Evidence for Reduced Hydrogen-Bond Cooperativity in Ionic Solvation Shells from Isotope-Dependent Dielectric Relaxation

We find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H₂O and D₂O. This isotope dependence allows us to reliably determine the kinetic contribution to the depolarization, which is found to be significantly smaller than predicted by existing theory. The discrepancy can be explained from a reduced hydrogen-bond cooperativity in the solvation shell: we obtain quantitative agreement between theory and experiment by reducing the Kirkwood correlation factor of the solvating water from 2.7 (the bulk value) to ~ 1.6 for NaCl and ~ 1 (corresponding to completely uncorrelated motion of water molecules) for CsCl.