New Phases Found in Reverse Micelle Systems with High Concentrations of AOT

Abstract

This paper discusses the phase behavior, rheology, and structure of self-assembled sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelle systems at high AOT concentrations. When the amount of AOT and w(o) (the molar ratio of water to AOT) were changed, many different phases were found, a fact which is not discussed in the literature. Opaque gel-like phase (phase separation) occurred with high concentrations of AOT in organic solvents without water. When the AOT concentration and w(o) were increased to 18-72 m and 2, respectively, the samples were gel-like and translucent. Dynamic rheological results indicate that the viscoelastic transition agreed with a multirelaxation time model. Small-angle X-ray scattering (SAXS) results imply that these samples showed a hexagonally close-packed cylindrical structure in which the diameter of a cylinder was ~2.5-3.0 nm, depending on the water contents. Moreover, these AOT cylinders self-assembled into fiber bundles with a diameter of 1-10 μm, as determined using a polarized optical microscope. As w(o) was increased to 2-6 in 72 m AOT samples, similar rheological and SAXS results were obtained. However, a different type of viscoelastic transition occurred, from multirelaxation to single-relaxation, when w(o) was increased to 7-11. The samples were in the transparent gel-like phase, and the structures determined by SAXS were a combination of hexagonally packed cylindrical and lamellar structure. The close-packed cylindrical structures had larger radii and shorter lengths with increasing w(o). Furthermore, when w(o) was increased to 12, the gel-like phase disappeared and a highly viscous solution was observed. This is because all the cylindrical structures collapsed and transformed into lamellar structures when the amount of water was further increased.