Structural and morphological consequences of high-temperature treatments of hydroxyapatite in the absence or presence of HCl vapor.

Hydroxyapatite [HAP; Ca5(PO4)3(OH)], a biocompatible, osteoconductive material, was perceived, in the present investigation, to mimic a healthy bone mineral. Structural and morphological properties of its bulk and surface were examined versus high-temperature (up to 900 degrees C) thermal treatments in air or wet HCl gas atmosphere, using thermogravimetry, X-ray powder diffractometry, N2 sorptiometry, scanning electron microscopy, X-ray energy dispersive spectroscopy, and ex- and in situ infrared spectroscopy. CO, CDCl3, and methylbutynol were used as infrared probe molecules. Results obtained revealed that, in the absence of HCl, the bulk crystalline structure and the chemical composition of HAP were stable during high-temperature treatments. The surface exposed isolated Lewis acid sites (Ca2+) and reactive base sites (O(x-) and/or OH-) that chemisorbed atmospheric CO2 molecules with the formation of surface carbonate species (CaCO3). It is assumed that surface OH groups may interact with atmospheric oxygen molecules, leading to the formation and incorporation of peroxide (O2(2-) species. In the atmosphere of wet HCl, HAP was shown to suffer loss of chemical integrity, facilitated by its carbonated domains, as well as disintegration (or erosion) of particle aggregates and creation of what appeared to be deep groves.