Microscopic dynamics of charge separation at the aqueous electrochemical 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, H₃O⁺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. However, 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 result in a rate of dissociation that is 40 times 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 patterns 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.

Keywords: catalysis; chemical kinetics; ion pairing; surface science.

Fig. 1.

A and B contain typical snapshots going from a recombined state (black-bordered panel) to a dissociated state (red-bordered panel) for classical ions and water ions, respectively. The positive ion is highlighted in green, and the negatively charged ion is highlighted in yellow. C and D contain plots of the free-energy profile as a function of $R$, the interionic separation for classical ions and water ions, respectively. The data plotted in red correspond to ions in the bulk, and the data plotted in blue correspond to ions at the electrode interface.

Fig. 2.

Mechanism of ion pair dissociation for NaI. (A) Free energy as a function of interionic separation, $R$, and the total Madelung potential, $\psi$, shown in the contour plot. Members of the TSE are shown as black points on this surface. (B) Free energy as a function of the Madelung potential for the bulk (red) and interface (blue). (C) Side–side correlation function for the classical ions for the bulk (red) and interface (blue), with linear fits shown in black.

Fig. 3.

Mechanism for water ion association for bulk (red) and the interface (blue). (A) The distributions of recombination times, $\tau$. (B) Distribution function of the net system charge. (C) Distribution of hydrogen bond lifetimes.