Structure and dynamics of water-in-oil microemulsions near the critical and percolation points

The three-component ionic microemulsion system consisting of AOT/water/ decane shows an interesting phase behavior in the vicinity of room temperature. The phase diagram in the temperature-volume fraction (of the dispersed phase) plane exhibits a lower consolute critical point at about 40°C and 8% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to the high-volume fraction side at progressively lower temperatures. This phase behavior can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via a hard-core plus a short-range attractive interaction, the strength of which increases with temperature. We show that Baxter's sticky-sphere model can account quantitatively for the phase behavior, including the percolation line, provided that the stickiness parameter is a suitable function of temperature. We use the structure factors measured by small-angle neutron scattering below the critical temperature to determine this functional dependence. We also investigate the dynamics of droplets, below and approaching the critical and percolation points, by dynamic light scattering. Both theQ dependence of the first cumulant and the time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, USA.