title: Direct observation of the dynamics of single metal ions at the interface with solids in aqueous solutions.
abstract: The dynamics of ions adsorbed at the surface of immersed charged solids plays a central role in countless natural and industrial processes such as crystal growth, heterogeneous catalysis, electrochemistry, or biological function. Electrokinetic measurements typically distinguish between a so-called Stern layer of ions and water molecules directly adsorbed on to the solid's surface, and a diffuse layer of ions further away from the surface. Dynamics within the Stern layer remain poorly understood, largely owing to a lack of in-situ atomic-level insights. Here we follow the dynamics of single Rb+and H3O+ions at the surface of mica in water using high-resolution atomic force microscopy with 25 ms resolution. Our results suggest that single hydrated Rb+ions reside τ1= 104 ± 5 ms at a given location, but this is dependent on the hydration state of the surface which evolves on a slower timescale of τ2= 610 ± 30 ms depending on H3O+adsorption. Increasing the liquid's temperature from 5 °C to 65 °C predictably decreases the apparent glassiness of the interfacial water, but no clear effect on the ions' dynamics was observed, indicating a diffusion-dominated process. These timescales are remarkably slow for individual monovalent ions and could have important implications for interfacial processes in electrolytes.
authors.affiliation: University of Cambridge, Cavendish Laboratory, Cambridge CB3 0HE, UK.
authors.forename: Maria
authors.initials: M
authors.lastname: Ricci
authors.affiliation: Department of Physics, Durham University, Durham DH1 3LE, UK.
authors.forename: William
authors.initials: W
authors.lastname: Trewby
authors.affiliation: Department of Physics, Durham University, Durham DH1 3LE, UK.
authors.forename: Clodomiro
authors.initials: C
authors.lastname: Cafolla
authors.affiliation: Department of Physics, Durham University, Durham DH1 3LE, UK.
authors.forename: Kislon
authors.initials: K
authors.lastname: Voïtchovsky