Stability of oil-in-water emulsions in a low interfacial tension system

The stability of oil-in-water emulsions to both creaming and coalescence was measured as a function of salt concentration in heptane + water mixture stabilized by sodium bis(2-ethylhexylsulfosuccinate) (AOT). Emulsions were prepared from pre-equilibrated phases in Winsor I systems. Up to 0.035 M NaCl, the creaming rate decreases with salt concentration, with no visible sign of coalescence. Above 0.035 M and approaching the phase inversion salt concentration of 0.055 M, the creaming rate increases quite markedly and coalescence becomes appreciable. The creaming at low salt concentrations is due mainly to the buoyancy motion of single drops. A simple model for the time evolution of resolved water is developed which successfully describes the behavior. The drop size changes observed are shown to be due to Ostwald ripening, the rate of which decreases with salt concentration. Experimental ripening rates are consistent with a mechanism by which oil is transported between emulsion drops via microemulsion droplets present in the continuous phase. We calculate the energy of interdrop interaction allowing for drop deformation using experimentally determined parameters of interfacial tension, drop radius, and zeta potential. At high [NaCl], due mainly to the low interfacial tension, the drops can deform and the attraction between them becomes significant. As a result, flocculation occurs which leads to coalescence instability.