The Hydrolysis of Anhydrous Dicalcium Phosphate into Hydroxyapatite

Hydrolytic conversion of dicalcium phosphate anhydrous (DCPA; CaHPO₄) to non-stoichiometric hydroxyapatite (HAP; Ca_10-x(HPO₄)_x(PO ₄)_6-x(OH)_2-x) was investigated under controlled-solution conditions for identification of solution factors which influence the nucleation, growth, and crystal morphology of the apatitic product phase. The hydrolysis experiments (1 mmol to 7.5 mmol CaHPO₄/250 mL) were carried out under CO₂-free N2 at fixed pH in the presence of 25-100 mmol/L CaCl2 by standard pH-stat techniques at 37°C. The pH values of the experimental solutions ranged from 6.35 to 9.0. The crystal size and shape of the HAP product were established by x-ray diffraction, and by scanning and transmission electron microscopy. Generally, hydrolysis times and the size of the HAP crystals attained upon completion of the conversion were inversely related to pH. However, crystal size decreased sharply below pH 6.5. Crystal morphology was also strongly affected by pH, with the most equidimensionally-shaped crystals developing at pH 7.0. Although the HAP first appeared on the surface of the DCPA, the amount of DCPA used did not affect the final HAP crystal size or the time required for the hydrolysis to be completed. These data suggest that DCPA initiated HAP formation but that nucleation density (i.e., number of nuclei/unit area DCPA surface) and subsequent growth events were controlled by solution factors.