The importance of formation of hydroxyl ions by dissociation of trapped water molecules for growth of calcium hydroxyapatite crystals

Abstract

The polynuclear mechanism of crystal growth is adapted to describe the growth of calcium hydroxyapatite, HAP. As in the model previously used for growth of fluorapatite, FAP, and dissolution of HAP and FAP, the rate of nucleation is assumed to be a function of mean ion activity, and the lateral growth rate of nuclei is assumed to be controlled by the integration of calcium ions, accompanied by phosphate ions. For growth of HAP, allowance is made for the relatively slow formation of hydroxyl ions from water molecules trapped under the crystal surface. Taking this into account, the frequency for calcium ions to make a diffusion jump into a kink and, simultaneously, to partly dehydrate, calculated from experimental results for growth of HAP, is, within a factor of about 2, in agreement with the theoretical value, 1.6×10 5/s, as found for growth of FAP and dissolution of HAP and FAP. Some misconceptions concerning the interpretation of specific surface areas of crystals are discussed.