The hydration structure of mineral-water interfaces plays a significant role in geochemical processes such as ion adsorption, mineral growth and dissolution, and particle aggregation. Here we resolve the three-dimensional structures of the two most common barite-water interfaces, the (001) and the (210), using a combination of high resolution, in situ x-ray reflectivity (XR) measurements, classical molecular dynamics (CMD) simulations, and density functional theory (DFT) simulations. While both surfaces are terminated by a 1:1 ratio of barium and sulfate groups, the surfaces differ in degree of corrugation and the height of oxygens with respect to that of the topmost bariums. There are similarities in the XR derived best fit models for the two surfaces: waters adsorb and compensate for a portion of the missing bonds generated during surface formation and surface bariums and sulfates relax laterally and vertically from their bulk positions. However, on the (210) there is a greater degree of relaxation (up to 0.3 Å laterally and 0.16 Å vertically, as compared with 0.13 Å laterally and 0.08 Å vertically for the (001) surface). The 2D packing density of water at the (210) surface is also greater (~14 H2O/nm2) than that of the (001) surface (~11 H2O/nm2), although the heights of the adsorbed water layers from the topmost surface bariums are similar. The positions of the water layers for the DFT and XR results are similar, although the DFT simulations have a water layer present slightly closer to the surfaces. The CMD simulations are also similar to that of the DFT and XR results and reveal a more complex hydration structure for the (210) as compared to the (001) surface. The differences in interfacial structures between the (210) and (001) surfaces may be a key parameter affecting ion adsorption to these two surfaces.

Hydration structures of the barite (210)- and (001)-water interfaces

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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