Effect of temperature and reactant concentration on calcium phosphate precipitation

Calcium orthophosphates (CaP) constitute an important group of biomaterials used in bone repair and tooth remineralization. In some applications, controlling particle surface area and/or dimensions is important to fine tune ion release kinetics. This study evaluated the interaction between synthesis temperature (22 degrees C or 45 degrees C) and reactant concentration ("Low": 0.1 mol.L-1, "Medium": 0.5 mol.L-1 or "High": 1.0 mol.L-1) on phase formation, particle morphology, size and surface area. Calcium and phosphate precursor solutions (Ca/P = 1.0, initial pH: 7.9-8.1) were mixed and the precipitate was obtained after 24 h. DCPD (CaHPO4 center dot 2H(2)O) was the only phase identified by X-ray diffraction at 22 degrees C/Medium and 22 degrees C/High, with large plate crystal morphology, while DCPD/OCP (Ca4H(PO4)(3)center dot 2.5H(2)O, as agglomerates of sub-micron crystals) mixtures were observed at 22 degrees C/Low, 45 degrees C/Medium and 45 degrees C/High. Small-plate, low-crystallinity HAP (Ca5OH(PO4)(3)) was obtained at 45 degrees C/Low. In spite of the different morphologies, particle hydrodynamic radius (as determined by laser light scattering) was not affected by the tested variables (median: 10-20 mu m). Surface area, on the other hand, varied according to particle morphology (37-135 m(2)/g). In conclusion, temperature and reactant concentration interacted to define the precipitated phases, each of them with a characteristic morphology. Thermodynamically more stable phases (OCP or HAP) were found in syntheses conducted at 45 degrees C due to accelerated DCPD hydrolysis.