The three-dimensional structure of the barite (210)-water interface was measured using in situ high resolution x-ray reflectivity by measuring specular and non-specular reflectivity and compared with the interfacial structure derived from molecular dynamics simulations. The (210) x-ray reflectivity measurements were analyzed by fitting structural models to the data using a MATLAB-based non-linear least-squares algorithm. Models included adsorbed water species and vertical and lateral relaxation of the topmost bariums and sulfates. The best fit x-ray reflectivity model included six distinct adsorbed waters, which overlapped vertically and were located at z = 1.7 and 2.4 Å. In comparison, results from the molecular dynamics simulations had adsorbed waters at z = 2.0 and 2.8 Å. The positions for the waters derived from the XR were similar to the positions derived for the barite (001) surface (Bracco et al., 2017), suggesting both surfaces have the same preferred distance for a water molecular from a surface barium or sulfate. The results from the MD simulations also indicated a complex water structure that was not within the resolution of the XR experiments to derive, indicating the importance of considering both computational and experimental results when determining interfacial hydration structures.

Interfacial structure of the barite (210) surface: A comparison of x-ray reflectivity and molecular dynamics simulations

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