Adsorption of cations at electrochemical interfaces revisited

At electrochemical interfaces, alkali-metal and alkaline-earth metal cations are usually described as solvated species that nonspecifically adsorb, whereas specifically adsorbed anions interact more directly with metal electrodes. However, the mode of cation-surface interaction remains poorly defined at the atomic scale because experiments typically measure averaged interfacial responses. Distinguishing hydration modulated polarization from localized ion-surface coupling therefore requires access to both the atomic structure and the redistribution of electronic charge. Using density functional theory molecular dynamics, we reexamine this distinction for hydrated K+, Na+, Ca2+, oand Mg2+ at Pt(111), Au(111), and Ag(111) metal-water interfaces. Charge neutral, symmetric 3 × 3 cells containing one cation and five to seven water molecules at each interface allow for the study of compact, ion-rich layer configurations, rather than dilute electrolytes or electrodes maintained at an applied potential. Within the sampled compact layer configurations, structural sampling and charge density difference analysis indicate that cation adsorption spans a continuum of interfacial responses, ranging from diffuse, hydration modulated polarization to localized, compact layer coupling. Divalent cations exhibit the most persistent direct contact, and the selected divalent configurations show localized electronic redistribution, while monovalent cations are more sensitive to water coverage and substrate identity. Even predominantly non-contact cations can remain electronically relevant by polarizing the interfacial water layer and metal surface. These results suggest that cation adsorption within the compact interfacial region requires a local, structure dependent description that goes beyond simple specific or nonspecific classifications.