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

Abstract

We have investigated the dissociation of AOT monolayers stabilising oil-in-water microemulsion drops in Winsor I systems where the drops are in equilibrium with an excess oil phase and the monolayers have their preferred curvature. For planar monolayers, we derive a relationship between the degree of dissociation definedin terms of the negative adsorption of the co-ions, α_1p, and that defined in terms of "‘free’' and "‘bound’' counterions, α_2p. For typical values of the surface potential, α_2p is commonly 5–10 times larger than α_1p. We describe the estimation of the degree of dissociation of microemulsion droplets from conductivity, emf and electrophoretic mobility measurements. With increasing concentration of either NaCl or AOT, the degree of dissociation decreases as phase inversion is approached when the preferred monolayer curvature becomes close to zero. For different oils, the NaCl concentration dependence of the degree of dissociation shows a clear correlation with that required for phase inversion. For the conductivity results, effects due to obstruction of small ions by the microemulsion drops and changes in drop mobility with drop volume fraction are discussed.