Flocculation Transitions of Weakly Charged Oil-in-Water Emulsions Stabilized by Different Surfactants

Abstract

This study concerns dodecane-in-water emulsions stabilized primarily by a range of nonionic surfactants. The emulsions are stable with respect to coalescence and Ostwald ripening but unstable with respect to flocculation and creaming. Sharp and reversible transitions between a flocculated state and a nonflocculated state can be induced by the addition of small mole fractions of either the anionic surfactant sodium octadecyl sulfate or the cationic surfactant octadecyl trimethylammonium bromide and were measured as a function of NaCl concentration. The flocculation transitions are modeled in terms of three types of colloidal forces: a short-range repulsion modeled as a “hard wall”, van der Waals attraction, and electrostatic repulsion. The flocculation transition is taken to correspond to the ionic surfactant concentration for which the maximum in disjoining pressure equals the capillary pressure of the drops. Comparison of calculated and experimental results shows reasonable agreement for emulsions stabilized by n-decyl-β-d-glucoside and 1-O-n-decyl-d,l-xylitol. Emulsions stabilized by n-decyl-N-methylglucamide and n-dodecyl octaoxyethylene and n-hexadecyl 20-oxyethylene glycol ethers (C12E8 and C16E20) reveal a significant negative charge on the drop surfaces in the absence of added ionic surfactant. Additionally, we find that the surface charging of the surfactants with sugar headgroups by addition of charged surfactants is correctly predicted from the concentration of added ionic surfactant with the assumption that the ionic surfactant distributes equally between the emulsion surface monolayer and the micelles present in the continuous phase. For C12E8 and C16E20, the added ionic surfactant is found to distribute preferentially to the emulsion surfaces.