The effect of hydration number on the interfacial transport of sodium ions

Ion hydration and transport at interfaces are relevant to a wide range of applied fields and natural processes^1-5. Interfacial effects are particularly profound in confined geometries such as nanometre-sized channels^6-8, where the mechanisms of ion transport in bulk solutions may not apply^9,10. 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 (one to five) to the Na⁺ ion using a combined scanning tunnelling microscopy and noncontact atomic force microscopy system. We found that the Na⁺ ion 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⁺ ion can rotate collectively with a rather small energy barrier. This scenario would apply even at room temperature according to our classical molecular dynamics simulations. 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.