Influence of thermal treatment on the structure of calcium hydroxyapatite

Synthetic calcium hydroxyapatite, Ca-10(PO4)(6)(OH)(2) (CaHap), prepared by a wet method was evacuated at various temperatures up to 900 degrees C and the influence of thermal treatment on the structure of this material was investigated by FTIR, determination of concentration of surface P-OH groups, adsorption of H2O and heat of immersion in water. The number of surface P-OH groups was 2.6 groups nm(-2) and the ionic ratio of surface HPO42- to H2PO4- was 31:69. The surface P-OH groups were essentially dehydroxylated by outgassing above 400 degrees C to produce surface P-O-P species. The heat of rehydroxylation per one P-O-P group was determined to be 1.43x10(-16) J group(-1). When outgassed above 600 degrees C, CaHap was rehydrated via the reaction: (1-x)Ca-10(PO4)(6)(OH)(2)+3x beta-Ca-3(PO4)(2)+xH(2)O --> Ca10-x(P2O7)(x)(PO4)(6-2x)(OH)(2).