Kinetics of growth and dissolution of calcium hydroxyapatite in suspensions with variable calcium to phosphate ratio

Abstract

Kinetics of growth and dissolution of calcium hydroxyapatite at constant pH (6.0 and 7.2), and with variable values of the calcium to phosphate mole ratio in solution (0.1<Ca/P<20) are reported. Both processes are shown to be controlled by the polynuclear surface mechanism in which nucleation is expressed as a function of mean-ion activity. For dissolution, the lateral growth rate of nuclei is proportional to the difference between the total concentration of calcium ions in a saturated solution and in the solution, and the rate constant is related to the frequency for a calcium ion to make a diffusion jump into a kink and simultaneously, partially dehydrate. For growth, a newly developed model for the lateral growth rate is applied. This model takes into account that hydroxyl ions are formed at hydroxyl sites slightly below the growing surface by dissociation of trapped water molecules. In this way the lateral growth rate consists of two consecutive reactions, the process of calcium ions entering calcium sites, accompanied by phosphate ions, being delayed by the relatively slow production of hydroxyl ions at hydroxyl sites. The frequency for ion integration is found to be within a factor of 2 of the expected value, 1.6×10 5/s, for both growth and dissolution, both values of pH and all values of Ca/P studied. As this frequency is practically constant, the frequency for a calcium ion to leave a kink decreases as the saturation concentration of calcium decreases. This reflects important changes in the dynamics of the crystal surface. Kinetically determined values for the surface tension are for growth σ=100 mJ/m 2 and for dissolution σ=40 mJ/m 2.