Heterogeneous crystallization of dicalcium phosphate dihydrate on titanium surfaces

The kinetics of nucleation and crystal growth of dicalcium phosphate dihydrate (DCPD, CaHPO4.2H2O) on titanium powders with different grain sizes have been investigated using the constant composition crystal growth method at 37 degrees C and pH = 5.50. Nucleation is independent of titanium powder size while crystal growth rate is strongly size-dependent. A minimum relative supersaturation ratio, sigma(min), required in the system for a detectable crystal growth rate to occur, taking into account the presence of the foreign substrate in the system, was determined. Based on the kinetic data and on the various characterizations by X-ray photoelectron spectroscopy and by scanning electron microscopy, we propose a crystal growth kinetic equation as a function of two parameters: the size of the substrate and the excess of supersaturation (sigma-sigma(min)) which is squared, indicating a spiral growth mechanism.