ROTATIONAL REORIENTATION DYNAMICS OF AEROSOL-OT REVERSE MICELLES FORMED IN NEARCRITICAL PROPANE

The rotational reorientation kinetics of two fluorescent solutes (rhod amine 6G, R6G, and rhodamine 101, R101) have been determined in sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT, AOT) reverse micelles f ormed in liquid and nearcritical propane. We show that the amount of w ater loading ([water]/[AOT], R), continuous phase density, and tempera ture all influence the solute rotational dynamics. fn all cases, the d ecay of anisotropy data (i.e., frequency-dependent differential polari zed phase angle and polarized modulation ratio) are well-described by a hi-exponential decay law. We find that the faster rotational correla tion times are similar to but slightly less than the values predicted for an individual AOT reverse micelle rotating in propane. The recover ed rotational correlation times range from 200 to 500 ps depending on experimental conditions. This faster rotational process is explained i n terms of lateral diffusion of the fluorophore along the water/headgr oup interfacial region within the reverse micelle. The recovered value s for the slower rotational correlation times range from 7 to 18 ns, T hese larger rotational reorientation times are assigned to varying mic elle-micelle (i.e., tail-tail) interactions in the low-density, highly compressible fluid region. We also quantify the contribution of the r everse micellar ''aggregate'' to the total decay of anisotropy.