Thermodynamic study of crown ether–lithium/magnesium complexes based on benz-1,4-dioxane and its homologues

Abstract

The synthesis and characterization of benz-1,4-dioxane crown ethers (CEs) and some of its homologues are described and analyzed. The effect of added C-atom within the CE ring (increasing the hydrophobicity of the CE ring by increasing the number of CH₂-units) on the Li⁺ and Mg²⁺ complexation within a liquid–liquid extraction (LLE) is investigated and thermodynamically analyzed. The complex stability constant K, the change of entropy ΔS and enthalpy ΔH, and the Gibbs energy ΔG are determined. The enhanced hydrophobicity of the CE ring results in stronger complexation stability of the Mg²⁺ complex, while the Li⁺ complexes are less favored. This effect mainly occurs due to the increased entropy term with improved hydrophobicity of the CE. These results indicate a stronger extraction of Li⁺ in Mg²⁺-containing aqueous resources if more hydrophilic CEs are used.