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.