Precipitation and phase transformation of dicalcium phosphate dihydrate in electrolyte solutions of simulated body fluids: Thermodynamic modeling and kinetic studies

The metastable and stable equilibria of a precipitation in the biomimetic system Simulated Body Fluid (SBF)-CaCl2-K2HPO4-KOH-H2O were modeled in the pH region 3-7 at a Ca/P molar ratio of 1 using a thermodynamic approach. Saturation indices (SI) of the solid phases were calculated and used to prognose the salt precipitation/dissolution processes. At pH < 4, the solutions are undersaturated (SI < 0) in respect of all solid phases; co-precipitation of dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HA) occurs at pH 4 while at pH > 4 the stable phase is DCPD but the number of other co-precipitated solid phases increases. This result is associated with the increase in HPO42-, CaHPO40, and KHPO4- species in the studied solution. The phase transformations of five model DCPD-based calcium phosphate precursors in three simulated body fluids differing in their composition, to more stable octacalcium phosphate and hydroxyapatite was thermodynamically prognosed and experimentally confirmed by kinetic studies, as well as by chemical, XRD, SEM, and IR methods.