Kinetic Model for α–Tricalcium Phosphate Hydrolysis

A mechanistic model for the kinetics of hydrolysis of α-tricalcium phosphate (α–Ca₃(PO₄)₂ or α-TCP) to hydroxyapatite (Ca_{10? x} (HPO₄) _x (PO₄)_{6? x} (OH)_{2? x} or HAp) has been developed. The model is based on experimental hydrolysis rate data obtained using isothermal calorimetry. Analysis of the kinetic data according to the general kinetics models in terms of the fractional degree of reaction and time suggests the hydrolysis to be controlled by different rate-limiting mechanisms as reaction proceeds. Initially, the hydrolysis kinetics depend on the surface area of the anhydrous α-TCP. Subsequently, they change to a dependence on the rate of HAp product formation controlled by a nucleation and growth mechanism. The model predicts that HAp nuclei form at essentially one time and growth occurs in two dimensions, leading to a platelike morphology. The change in the reaction mechanism occurs at a fractional degree of hydrolysis, which does not change significantly with temperature in the range of 37°–56°C.