Thermodynamic insights into the interplay between calcium and iron(ii) hydroxycarboxylates: impacts on solubility, speciation, and bioavailability

Abstract

This study investigated the aqueous solubility, speciation, and thermodynamic properties of calcium and iron(II) hydroxycarboxylates to elucidate their complex interactions and optimize mineral supplement formulations. The results showed that the solubility of these compounds increased with temperature, with calcium lactate exhibiting higher solubility than calcium gluconate, while iron(II) gluconate had higher solubility than iron(II) lactate. The thermodynamic analysis revealed that the dissolution processes were endothermic and entropy-driven. Additionally, the presence of iron(II) ions was found to enhance the solubility of calcium hydroxycarboxylates, likely due to competitive complexation and suppression of ion pairing. In mixed systems containing both calcium and iron(II) hydroxycarboxylates, homologous systems (same anion) exhibited decreased solubilities due to the common ion effect, and the presence of a competing salt significantly altered the dissolution thermodynamics, with the effect being more pronounced for the less soluble salt. While cross-ligand systems (different anions) showed complex dissolution behavior involving rapid one-step and step-wise ion exchange processes, leading to supersaturation and precipitation. Isothermal titration calorimetry provided insights into the thermodynamics driving these ion exchange reactions. The findings have important implications for understanding the bioavailability and stability of these mineral supplements and fortified food products.