Ion Distribution and Cation Exchange at Mica-Electrolyte Interfaces Probed with Deep Potential Molecular Dynamics

An accurate description of the ion distribution and dynamics at mineral-aqueous solution interfaces is crucial to understanding a variety of technological and environmental phenomena. To obtain better insight, many studies have combined experimental measurements with classical molecular dynamics simulations, yet their interpretation remains limited by the empirical nature of the classical force fields. Here, we investigate the Stern layer structure and cation exchange mechanism at muscovite mica-electrolyte interfaces using nanosecond time scale molecular dynamics simulations based on deep neural network interatomic potentials trained on density functional theory (DFT) data. Focusing on mica with exposed surface K⁺ interfaced with aqueous NaCl and mica with surface Na⁺ interfaced with KCl solution, we find that K⁺ remains predominantly in inner-sphere configurations, while Na⁺ exhibits notable populations in outer-sphere states. Most importantly, our simulations show that contact with an electrolyte solution results in the coadsorption of multiple cation species, making the mica surface locally overcharged and thus reshaping the cation speciation in a manner that enhances the tendency of neighboring surface cations to desorb. These findings are consistent with recent experimental observations that coadsorption of different cation species induces changes in cation speciation and slow kinetics of cation exchange at the muscovite-water interface, providing a basis for their detailed understanding.