Properties of aqueous electrolyte solutions at carbon electrodes: effects of concentration and surface charge on solution structure, ion clustering and thermodynamics in the electric double layer

TL;DR

This study uses molecular dynamics simulations to explore how concentration and surface charge influence the structure, ion clustering, and thermodynamics of aqueous NaCl solutions at graphite electrodes, revealing asymmetric effects and the importance of electric fields.

Contribution

It provides new insights into the complex interplay of concentration and surface charge on interfacial properties beyond mean-field theories, highlighting ion clustering and thermodynamic behavior.

Findings

Negatively charged surfaces increase ion cluster size and concentration.

Capacitance is largely unaffected by bulk solution concentration.

Electric fields significantly influence ion and water activity coefficients.

Abstract

Surfaces are able to control physical-chemical processes in multi-component solution systems and, as such, find application in a wide range of technological devices. Understanding the structure, dynamics and thermodynamics of non-ideal solutions at surfaces, however, is particularly challenging. Here, we use Constant Chemical Potential Molecular Dynamics simulations to gain insight into aqueous NaCl solutions in contact with graphite surfaces at high concentrations and under the effect of applied surface charges: conditions where mean-field theories describing interfaces cannot (typically) be reliably applied. We discover an asymmetric effect of surface charge on the electric double layer structure and resulting thermodynamic properties, which can be explained by considering the affinity of the surface for cations and anions and the cooperative adsorption of ions that occurs at higher concentrations. We characterise how the sign of the surface charge affects ion densities and water structure in the double layer and how the capacitance of the interface - a function of the electric potential drop across the double layer - is largely insensitive to the bulk solution concentration. Notably, we find that negatively charged graphite surfaces induce an increase in the size and concentration of extended liquid-like ion clusters confined to the double layer. Finally, we discuss how concentration and surface charge affect the activity coefficients of ions and water at the interface, demonstrating how electric fields in this region should be explicitly considered when characterising the thermodynamics of both solute and solvent at the solid/liquid interface.