Non-Gaussian statistics of electrostatic fluctuations of hydration shells

TL;DR

This study investigates non-Gaussian electric field fluctuations in water near a solute, revealing complex free energy landscapes and the role of hydrogen-bond network defects influenced by solute dipoles.

Contribution

It provides new insights into the non-Gaussian statistics and free energy surface transformations of interfacial water electric fields influenced by solute dipoles.

Findings

Electric field fluctuations are non-Gaussian and exceed linear-response predictions.

The free energy surface transitions from single-well to double-well with increasing dipole.

Excited surface states involve hydrogen-bond network defects and are affected by external electric fields.

Abstract

We report the statistics of electric field fluctuations produced by SPC/E water inside a Kihara solute given as a hard-sphere core with a Lennard-Jones layer at its surface. The statistics of electric field fluctuations, obtained from numerical simulations, are studied as a function of the magnitude of a point dipole placed close to the solute-water interface. The free energy surface as a function of the electric field projected on the dipole direction shows a cross-over with the increasing dipole magnitude. While it is a single-well harmonic function at low dipole values, it becomes a double-well surface at intermediate dipole moment magnitudes, transforming to a single-well surface, with a non-zero minimum position, at still higher dipoles. A broad intermediate region where the interfacial waters fluctuate between the two minima is characterized by intense field fluctuations, with non-Gaussian statistics and the variance far exceeding the linear-response expectations. The excited state of the surface water is found to be lifted above the ground state by the energy required to break approximately two hydrogen bonds. This state is pulled down in energy by the external electric field of the solute dipole, making it readily accessible to thermal excitations. The excited state is a localized surface defect in the hydrogen-bond network creating a stress in the nearby network, but otherwise relatively localized in the region closest to the solute dipole.