Influence of restricted environment and ionic interactions on water solvation dynamics

Polar solvation dynamics of water sequestered inside Aerosol OT (AOT) reverse micelles have been investigated as a function of the surfactant countercation, specifically replacing ${\mathrm{Na}}^{\mathrm{+}}$ for ${\mathrm{K}}^{\mathrm{+}}$ and ${\mathrm{Ca}}^{\mathrm{2+}}.$ For Ca-AOT reverse micelles, the solvation dynamics for the smallest micelles probed occurs on a subnanosecond time scale. The K-AOT reverse micelles display an additional ultrafast component that is attributable to bulklike water motion. As previously 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 dynamics in strongly ionic aqueous solutions of ${\mathrm{Ca}}^{\mathrm{2+}}$ and ${\mathrm{K}}^{\mathrm{+}}$ have also been investigated. The 10 M electrolyte solutions display water motion on significantly shorter time scales with substantial ultrafast components. These results show that the micellar interfacial structure plays a significant role in immobilizing intramicellar water and that solvent immobilization in the reverse micelles is not merely a result of solvent–ion interactions.