Insights on the Alumina-Water Interface Structure by Direct Comparison of Density Functional Simulations with X-ray Reflectivity

Density functional theory molecular dynamics (DFT-MD) simulations are frequently used to predict the interfacial structures and dynamical processes at solid-water interfaces in efforts to gain a deeper understanding of these systems. However, the accuracy of these predictions has not been rigorously quantified. Here, direct comparisons between large-scale DFT-MD simulations and high-resolution X-ray reflectivity (XR) measurements of the well-defined Al₂O₃(001)/water interface reveal the relative accuracy of these two methods to describe interfacial structure, a comparison that is enabled by XR's high sensitivity to atomic-scale displacements. The DFT-MD simulated and XR model-fit structures are qualitatively similar, but XR signals calculated directly from the DFT-MD predictions deviate significantly from the experimental data, revealing discrepancies in these two approaches. Differences in the derived interfacial Al₂O₃ relaxation profiles of ∼0.02 Å within the top five layers are significant to XR, but at the limit of the accuracy of DFT. Further differences are found in the surface hydration layer with a simulated average water layer height ∼0.2 Å higher than that observed experimentally. This is outside the accuracy of both XR and DFT and is not improved by the inclusion of a phenomenological correction for hydrogen bonding (e.g., Grimme).