REVERSE MICELLES IN SUPERCRITICAL FLUIDS .2. FLUORESCENCE AND ABSORPTION SPECTRAL PROBES OF ADJUSTABLE AGGREGATION IN THE 2-PHASE REGION

The properties of bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles and microemulsions in supercritical fluid (SCF) ethane, liquid propane, and other alkanes are reported. The microscopic environment inside the reverse micelles was investigated with the absorption probe pyridine N-oxide and the fluorescence probe 8-anilino-1-naphthalenesulfonic acid (ANS). The microscopic behavior is related directly to a macroscopic property, the water-to-surfactant ratio Wo. In the one-phase region, a reverse micelle in a SCF is much like that in a liquid solvent. However, in the two-phase region, both the microscopic and macroscopic properties may be adjusted with pressure in ethane and propane, because of changes in the partitioning of the components between the phases. The large effect of pressure on Wo at saturation, Wosat, and likewise on the micelle radius, is described in terms of the repulsive solvent penetration of the surfactant tails and the attractive tail-solvent interactions. © 1990 American Chemical Society.