Molecular dynamics simulation of adsorption of ions from aqueous media onto charged electrodes

Abstract

Molecular dynamics simulation of 216 water molecules (ST2 model) between charged flat electrodes 2.362 nm apart showed layering with a few molecules at each surface that broke H bonds with the bulk and oriented their charges towards the electrode. Compared to uncharged electrodes, the atomic and molecular distributions were unsymmetric. When a lithium and an iodide ion were substituted at random for two water molecules, the iodide ion contact adsorbed on the anode with no water molecules between it and the electrode. The iodide ion appeared weakly solvated on the solution side to water molecules that preferred to engage in hydrogen bonding with the network of the bulk solvent. In contrast, the lithium ion adsorbed without losing its primary solvation shell of six water molecules and was never observed further than two water molecules removed from the electrode. Its average position corresponded to an ion supported on a tripod of three waters. The average solvation number was not changed upon adsorption in this configuration. These qualitative observations and some quantitative results afford striking confirmation on the one hand and new insight on the other of some aspects of the standard model of the adsorption of ions on electrode surfaces. Time durations for simulations were generally between 200 and 800 ps with a basic integration time step of 2 fs.