The Calcium Phosphate−Calcium Carbonate System: Growth of Octacalcium Phosphate on Calcium Carbonates

The kinetics of nucleation and crystal growth of octacalcium phosphate [Ca 8 (PO 4 ) 6 H 2 ·5H 2 O, OCP] from calcium phosphate supersaturated solutions inoculated with seed crystals was investigated at 37 °C, pH 7.40, at conditions of constant solution supersaturation. Stable calcium phosphate solutions, supersaturated with respect to OCP and hydroxyapatite [Ca 5 (PO 4 ) 3 OH, HAP] were inoculated with calcium carbonate-based bone cement powder consisting of mixed aragonite and calcite crystals and with well-characterized calcite seed crystals. On all substrates tested, OCP nucleated followed by crystal growth of the nuclei formed past the lapse of induction times, inversely proportional to the solution supersaturation. From the dependence of the induction time on the solution supersaturation with respect to OCP, a value of 10 mJ m −2 was calculated for the nucleating phase. The rates of OCP crystal growth on the carbonate substrates showed linear dependence on the solution supersaturation that in combination with the independence from the fluid dynamics in the reactor suggested a surface diffusion-controlled mechanism. Moreover, the independence of the crystallization rates on the amount of the inoculating seed crystals suggested that nucleation and growth took place exclusively on the crystalline substrates. The transient calcium phosphate phase, OCP, was stabilized in our experiments, and it was the only phase growing at constant driving force, despite the fact the solutions were supersaturated with respect to HAP as well.