The kinetics of calcium deficient and stoichiometric hydroxyapatite formation from CaHPO4·2H2O and Ca4(PO4)2O

Isothermal calorimetry was performed on intimate mixtures of CaHPO₄·2H₂O and Ca₄(PO₄)₂O constituted at Ca/P molar ratios of 1.50 and 1.67 to form the hydroxyapatite compositions Ca₉HPO₄(PO₄)₅OH and Ca₁₀(PO₄)₆(OH)₂, respectively, at complete reaction. The temperature range investigated was 15–70°C. The effects of the reaction temperature on the rates of heat evolution during hydroxyapatite formation were determined. Reactions were carried out utilizing a liquid-to-solids weight ratio of 1.0. A two-stage reaction mechanism was observed regardless of the Ca/P ratio as indicated by the presence of two reaction peaks in the plots of the rates of heat evolution against time. An Arrhenius relationship was found between the rate and temperature for each reaction stage for both compositions. Apparent activation energies of 120 and 90 kJ/mol (Ca/P=1.67) and 118 and 83 kJ/mol (Ca/P=1.50), respectively, were calculated for the first and second reaction peaks. An Arrhenius relationship was also found between the time of maximum rate and temperature. The following qualitative reaction mechanism is proposed for each of the two reaction stages for both compositions studied. The first stage involves the complete consumption of CaHPO₄·2H₂O and the partial consumption of Ca₄(PO₄)₂O to form a noncrystalline calcium phosphate and nanocrystalline hydroxyapatite. During the second stage the remaining Ca₄(PO₄)₂O reacts with the noncrystalline calcium phosphate to form the final product, stoichiometric or calcium deficient hydroxyapatite.