THE KINETICS OF CALCIUM-DEFICIENT AND STOICHIOMETRIC HYDROXYAPATITE FORMATION FROM CAHPO4-CENTER-DOT-2H(2)O AND CA-4(PO4)(2)O

Isothermal calorimetry was performed on intimate mixtures of CaHPO4 . 2H(2)O and Ca-4(PO4)(2)O constituted at Ca/P molar ratios of 1.50 and 1.67 to form the hydroxyapatite compositions Ca9HPO4(PO4)(5)OH and Ca- 10(PO4)(6)(OH)(2), respectively, at complete reaction. The temperature range investigated was 15-70 degrees C. The effects of the reaction t emperature on the rates of heat evolution during hydroxyapatite format ion were determined. Reactions were carried out utilizing a liquid-to- solids weight ratio of 1.0. A two-stage reaction mechanism was observe d regardless of the Ca/P ratio as indicated by the presence of two rea ction peaks in the plots of the rates of heat evolution against time. An Arrhenius relationship was found between the rate and temperature f or each reaction stage for both compositions. Apparent activation ener gies of 120 a nd 90 kJ/mol (Ca/P = 1.67) a nd 118 and 83 kJ/mol (Ca/P = 1.50), respectively, were calculated for the first and second reacti on peaks. An Arrhenius relationship was also found between the time of maximum rate and temperature. The following qualitative reaction mech anism is proposed for each of the two reaction stages for both composi tions studied. The first stage involves the complete consumption of Ca HPO4 . 2H(2)O and the partial consumption of Ca-4(PO4)(2)O to form a n oncrystalline calcium phosphate and nanocrystalline hydroxyapatite. Du ring the second stage the remaining Ca-4(PO4)(2)O reacts with the nonc rystalline calcium phosphate to form the final product, stoichiometric or calcium deficient hydroxyapatite.