Chemical Equilibria Modeling of Calcium Phosphate Precipitation and Transformation in Simulated Physiological Solutions

The metastable and stable equilibria during the precipitation of calcium phosphates in three biomimetic systems, namely SBFc-CaCl2–K2HPO4–H2O, SBFc-CaCl2–MgCl2–K2HPO4–H2O and SBFc-CaCl2–ZnCl2–K2HPO4–H2O, where SBFc denotes a conventional simulated body fluid, were modeled by a thermodynamic approach (ion-association model, computer program PHREEQCI v.2.14.3). In all cases the highest saturation indices (SI) and the thermodynamic stability were calculated for hydroxyapatite, Ca5(PO4)3OH. Co-precipitation of metastable phases of seven salts in the first system, co-precipitation of additional four magnesium salts in the second system, and no co-precipitation of zinc salts in the third system were calculated at pH 8. Precipitation of amorphous calcium phosphate incorporating Mg2+, Na+, K+, \( {\text{CO}}_{3}^{2 - } \) and Cl− ions at levels close to those of natural enamel, dentin and bone, instead of the thermodynamically stable hydroxyapatite, was experimentally found. This reveals that kinetic factors are decisive for the precipitation processes. In addition, the phase transformations of the precipitated metastable amorphous products (9.94 > SI > 0) during their maturation in the three simulated body fluids, differing in their concentrations of \( {\text{HCO}}_{3}^{ - } \) and Cl− ions and organic macromolecules, were also modeled. Dissolution of all magnesium salts was found to occur and equilibrium of the more stable calcium salts was established, depending on the \( {\text{HCO}}_{3}^{ - } \) concentration. In contrast, transformation of the stable equilibrium products (SI > 0) to the thermodynamically stable hydroxyapatite (SI = 0), independent of the \( {\text{HCO}}_{3}^{ - } \) concentration, was both calculated and experimentally proven. Analogous distribution of the species was calculated for the initial and equilibrium solutions of the studied SBFs, while it was different for the metastable solutions. The predominance of free Me2+ ions (Me = Ca, Mg) was calculated in all studied cases.