Water immobilization at surfactant interfaces in reverse micelles

The mobility of water in water/Aerosol OT (AOT)/isooctane reverse micelles has been investigated via polar solvation dynamics measurements using ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w(0) = 1.1, no solvation dynamics are observed on the time scale of the instrument, suggesting that the water is essentially frozen. As w(0) increases, the mobility of water increases, and the time-correlation functions exhibit multiexponential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion.(1) The other components decay on time scales of a few picoseconds to hundreds of picoseconds. The amplitude of the solvent response increases as w(0) increases, which is attributed to increased mobility of the water inside the reverse micelles. Because the micellar interior is highly ionic, the solvation dynamics of coumarin 343 in a 1 M Na+ (Na2SO4) solution were also investigated to compare the dynamics of water in the reverse micelles with the dynamics of bulk water in an ionic solution.