Real-time observation of cation exchange kinetics and dynamics at the muscovite-water interface

Ion exchange at charged solid-liquid interfaces is central to a broad range of chemical and transport phenomena. Real-time observations of adsorption/desorption at the molecular-scale elucidate exchange reaction pathways. Here we report temporal variation in the distribution of Rb⁺ species at the muscovite (001)-water interface during exchange with Na⁺. Time-resolved resonant anomalous X-ray reflectivity measurements at 25 °C reveal that Rb⁺ desorption occurs over several tens of seconds during which thermodynamically stable inner-sphere Rb⁺ slowly transforms to a less stable outer-sphere Rb⁺. In contrast, Rb⁺ adsorption is about twice as fast, proceeding from Rb⁺ in the bulk solution to the stable inner-sphere species. The Arrhenius plot of the adsorption/desorption rate constants measured from 9 to 55 °C shows that the pre-exponential factor for desorption is significantly smaller than that for adsorption, indicating that this reduced attempt frequency of cation detachment largely explains the slow cation exchange processes at the interface.