Importance of Ion Size on the Dominance of Water–Ion Versus Water–Water Interactions in Au-Supported Solvatomers

The solvation of ions at interfaces is important to areas as diverse as atmospheric sciences, energy materials, and biology. Despite the significance, fundamental understanding, particularly at the molecular level, remains incomplete. Here, we probe the initial solvation of two singly charged but differently sized ions (Li and Cs) on a Au(111) by combining low-temperature scanning tunneling microscopy with density functional theory. Real-space molecular scale information reveals that water–ion interactions dominate the Li−water system, whereas water–water interactions dominate in the Cs case, and in both cases, the Au(111) surface confines the formed solvatomers to two dimensions. The difference in prevalent interactions leads to disparate symmetry and binding patterns of the solvation shells observed, as well as significantly different ion−surface interactions. The relationship between water number, geometry, and electronic structure of the solvatomers obtained here is an essential step toward understanding heterogeneous interfaces on the nanoscale.