Hydration Structure of the Barite (001)-Water Interface: Comparison of X-ray Reflectivity with Molecular Dynamics Simulations

The three-dimensional structure of the barite (001)-water interface was studied using in situ specular and nonspecular X-ray reflectivity (XR). Displacements of the barium and sulfate ions in the surface of a barite crystal and the interfacial water structure were defined in the analyses. The largest relaxations (0.13 Å lateral and 0.08 Å vertical) were observed for the barium and sulfate ions in the topmost unit cell layer, which diminished rapidly with depth. The best fit structure identified four distinct adsorbed species, which in comparison with molecular dynamics (MD) simulations reveals that they are associated with positions of adsorbed water, each of which coordinates one or two surface ions (either barium, sulfate, or both). These water molecules also adsorb in positions consistent with those of bariums and sulfates in the bulk crystal lattice. These results demonstrate the importance of combining high-resolution XR with MD simulations to fully describe the atomic structure of the hydrated mineral surface. The agreement between the results indicates both the uniqueness of the structural model obtained from the XR analysis and the accuracy of the force field used in the simulations.