Influence of restricted environment and ionic interactions on water solvation dynamics

Polar solvation dynamics of water sequestered inside Aerosol OT (AOT) rever se micelles have been investigated as a function of the surfactant counterc ation, specifically replacing Na+ for K+ and Ca2+. For Ca-AOT reverse micel les, the solvation dynamics for the smallest micelles probed occurs on a su bnanosecond time scale. The K-AOT reverse micelles display an additional ul trafast component that is attributable to bulklike water motion. As previou sly reported for Na-AOT reverse micelles [Riter, Willard, and Levinger, J. Phys. Chem. B 102, 2705 (1998)], solvent mobility increases with increasing micellar size for both Ca-AOT and K-AOT reverse micelles. The solvation dy namics in strongly ionic aqueous solutions of Ca2+ and K+ have also been in vestigated. The 10 M electrolyte solutions display water motion on signific antly shorter time scales with substantial ultrafast components. These resu lts show that the micellar interfacial structure plays a significant role i n immobilizing intramicellar water and that solvent immobilization in the r everse micelles is not merely a result of solvent-ion interactions. (C) 199 8 American Institute of Physics. [S0021-9606(98)52446-4].