Kinetic Study on the Crystal Growth of Hydroxyapatite

The kinetics of hydroxyapatite (HAP) crystallization, which is thermodynamically the most stable calcium phosphate salt, has been investigated at physicochemically defined physiological conditions. The crystal growth process involved metastable supersaturated solutions, and for the study the constant composition method was employed. The kinetic results were tested over the most important crystal growth models. At the experimental conditions with the relatively low supersaturation level, the spiral growth model introduced by the Burton−Cabrera−Frank theory was found to better interpret the kinetic data. With the formulation of the model, thermodynamic and morphological parameters of hydroxyapatite were calculated. The surface kink site density was found to be 4.33 × 10 14 kinks/m 2, which means that the active growth sites represent less than 1 ‰ of the total number of HAP growth units on the crystal surface. The mean linear growth rate of the HAP crystal surface was found to be 1.34 monolayers per hour.