THE IMPORTANCE OF FORMATION OF HYDROXYL IONS BY DISSOCIATION OF TRAPPED WATER-MOLECULES FOR GROWTH OF CALCIUM HYDROXYAPATITE CRYSTALS

The polynuclear mechanism of crystal growth is adapted to describe the growth of calcium hydroxyapatite, HAP. As in the model previously use d 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 t he integration of calcium ions, accompanied by phosphate ions. For gro wth of HAP, allowance is made for the relatively slow formation of hyd roxyl ions from water molecules trapped under the crystal surface. Tak ing this into account, the frequency for calcium ions to make a diffus ion jump into a kink and, simultaneously, to partly dehydrate, calcula ted from experimental results for growth of HAP, is, within a factor o f about 2, in agreement with the theoretical value, 1.6 x 10(5)/s, as found for growth of FAP and dissolution of HAP and FAP. Some misconcep tions concerning the interpretation of specific surface areas of cryst als are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.