WATER IMMOBILIZATION AT SURFACTANT INTERFACES IN REVERSE MICELLES

The mobility of water in water/Aerosol OT (AOT)/isooctane reverse mice lles has been investigated via polar solvation dynamics measurements u sing ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w(0) = 1.1, no solvation dy namics are observed on the time scale of the instrument, suggesting th at the water is essentially frozen. As w(0) increases, the mobility of water increases, and the time-correlation functions exhibit multiexpo nential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion.(1) The other component s decay on time scales of a few picoseconds to hundreds of picoseconds . The amplitude of the solvent response increases as w(0) increases, w hich is attributed to increased mobility of the water inside the rever se micelles. Because the micellar interior is highly ionic, the solvat ion 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.