Magic-number effect on the interfacial transport of hydrated ion clusters*

Ion hydration at interfaces is important for an extremely wide range of fields and processes, one of the key issues is to uncover the microscopic factors that govern the transport of the interfacial hydrated ions. Using a combined scanning tunneling microscopy and noncontact atomic force microscopy system, we can construct individual Na⁺ hydrates (1-5 water molecules) on a NaCl(001) surface and investigate their mobility in a controlled manner. We found that Na⁺●3H₂O diffuses orders of magnitude faster than other ion hydrates. Ab initio calculations revealed that such high mobility arises from the existence of a peculiar metastable state, in which the three water molecules around the Na⁺ can rotate collectively with an exceptionally small barrier. The above picture applies even at room temperature according to the classical molecular dynamics simulations. Our work suggests that there may exist a magic-number effect on the transport of interfacial hydrated ions, which is determined by the degree of symmetry match between the hydrates and the interface.