Nanosecond reorganization of water within the interior of reversed micelles revealed by frequency-domain fluorescence spectroscopy

The fluorescence properties of 1,8-ANS (1-anilino-8-naphthalenesulfonic acid) in AOT (Aerosol-OT; 1,4-bis[2-ethylhexyl] sulfosuccinate) reverse micelles have been studied. Using steady-state and time-resolved methods we show that water content and temperature play key roles in the photophysics of the emission process. For example, a continuous red sheft of the emission spectrum is observed as the water content within the AOT reverse micelle increases. New multifrequency phase and modulation experiments show clearly that the 1,8-ANS emission spectrum is continuously evolving on a nanosecond time scale. From these experiments, we find that the solvation correlation functions (S(t)) are characterized by a pair of so vent relaxation rates when the water content is at or below R ≈ 2.5 (R, molar ratio of water to AOT). Above R ≈ 2.5 (r_w ≈ 2R ≈ 5 Å, r_w is the core radius of the water droplet) we observe a single relaxation process. We propose that, at lower water levels, the two solvation rates are a consequence (in part) of interfacial (type 1) and core (type 2) associated water, each characterized by its own relaxation rate. Upon increasing the water content within the reverse micelle, the relative fraction of type 2 water increases and the solvent relaxation process collapses into a single relaxation rate. From temperature-dependent studies we determine the activation barriers for each of these solvent reorganization events.