Why does dissolving salt in water decrease its dielectric permittivity

TL;DR

This study uses a deep neural network trained on density functional theory data to explain why increasing salt concentration in water reduces its dielectric permittivity, highlighting the disruption of water's hydrogen-bond network.

Contribution

The paper introduces a DNN-based approach to accurately model dielectric permittivity changes in salt water, revealing the microscopic mechanism behind the phenomenon.

Findings

DNN predictions align well with experimental data.

Disruption of water's hydrogen-bond network reduces dielectric response.

Ionic hydration shells interfere with water dipolar correlations.

Abstract

The dielectric permittivity of salt water decreases on dissolving more salt. For nearly a century, this phenomenon has been explained by invoking saturation in the dielectric response of the solvent water molecules. Herein, we employ an advanced deep neural network (DNN), built using data from density functional theory, to study the dielectric permittivity of sodium chloride solutions. Notably, the decrease in the dielectric permittivity as a function of concentration, computed using the DNN approach, agrees well with experiments. Detailed analysis of the computations reveals that the dominant effect, caused by the intrusion of ionic hydration shells into the solvent hydrogen-bond network, is the disruption of dipolar correlations among water molecules. Accordingly, the observed decrease in the dielectric permittivity is mostly due to increasing suppression of the collective response of solvent waters.