Structure and Dynamics of Aqueous Electrolytes at Quartz (001) and (101) Surfaces

Understanding and describing reactivity at mineral–water interfaces, such as ion adsorption, the kinetics of dissolution, or surface charge development, depends on our ability to improve the accuracy of electrical double-layer (EDL) models. While molecular dynamics (MD) simulations are routinely used to investigate the structure and energetics of adsorbed ions comprising the EDL, less attention has been paid to their self-diffusion dynamics, which can uniquely inform coupling to interfacial reactions. Here, we use MD to investigate both the organization and diffusion dynamics of water and electrolyte ions (NaCl, KCl, CaCl 2 ) on hydroxylated quartz (001) and (101) surfaces, which allow us to assess surface structural effects of corrugation and silanol density. Complementary atomic force microscopy measurements are also used to probe the interfacial solution structure. We found that the inner- versus outer-sphere complex formation depends on the cation size and charge but not necessarily on hydration energies. The participation of surface silanols in the hydration spheres of Na + and K + generally indicates their preference for inner-sphere complexation, but this is strongly dependent on the orientation of the surface considered through its influence on the organization and dynamics of adsorbed water layers. In particular, the surface orientation substantially affects the diffusive behavior of near-surface water. Na + is found to decrease the mobility of water in the first layer, consistent with the increasing frequency of hydrolysis implied by the faster quartz dissolution rates observed in experiments via the well-known salt effect. Our results are also in good agreement with the observed dissolution rate of quartz vs the surface adsorption strength measured by Dove and Nix. This study sets the stage for a forthcoming study examining how the dynamics at quartz/electrolyte interfaces are influenced by externally applied electric fields.