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

The stability of oil-in-water emulsions to both creaming and coalescence wa s measured as a function of salt concentration in heptane + water mixture s tabilized by sodium bis(2-ethylhexylsulfosuccinate) (AOT). Emulsions were p repared from pre-equilibrated phases in Winsor I systems. Up to 0.035 M NaC l, the creaming rate decreases with salt concentration, with no visible sig n of coalescence. Above 0.035 M and approaching the phase inversion salt co ncentration of 0.055 M, the creaming rate increases quite markedly and coal escence 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 rip ening, the rate of which decreases with salt concentration. Experimental ri pening rates are consistent with a mechanism by which oil is transported be tween emulsion drops via microemulsion droplets present in the continuous p hase. We calculate the energy of interdrop interaction allowing for drop de formation using experimentally determined parameters of interfacial tension , drop radius, and zeta potential. At high [NaCl], due mainly to the low in terfacial tension, the drops can deform and the attraction between them bec omes significant. As a result, flocculation occurs which leads to coalescen ce instability.