ROTATIONAL-DYNAMICS OF AOT REVERSED MICELLES IN NEAR-CRITICAL AND SUPERCRITICAL ALKANES

Time-resolved fluorescence depolarisation studies of Aerosol-OT (AOT) reversed micelles containing the hydrophilic probe perylene tetracarboxylate in ethane and propane at high pressure are reported. Such measurements have been made at sub-nanosecond resolution and support the concept of a simple spherical structure for AOT-stabilised aggregates in supercritical and near-critical fluids at high pressure. The results show a negligible pressure dependence for both the micellar rotational correlation time, phi-mic, and the rotational correlation length, zeta-r. For w = 3 (w = [water]/[AOT]) reversed micelles far from the one-phase (L2) --> two-phase boundary in both supercritical ethane and near-critical propane, the results obtained for the hydrodynamic radius of the reverse micelles, r(h), are consistent with those obtained from other techniques. As the micro-emulsion phase boundary is approached by lowering the pressure both the solvent density and viscosity decrease, but the rotational correlation length, zeta-r, increases only slightly. As w increases in the region between w = 3 and w = 10, the internal motion of the probe molecule, 3,4,9,10-perylene sodium tetracarboxylate (PTC) becomes dominant, and the technique becomes less sensitive to overall droplet rotation and therefore to the determination of the droplet size.