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, 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. 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.chemical kinetics | catalysis | surface science | ion pairing I n aqueous solution, the association or dissociation of oppositely charged ions requires the collective rearrangement of surrounding water molecules (1, 2), which solvate bound ion pairs differently than individual ions (3-6). The solvent fluctuations that enable these collective rearrangements drive the dynamics of ion pairing and unpairing and therefore play a fundamental role in many chemical reactions. Near the surface of an electrode, these collective solvent fluctuations can differ significantly from that of the bulk liquid (7,8), and these differences can affect the rates and mechanisms of aqueous electrochemical reactions. In this work, we use molecular simulation to investigate the microscopic processes of aqueous ion pairing when it takes place near, but not in direct contact with, an extended metallic electrode. We identify the specific effects of the electrode interface by comparing our results to those generated in the environment of the bulk liquid. We find that the presence of an electrode has little effect on the mechanistic details of ionic charge separation, but can significantly influence the thermodynamics and kinetics of the process. We highlight that this influence is different for simple monovalent salts like Na + and I − , whose transport is limited by the mobility of an aqueous solvation shell (9), than it is for water ions, like H3O + and OH − , whose transport is limited by the concerted hopping of protons along hydrogen-bonding chains (10). This fundamental difference is controlled by the microscopic details of electrode-water interactions and thus has implications for...

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