Ion transport in water film on silica and mica surfaces: Insights from microsecond molecular dynamics and logarithmic mean-force dynamics

Ion transport in water films on mineral surfaces is crucial for geophysical and engineering applications. However, probing these nanoscale phenomena is challenging for both experiments and conventional molecular dynamics (MD) simulations, which struggle to sample the rare ion transport events due to high computational cost. Consequently, the free energy barrier, which governs an ion's lateral entry into the film from the adjacent water-saturated region on the mineral surface, remained unevaluated. To address this, we employed microsecond MD and advanced free energy calculation methods (LogMFD/LogPD) to investigate ion energetics on silica and mica surfaces. Our results reveal a profound difference: a discernible free energy barrier of ∼3 kJ/mol for Cl- transport into the film on α-quartz, whereas the barrier is negligible on the muscovite mica. This difference is attributed to the local ionic environment; mica's dense Na+ counterion layer stabilizes incoming Cl-, while its absence on α-quartz creates an unstable state. This mechanism was further confirmed in infinitely dilute systems, where the transport barrier on both mineral surfaces is strongly modulated by the presence and location of the counterion. These findings provide molecular-level insights into ion distributions and diffusivities in heterogeneous geological formations and highlight the utility of advanced sampling to resolve the complex energy landscapes governing nanoscale transport.