Surface modification of calcium hydroxyapatite with pyrophosphoric acid

The surface of synthetic colloidal calcium hydroxyapatite (Ca 10(PO 4) 6(OH) 2; CaHap) was treated with pyrophosphoric acid (H 4P 2O 7; PP) in acetone and the materials were characterized by XRD, thermal analysis, N 2 adsorption, TEM, and FTIR. No remarkable change in XRD patterns or in particle morphology by the modification was observed. The additional amount of PO 4 of CaHap was increased with an increase in PP concentration. The Ca/P molar ratio of CaHap was decreased from 1.62 to 1.35 by the modification. IR results indicated that the PP reacts with surface POH groups of CaHap to form additional surface POH groups as follows: surface POH+H 4P 2O 7→ surface POPO(OH) 2 + H 3PO 4. When the modified CaHap with Ca/P molar ratio less than 1.50 was treated at 850 °C in air, the materials transformed into β-Ca 3(PO 4) 2.