Formation of fluoridated hydroxyapatite by competitive attack of OH- and F- ions onto α- or β-tricalcium bis (orthophosphate)

Abstract

The process of formation of fluoridated hydroxyapatite, [Ca10(PO4)6(OH)2-2xF2x] (FHAp) through the hydrolysis of α- or β-tricalcium bis (orthophosphate), [Ca3(PO4)2] (α- or β-TCP) in the presence of fluoride ions (F-) was examined. The presence of F- ions improved the hydrolysis of both TCPs. The quantity of F- ions incorporated into the apatite increased with the molar ratio (R) of ammonium fluoride to TCP (R = NH4F/TCP). When R = 0.7, the molar ratio of Ca/P in the products, which contained small amount of CO32- ion, was 1.68. On the other hand, for R = 0.18 or 0.35, the Ca/P ratio decreased to 1.61-1.64. This phenomenon indicates the formation of Ca-deficient FHAps containing HPO42- (and CO32-) ions. FHAp formed from α-TCP contained fine particles with a size smaller than 0.5 μm, and needle-like crystals whose width depended on the value of R. FHAp formed from β-TCP was needle-like for R = 0.18, and spindle-like for R = 0.70. Differences in the hydrolysis process between α-TCP and β-TCP were observed. α-TCP is attacked by OH- and F- ions which compete in the formation of FHAp crystals on the surface of the α-TCP particles. On the other hand, β-TCP is attacked by F- ions and forms clusters, and then some of the F- ions that are incorporated in the clusters are liberated, depending on the F- ion concentration in the solution. The clusters loosing F- ions incorporate OH- ions to form FHAp. The F- ions liberated from the clusters the gain attack the residual β-TCP. When R = 0.18, at an early stage, FHAp with a composition close to fluoropatite [Ca10(PO4)6F2] (FAp) was formed. Then, following the reduction of the F- ion concentration in the reaction media, FHAp with a composition close to hydroxyapatite, [Ca10(PO4)6(OH)2] (HAp), was formed.