If it fits, it sits: Geometric ordering of cations at electrochemical interfaces

The atomic-scale structure of solid–liquid interfaces is a cornerstone of many scientific fields, including electrochemistry, environmental science, and biology. Despite its importance, current models often struggle to accurately describe how ions and solvent molecules arrange near charged surfaces. Using 1M alkali chloride solutions at charged Pt(111) interfaces as model system, we investigate the behavior of cations at the surface. Since alkali cations can critically influence electrochemical reactions, understanding their arrangement at the Pt interface, including their arrangement relative to the water and their crowding behavior, is thus essential. Using classical molecular dynamics simulations, we examine the spatial distribution, solvation structure, and the phenomena governing interfacial ordering for lithium, sodium, and cesium ions near the electrode. To account for electrode charges, we use a polarizable electrode model featuring fluctuating atomic charges and fixed total surface charge. Our results reveal that even without any applied surface charge, ions preferentially occupy specific positions. These positions are defined by the geometric compatibility between the ions’ hydration shells and the layered water structure at the Pt(111) interface. The layered water essentially functions as a structural template for ions, leading to interfacial ion layering and ordering. Applied charge strengthens this ordering, promoting the formation of cation – cation pairs and cation clusters in which cations share their solvation shell. Our results shed more light on the crowding of ions at the interface and furthermore suggest a geometric “fit” mechanism governing ion layering that expands beyond purely electrostatic explanations, while, at the same time, providing an intuitive explanation of the cation’s behavior. • Alkali ion layering at heterogeneous surfaces results from water layering. • The ionic positions are constrained even in the absence of an applied surface charge. • Preferred solvation structures impact in- and out-of-plane ordering of cations. • Cation-cation pairs and cation clusters present at interface.