Kinetics of Hydroxyapatite Formation at Low Temperature

The influence of temperature on the kinetics of formation of calcium‐deficient hydroxyapatite by an acid‐base reaction has been determined. Calcium‐deficient hydroxyapatite is compositionally similar to bone apatite. Reactants used in this study were tetracalcium phosphate, a calcium phosphate more basic than hydroxyapatite, and dicalcium phosphate, which is more acidic than hydroxyapatite. The kinetics of hydroxyapatite formation are initially controlled by the surface areas of the reactants; however, eventually the rate of hydroxyapatite formation becomes diffusionally controlled. The origin of diffusion control appears to be associated with the epitaxial formation of hydroxyapatite on the surface of the dicalcium phosphate reactant. The microstructure of hydroxyapatite formed at 5°, 15°, and 25°C were generally similar and are characterized by the formation of a porous acicular product which covers the surfaces of the reactants. The microstructure of the hydroxyapatite formed at 38°C was distinct from this and exhibited a pseudomorphic relationship with the dicalcium phosphate reactant. Seeding with hydroxyapatite accelerated the initial reaction but did not appear to have major long‐term effects on the fractional degree of reaction or on microstructural development. Reaction was also accelerated in a neutral salt solution of high ionic strength.