STRUCTURAL EVOLUTION OF BICONTINUOUS MICROEMULSIONS

The phase behavior and microstructure of a three-component microemulsion system composed of sodium bis(ethylhexyl) sulfosuccinate (AOT)/water (0.6 wt % NaCl)/decane, have been extensively studied in the recent years. In this paper we investigate the structural inversion of a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at high temperatures through a bicontinuous structure at the intermediate temperature using small-angle neutron scattering (SANS) technique. Specifically we study the temperature evolution of the microstructure for a system having an equal water and decane volume fraction of 45% and the surfactant volume fraction of 10%. SANS intensity distributions for seven temperatures ranging from 37.3 to 50.3-degrees-C have been measured with two samples, one having an oil-water contrast (the bulk contrast) and the other a surfactant-oil/water contrast (the interface contrast). We analyze the data using the Teubner-Strey model, for the bulk case, and a modified Berk model, for both the bulk and the interface cases. It is shown that a least two length scales are required in the theory to quantitatively fit the experimental data. Clear evidence for a structural inversion through a "disordered bicontinuous" microstructure around 40-degrees-C is established. For the disordered bicontinuous microemulsions, the modified Berk model is shown to be capable of reproducing the scattering intensity distributions of both the bulk and the interface contrast cases with the same set of parameters.