  Understanding the molecular and electronic structure of electrolytes at
interfaces requires an analysis of the interactions between the electrode
surface, the ions, and the solvent environment on equal footing. Here, we
tackle this challenge by exploring the initial stages of Cs+ hydration on a
Cu(111) surface by combining experiment and theory. Remarkably, we observe
"inside out" solvation of Cs ions, i.e, their preferential location at the
perimeter of the water clusters on the metal surface. In addition, water-Cs
complexes containing multiple Cs+ ions are observed to form at these surfaces.
Established models based on maximum ion-water coordination and the double layer
notion cannot account for this situation and the complex interplay of
microscopic interactions is the key to a fundamental understanding.
