THE EFFECTS OF ELECTROLYTES ON THE RATES OF HYDROXYAPATITE FORMATION AT 25 AND 38-DEGREES-C

The effects of electrolytes on the rates of hydroxyapatite (HAp) forma tion at 25 and 38 degrees C were investigated. Solutions were selected to contain ions in common with HAp lattice ions or to contain ions ca pable of substituting into HAp. The effects of phosphate, calcium, chl oride, and fluoride were studied in particular. The reactants from whi ch HAp was formed were a mixture of the particulate solids CaHPO4 and Ca-4(PO4)(2)O. These reactants were proportioned to form the calcium d eficient composition Ca9HPO4(PO4)(5)OH at complete reaction. The rates of HAp formation were examined by determining rates of heat liberatio n at 25 and 38 degrees C using isothermal calorimetry and by analyzing the variations in solution chemistry. HAp formation initially occurs by a mechanism which is interfacially controlled. However, because the reactants dissolve incongruently, HAp overgrows these particles and e ventually the conversion becomes diffusionally controlled. The presenc e of electrolytes influences HAp formation but in differing ways. Solu tions containing phosphate salts initially accelerate the rate of HAp formation by reducing the incongruency of the CaHPO4 dissolution. Sodi um fluoride accelerates reaction by improving the crystallinity of the apatite overgrowths as a result of fluoride incorporation into the HA p, thereby making them less effective as diffusion barriers. Calcium c hloride solutions tend to reduce the proportion of HAp formed prior to the onset of the diffusionally controlled reactions. Although the rea ctants used were proportioned to produce calcium-deficient HAp at comp lete reaction, no evidence was obtained to indicate the uptake of calc ium and chloride from CaCl2 solutions to form a chloroapatite having a Ca/P ratio > 1.5. (C) 1996 John Wiley & Sons, Inc.