Structure of the Fluorapatite (100)-Water Interface by High Resolution X-ray Reflectivity

For more than two decades it has been noted that a complete understanding of the surface chemistry and dissolution dynamics of the apatite-water system requires direct observation of interfacial structure at the molecular scale. Here we provide the first results on the structure of the apatite(100)-water interface using high resolution x-ray reflectivity. The specular x-ray reflectivity from a natural growth surface of Durango fluorapatite was measured at beamline, 11-ID-D (BESSRC-CAT, Advanced Photon Source). The natural growth surface exhibits a uniform crystallographic termination corresponding to the unit cell border of the apatite crystal. Atomistic model of the interfacial structure were obtained by comparing the experimental results with calculated reflectivities and optimized through non-linear least-square fitting, in which the structural parameters were selected to be both physically and chemically plausible. The derived structure indicates the presence of a surface hydration layer at the apatite-water interface which is formed through the adsorption of water molecules on the solid surface.