Alteration of gas phase ion polarizabilities upon hydration in high dielectric liquids

TL;DR

This study develops a classical electrostatic model to show that ion polarizabilities increase upon hydration in high dielectric liquids, explaining the high permittivity of ionic liquids through a collective solvation mechanism.

Contribution

It introduces a new electrostatic theory modeling polarizable ions and solvent molecules, revealing how hydration enhances ion polarizability and dielectric permittivity in ionic liquids.

Findings

Hydration causes ions to acquire dipole moments and increased electron cloud rigidity.

Ion polarizability increases upon solvation in polar solvents and ionic liquids.

Collective solvation amplifies dielectric permittivity in ionic liquids.

Abstract

We investigate the modification of gas phase ion polarizabilities upon solvation in polar solvents and ionic liquids. To this aim, we develop a classical electrostatic theory of charged liquids composed of solvent molecules modeled as finite size dipoles, and embedding polarizable ions that consist of Drude oscillators. In qualitative agreement with ab-initio calculations of polar solvents and ionic liquids, the hydration energy of a polarizable ion in both type of dielectric liquid is shown to favor the expansion of its electronic cloud. Namely, the ion carrying no dipole moment in the gas phase acquires a dipole moment in the liquid environment, but its electron cloud also reaches an enhanced rigidity. We find that the overall effect is an increase of the gas phase polarizability upon hydration. In the specific case of ionic liquids, it is shown that this hydration process is driven by a collective solvation mechanism where the dipole moment of a polarizable ion induced by its interaction with surrounding ions self-consistently adds to the polarization of the liquid, thereby amplifying the dielectric permittivity of the medium in a substantial way. We propose this self-consistent hydration as the underlying mechanism behind the high dielectric permittivities of ionic liquids composed of small charges with negligible gas phase dipole moment. Hydration being a correlation effect, the emerging picture indicates that electrostatic correlations cannot be neglected in polarizable liquids.