NONEXPONENTIAL RELAXATION IN DENSE MICROEMULSIONS

Three-component mixtures of sodium di-2-ethyl-hexylsulfosuccinate (AOT), water and decane, having a fixed molar ratio of water to AOT equal to 40 but with different amount of decane, form one-phase microemulsions in the vicinity of room temperature. The microemulsion consists of spherical droplets of water, coated by mono-layers of AOT, dispersed in continuous medium of decane. The average radius of the droplets is about 85 angstrom with a polydispersity of 25%. By varying the relative amount of decane, the volume fraction of the droplets can be varied continuously from 10 to 70%, simulating gas and liquid phases of simple fluids. The phase diagram of this model microemulsion system, in the temperature-volume fraction (T-phi)-plane, shows a lower miscibility gap at low phi with a lower consolute temperature of 40-degrees-C at a critical volume fraction of 10%. For phi > 40%, the isotropic, disordered microemulsions undergo a first order phase transformation to a lamellar structure upon heating to sufficiently high T. In addition, there is a percolation line cutting across the one-phase region from low to high volume fractions starting near the consolute point. Extensive dynamic light scattering measurements have been used to study time-dependent droplet density correlation functions below and above the percolation temperatures, in both dilute and dense microemulsions. Evolution of the time correlation functions from an exponential to a stretched exponential decay, as the volume fraction and temperature are varied, is discussed in the light of the critical phenomenon, the percolation transition and a glass-like transition at volume fraction of 60%.