Dissolution kinetics of dicalcium phosphate dihydrate under pseudophysiological conditions

Abstract Dissolution kinetics of the (0 1 0) face of dicalcium phosphate dihydrate (DCPD) and the transformation of DCPD to hydroxyapatite were observed using in situ atomic force microscopy. Step velocities for [2 0 1]- and [1 0 1]′-oriented steps were measured in pseudophysiological solutions. The latter step was inclined 5–7° toward [0 0 1] from [1 0 1] directions. The step kinetic coefficients calculated for the [2 0 1] and [1 0 1]′ steps were, respectively, 0.70×10 −4 and 1.43×10 −4  m/s, values comparable to those for soluble inorganic crystals. The solubility product for DCPD accurately estimated from the relation between the step velocity and the undersaturation was (4.13±0.1)×10 −7 , a value 1.6 times larger than that previously reported. It was shown that the precipitation of hydroxyapatite occurred after the dissolution of DCPD, and no evidence of direct structural transformation from DCPD to hydroxyapatite was observed. This indicates that DCPD acts as a simple heterogeneous growth center for hydroxyapatite without requiring any structural resemblance.