Single ion hydrates under the SPM tip*

Ion hydration and transport at interfaces are relevant to a wide range of applied fields and natural processes. To correlate atomic structure with the transport properties of hydrated ions, both the interfacial inhomogeneity and the complex competing interactions among ions, water and surfaces require detailed molecular-level characterization. Here we constructed individual sodium ion (Na+) hydrates on a NaCl(001) surface by progressively attaching single water molecules to the Na+ using a combined scanning tunnelling microscopy(STM) and atomic force microscopy(AFM) system. We found that the Na+ hydrated with three water molecules diffuses orders of magnitude more quickly than other ion hydrates. Ab initio calculations revealed that such high ion mobility arises from the existence of a metastable state, in which the three water molecules around the Na+ can rotate collectively with a rather small energy barrier. Our work suggests that anomalously high diffusion rates for specific hydration numbers of ions are generally determined by the degree of symmetry match between the hydrates and the surface lattice.
Reference: Nature 557, 701 (2018)