Ion-size effect within the aqueous solution interface at the Pt(111) surface: molecular dynamics studies

All-atom classical force-field based molecular dynamics simulations have been employed to investigate the structure and dynamics of interfacial water in systems of pure water, 1 M LiOH and 1 M KOH aqueous solutions at an uncharged Pt(111) surface. Results indicate that the ordering of water molecules is affected as far as 9 Å from the Pt surface, corresponding to three layers of water molecules. Specific packing geometries of water in electrolyte solutions depend on the ionic radius, and both Li+ and K+ ions are found to adsorb directly onto the Pt surface. Significantly higher values of the water-dipole autocorrelation function in the adlayer are found for the system with Li+ ions compared to the systems with K+ ions or pure water. Also strongly reduced translational motion is observed in the case of Li+, both in-plane and perpendicular to the surface. This result suggests a strong stabilizing role of Li+ ions on water molecules. Decreased mobility of the water adlayer makes it difficult for other compounds in the aqueous solution to access the Pt surface. This implies that the reason for the reduced catalytic activity of Pt(111) surface in the presence of LiOH is due to the freezing effect Li+ ions have on water.