Hydration and dehydration of monovalent cations near an electrode surface

The mechanism of hydration and dehydration of monovalent ions, Li⁺, Na⁺, K⁺, and Cs⁺, in a dilute solution near an electrode surface was studied by molecular dynamics simulations. The potentials of mean force for these ions were calculated as a function of the distance from the electrode surface and the potential barriers for dehydrating the first and the second hydration shell near the electrode surface and were estimated for each ion species. It was found that the mechanism of hydration for Li⁺ is distinct from those for Na⁺, K⁺, and Cs⁺. Penetration of ions into the first layer of water molecules on the electrode surface is unlikely to occur for the case of Li⁺, while that would occur with certain probabilities for the case of Na⁺, K⁺, or Cs⁺, whether or not voltage is applied to the electrode. Li⁺ ions would be adsorbed on the electrode surface in a doubly hydrated form with a significant probability, while Na⁺, K⁺, and Cs⁺ ions would be adsorbed most likely in a singly hydrated form. Furthermore, the theory of ionic radii, which has been successfully used in the analysis of bulk solutions, was applied to the electrode/electrolyte interface. It was found that the theory of ionic radii is also useful in explaining the structural behaviors of ions near an electrode surface. The distance between an ion and the layers of water molecules on the electrode surface showed almost linear dependence on the radius of the ion, as predicted by the theory of ionic radii. Analysis of the deviation from the linearity showed that Li⁺ ions are most likely adsorbed in the first layer of water molecules on the electrode surface, while Na⁺, K⁺, and Cs⁺ ions are adsorbed on the second layer of water molecules. These analyses indicate that Li⁺ is a structure maker, while Na⁺, K⁺, and Cs⁺ are structure breakers, which is consistent with the widely accepted idea in explaining the behaviors of the bulk solutions.