Molecular Simulation of Structure and Diffusion at Smectite-Water Interfaces: Using Expanded Clay Interlayers as Model Nanopores

In geologic Settings relevant to a nuniber of extraction and, potential sequestration processes, nanopores botifided by clay mineral surfaces play a critical role in the transport of aqueous, species, Solution, structure and dynamics at,clay water interfaces are, quite different front, their bulk values, and the spatial extent of this disruption remains a topic of current interest. We have used molecular dynamics simulations to investigate the structure and diffusion of aqueous solutions in clay nanopores approximately 6 nm thick, comparing the effect of clay composition with model Na-hectorite and Na-montmorillonite surfaces. In addition to structural properties at the interface, water and ion diffusion coefficients were calculated within each aqueous layer at the interface, as well as in the central bulk-like region of the nanopore. The results show similar solution structure and diffusion properties at each surface, with:, subtle differences in sodium ad-sorption complexes and water structure in the first adsorbed layer due to different arrangements of layer hydroxyl groups in the two clay models. Interestingly, the extent of Surface disruption on bulk-like solution structure and diffusion extends to only a few water layers. A comparison of sodium ion residence times confirms similar behavior of inner-sphere and outer-sphere surface complexes at each clay surface, but similar to 1% of sodium ions adsorb in ditrigonal cavities on the hectorite surface. The presence of these anhydrous ions is consistent with highly immobile anhydrous ions seen in previous nuclear magnetic resonance spectroscopic measurements of hectorite pastes.