Structure and dynamics of dense water-in-oil microemulsions below percolation threshold

Abstract

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 around 10% volume fraction and a percolation line, cutting across the plane starting from the vicinity of the critical point, extends to 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 for the phase behavior including the percolation line, quantitatively, 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 percolation line by dynamic light scattering. Both the first cumulant and the droplet density time correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters form by the microemulsion droplets due to the attraction. Analysis of SANS data indicates that above the percolation line, the clustered droplets structure gradually transforms into an ordered bicontinuous structure