A computer modelling study of the uptake and segregation of fluoride ions at the hydrated hydroxyapatite (0001) surface: introducing a Ca10(PO4)6(OH)2 potential model

A combination of electronic structure calculations and interatomic potential-based methods are employed to study the uptake and segregation of fluoride ions from solution into the hydroxyapatite lattice. The hydroxyapatite potential model derived for this work accurately reproduces experimental properties and relative enthalpies of formation and is further validated by Density Functiontional Theory calculations of fluoride defects in bulk hydroxyapatite. Calculations of solid solutions of fluor- and hydroxy-apatite show that hydroxy groups are easily replaced by fluoride ions on thermodynamic grounds (ΔEx = −0.4 to −6.4 kJ mol−1), forming sheets of fluoride in the a/b plane. Molecular Dynamics simulations of the incorporation of fluoride into hydroxyapatite show that fluoride ions are also easily incorporated from solution into the surface of hydroxyapatite (ΔE = −193 kJ mol−1), but they do not segregate into the bulk crystal beyond approximately 10 Å. The hydroxy-groups, which remain aligned in the c-direction in the bulk material, due to the formation of pairs of OH− groups, are found to reverse randomly in the surface region when a vacuum interface is introduced. This surface region shows considerable relaxation and reconstruction of the calcium and phosphate sub-lattices, leading to the disintegration of the OH− pairs and hence their re-orientation. However, when the vacuum is filled with water, the surface relaxation is less extensive and the hydroxy groups remain aligned, although surface OH− groups dissolve into the water. When surface OH− groups are replaced by fluoride ions, this fluoride remains in the crystal lattice and further anchors surface calcium ions. As such this surface fluoride prevents the onset of apatite dissolution, although the fluorapatite layer is only superficial.