Structure of synthetic calcium hydroxyapatite particles modified with pyrophosphoric acid

Abstract

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, TEM, FTIR, and N 2 and H 2O adsorption measurements. XRD patterns and morphology of CaHap particles were essentially not changed by the modification. The additional amount of PO 4 of CaHap was increased with an increase of PP concentration and the Ca/P molar ratio of the particles decreased from 1.62 to 0.81. IR results indicated that the isolated surface P OH band developed with increasing the PP concentration up to 6.0 mmol dm −3 by the reaction of isolated surface P OH groups of CaHap and pyrophosphoric acids. Above 10.2 mmol dm −3 , a hydrogen-bonding surface P OH band appeared at 2913 cm −1 and enlarged with increasing the PP concentration, while the isolated surface P OH band was weakened. The results of N 2 and H 2O adsorption measurements revealed that the modified particles aggregated compared to the unmodified ones, which would be due to the formation of hydrogen-bonding surface P OH groups among the particles.