Rotational dynamics of AOT reversed micelles in near-critical and supercritical alkanes

Abstract

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, ϕ_mic, and the rotational correlation length, ξ_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 microemulsion phase boundary is approached by lowering the pressure both the solvent density and viscosity decrease, but the rotational correlation length, ξ_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.