Structure of the fluorapatite (100)-water interface by high-resolution X-ray reflectivity

A complete understanding of the surface chemistry of the apatite-water system requires direct observation of the interfacial structure at the molecular scale. We report results for the structure of the apatite (100)-water interface obtained with high-resolution specular X-ray reflectivity from a natural growth surface of Durango fluorapatite. A uniform termination at the crystallographic unit-cell boundary was determined. An atomistic model of the interfacial structure is compared to the experimental results and optimized through non-linear least-squares fitting in which the structural parameters were selected to be both physically and chemically plausible. The best-fit structure includes a Ca- and/or F-deficient outermost surface, minimal structural relaxations of the near-surface apatite crystal, and the presence of a layered interfacial water structure exhibiting two distinct water layers. The height of the first water layer is 2.64(9) Angstrom relative to the relaxed surface with 3.5(1.3) water molecules per surface unit-cell area (64.9 Angstrom(2)). A second layer of adsorbed water is found 1.53(5) Angstrom above the first layer, followed by a nearly featureless profile of the bulk liquid water. The layered structure of water is interpreted as being due to hydrogen bonding at the solid-water interface. The interfacial structure shows a strong similarity with the octacalcium phosphate structure projected along a surface normal direction.