Structural and dynamical transformations between neighboring dense microemulsion phases

A small-angle x-ray scattering (SAXS) study of dense AOT-water-decane microemulsions [AOT denotes sodium bis(2-ethylhexyl) sulfosuccinate] was undertaken in order to delineate clearly the phase behavior and corresponding structural transitions for AOT-plus-water volume fractions ranging from φ=0.60 to 0.95. Spectra were collected for temperatures between T=3 and 65 °C. The resulting T-vs-φ phase diagram indicates three distinct structural domains when the water-to-AOT molar ratio is fixed at W=40.8, namely, the previously investigated ${\mathit{L}}_2$ droplet phase, a high-temperature ${\mathit{L}}_{\mathrm{\alpha }}$ lamellar phase, and a low-temperature ${\mathit{L}}_3$ phase consisting of randomly connected lamellar sheets. A significantly wide coexistence region accompanies the droplet-to-lamellar phase transition, which is demonstrated to be first order. For W between 15 and 40, an analysis of the lamellar structure using a one-dimensional paracrystal model produces a Hosemann g factor indicative of an approximately constant variation in the lamellar spacing of about 8%. The SAXS study was supplemented by dielectric-relaxation, shear-viscosity, and quasielastic light-scattering measurements in order to substantiate the observed phase transitions and further our understanding of the structural and dynamical properties of the ${\mathit{L}}_3$ phase. It was found that the ${\mathit{L}}_3$ phase exhibits Newtonian behavior up to a shear rate of 790 ${\mathrm{s}}^{\mathrm{-}1}$, in contradiction to previous theoretical considerations. The phase exhibits two distinct relaxation modes. A relaxation time of ∼1 ms characterizes the Brownian motion of a single lamellar sheet, while the motion of the entire interconnected sheet assembly has a relaxation time on the order of 1 s.