Microscopic dynamics of charge separation at the aqueous electrochemical interface

TL;DR

This study uses molecular simulations to explore how charge separation occurs at water-metal interfaces, revealing that while the microscopic mechanisms are similar to bulk water, the thermodynamics and kinetics vary with ion type and environment.

Contribution

The paper demonstrates that the microscopic mechanism of charge separation is conserved at interfaces, but thermodynamic and kinetic details differ based on ion type and environment.

Findings

Classical ions have a 40x slower dissociation rate at the interface.

Water ions exhibit similar association rates at and away from the interface.

Water's solvation and reorganization are altered near the metal interface.

Abstract

We have used molecular simulation and methods of importance sampling to study the thermodynamics and kinetics of ionic charge separation at a liquid water-metal interface. We have considered this process using canonical examples of two different classes of ions: a simple alkali-halide pair, Na$^+$I$^-$, or classical ions, and the products of water autoionization, H$_3$O$^+$OH$^-$, or water ions. We find that for both ion classes, the microscopic mechanism of charge separation, including water's collective role in the process, is conserved between the bulk liquid and the electrode interface. Despite this, the thermodynamic and kinetic details of the process differ between these two environments in a way that depends on ion type. In the case of the classical ion pairs, a higher free energy barrier to charge separation and a smaller flux over that barrier at the interface, results in a rate of dissociation that is 40x slower relative to the bulk. For water ions, a slightly higher free energy barrier is offset by a higher flux over the barrier from longer lived hydrogen bonding patters at the interface, resulting in a rate of association that is similar both at and away from the interface. We find that these differences in rates and stabilities of charge separation are due to the altered ability of water to solvate and reorganize in the vicinity of the metal interface.

Figures

Full text

See pages 1-last of arxiv.pdf See pages 1-last of SI_arxiv.pdf