Waals attraction, and a abort-range repulsion modeled as a "hard wall", we show how the flocculation transition can be quantitatively predicted using no adjustable parameters.