Dissociation of AOT monolayers stabilising oil-in-water microemulsions in Winsor I systems

We have investigated the dissociation of AOT monolayers stabilising oil-in- water microemulsion drops in Winsor I systems where the drops are in equili brium with an excess oil phase and the monolayers have their preferred curv ature. For planar monolayers, we derive a relationship between the degree o f dissociation defined in terms of the negative adsorption of the co-ions, alpha(1p), and that defined in terms of "free" and "bound " counterions, al pha(2p). For typical values of the surface potential, alpha(2p) is commonly 5-10 times larger than alpha(1p). We describe the estimation of the degree of dissociation of microemulsion droplets from conductivity, emf and elect rophoretic mobility measurements. With increasing concentration of either N aCl or AOT, the degree of dissociation decreases as phase inversion is appr oached when the preferred monolayer curvature becomes close to zero. For di fferent oils, the NaCl concentration dependence of the degree of dissociati on shows a clear correlation with that required for phase inversion. For th e conductivity results, effects due to obstruction of small ions by the mic roemulsion drops and changes in drop mobility with drop volume fraction are discussed.