Abstract
We find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H_{2}O and D_{2}O. 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.
Highlights
The solvation of ions in water plays a crucial role in numerous physical, chemical, and biological processes, ranging from ion transport in fuel cells to the electrostatic screening of DNA
Ions generate strong local electric fields, orienting the dipole moments of the neighboring water molecules in solution, and causing a reduction in the dielectric response of the water, an effect generally referred to as depolarization. This depolarization is the sum of three contributions: (i) the dilution of the water [18,19,20], (ii) the strongly reduced mobility of water molecules solvating the ions [21], and (iii) the reaction of water molecules to the moving ions: an ion moving in the direction of an externally applied electrical field causes a reorientation of the surrounding water molecules such that their dipoles are directed opposite to the applied field [22,23,24,25,26,27]
The amplitude of the static depolarization can be used to estimate the number of water molecules immobilized per solvated cation, the so-called hydration number [8,10,14,21,28,29,30,31,32,33]
Summary
The solvation of ions in water plays a crucial role in numerous physical, chemical, and biological processes, ranging from ion transport in fuel cells to the electrostatic screening of DNA. We find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H2O and D2O. Ions generate strong local electric fields, orienting the dipole moments of the neighboring water molecules in solution, and causing a reduction in the dielectric response of the water, an effect generally referred to as depolarization.
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