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) formation at 25 and 38 degrees C were investigated. Solutions were selected to contain ions in common with HAp lattice ions or to contain ions capable of substituting into HAp. The effects of phosphate, calcium, chloride, and fluoride were studied in particular. The reactants from which HAp was formed were a mixture of the particulate solids CaHPO4 and Ca4(PO4)2O. These reactants were proportioned to form the calcium deficient composition Ca9HPO4(PO4)5OH at complete reaction. The rates of HAp formation were examined by determining rates of heat liberation 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 eventually the conversion becomes diffusionally controlled. The presence of electrolytes influences HAp formation but in differing ways. Solutions containing phosphate salts initially accelerate the rate of HAp formation by reducing the incongruency of the CaHPO4 dissolution. Sodium fluoride accelerates reaction by improving the crystallinity of the apatite overgrowths as a result of fluoride incorporation into the HAp, thereby making them less effective as diffusion barriers. Calcium chloride solutions tend to reduce the proportion of HAp formed prior to the onset of the diffusionally controlled reactions. Although the reactants used were proportioned to produce calcium-deficient HAp at complete reaction, no evidence was obtained to indicate the uptake of calcium and chloride from CaCl2 solutions to form a chloroapatite having a Ca/P ratio > 1.5.