Mineral- and Ion-Specific Effects at Clay-Water Interfaces: Structure, Diffusion, and Hydrodynamics

We use molecular dynamics to investigate how the structure, diffusion, and hydrodynamic properties of clay interfaces with aqueous solutions depend on the nature of the clay, the nature of the counterions, and the salt concentration in the solution. Specifically, we study water-filled nanopores between uncharged (pyrophyllite) and charged (mont-morillonite and beidellite, with substitutions located in the octahedral and tetrahedral layers, respectively) clays, with sodium or cesium as counterions, in the absence and in the presence of added salt. We discuss how the balance between solvation and attraction of the cations to the surface results in various distributions between inner- and outer-sphere complexes, and how this influences the dynamics of water near the surface, as well as the hydrodynamic flow in the presence of an external force. In the latter case, the discussion based on mapping the molecular velocity profiles to a continuous description (parabolic Poiseuille flow) shows that the larger effects come from the presence/absence of charge in the mineral, as well as the localization of substitutions within the clay layer. The salt concentration and the nature of the counterions have a comparatively less important impact far from the surface-even though some differences are observed in its close vicinity, which are not properly captured by the continuous description.