Structure and properties of AOT reversed micelles as studied by the fluorescence probe technique

The microviscosity in the water pool of Aerosol OT (AOT) reversed micelles was estimated with the fluorescence yield of a viscosity-sensitive fluorescence probe, auramine O (AuO), and the steady-state fluorescence depolarization of xanthene dyes solubilized in the micellar core. The fluorescence yield measurement of AuO showed that the microviscosity eta(W) at the vicinity of the AOT anionic head groups falls abruptly up to R(W) ([H2O]/[AOT])=10 and then gradually decreases until the upper limit of the water solubilization. The eta(W)-T-W curve almost coincides with the curve obtained from the steady-state fluorescence depolarization of RhB. Thus, the availability of the AuO method was confirmed. Fluorescence depolarization of xanthene dyes with different ionic characters evidently demonstrated the heterogeneity of the water pool. The effects of temperature, surfactant concentration, solvent, and counterion on the microviscosity were examined to obtain a more detailed picture of the static structure of AOT reversed micelles. In addition, the fluorescence depolarization of a solubilized dye demonstrated that dynamic fluctuation occurs at the vicinity of the phase separation in the cooling process. Finally, the intermicellar aggregation state was studied by utilizing the excitation energy transfer phenomenon. A suitable combination of donor and acceptor molecules solubilized in the water pool illustrates that AOT reversed micelles are not randomly dispersed in an isolated state in a nonpolar medium, but are clustered through intermicellar coagulation.