On the molecular correlations that result in field-dependent conductivities in electrolyte solutions

TL;DR

This study uses advanced response theory to analyze how ionic correlations and solvent effects influence electric field-dependent conductivities in electrolyte solutions, revealing conditions where nonlinear responses are significant or suppressed.

Contribution

It applies nonequilibrium ensemble reweighting and response theory to elucidate the molecular origins of field-dependent conductivities in different electrolyte models, including implicit and explicit solvents.

Findings

Implicit solvent models show strong field dependence at low concentrations.

Explicit solvent effects suppress nonlinear conductivity responses.

Discrepancies with Onsager-Wilson theory are due to increased ion correlations.

Abstract

Employing recent advances in response theory and nonequilibrium ensemble reweighting, we study the dynamic and static correlations that give rise to an electric field-dependent ionic conductivity in electrolyte solutions. We consider solutions modeled with both implicit and explicit solvents, with different dielectric properties, and at multiple concentrations. Implicit solvent models at low concentrations and small dielectric constants exhibit strongly field-dependent conductivities. We compared these results to the Onsager-Wilson theory of the Wien effect, which provides a qualitatively consistent prediction at low concentrations and high static dielectric constants, but is inconsistent away from these regimes. The origin of the discrepancy is found to be increased ion correlations under these conditions. Explicit solvent effects act to suppress nonlinear responses, yielding a weakly field-dependent conductivity over the range of physically realizable field strengths. By decomposing the relevant time correlation functions, we find that the insensitivity of the conductivity to the field results from the persistent frictional forces on the ions from the solvent. Our findings illustrate the utility of nonequilibrium response theory in rationalizing nonlinear transport behavior.