Journal of Dental Research, Vol 72, 474-480, Copyright © 1993 by International & American Associations for Dental Research Online Journals
The hydrolysis of anhydrous dicalcium phosphate into hydroxyapatite
K. Ishikawa and E. D. Eanes
National Institute of Dental Research's, National Institute of Standards and Technology, Gaithersburg, Maryland 20899.
Hydrolytic conversion of dicalcium phosphate anhydrous (DCPA; CaHPO4) to
non-stoichiometric hydroxyapatite (HAP; Ca10-x(HPO4)x(PO4)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 degrees 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 equi-dimensionally-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.
