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

Abstract

The fluorescence properties of 1,8-ANS (1-anilino-9-naphthalenesulfonic acid) in AOT (Aerosol-OT; 1,4-bis(2-ethylhexyl) sulfosuccinate) reverse micelles have been studied. Steady-state and time-resolved methods show that water content and temperature play key roles in the photophysics of the emission process. For example, a continuous red shift 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, the authors find that the solvation correlation functions (S(t)) are characterized by a pair of solvent relaxation rates when the water content is at or below R {approx} 2.5 (R, molar ratio of water to AOT). Above R {approx} 2.5 (r{sub w} {approx} 2R {approx} 5{angstrom}, r{sub w} is the core radius of the water droplet) the authors observe a single relaxation process. It is proposed that, as 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 solventmore » relaxation process collapses into a single relaxation rate. From temperature-dependent studies they determine the activation barriers for each of these solvent reorganization events.« less