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 themselves near charged surfaces. We study the Pt(111) electrode in 1M alkali chloride solutions, a prototypical model system that encapsulates the essential physics of electrochemical interfaces. Alkali ions can critically influence electrochemical reactions even if they do not specifically adsorb. Understanding their behavior at the interface 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 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.
Moss et al. (Thu,) studied this question.