Thermodynamics of solvent extraction of metal picrates with crown ethers: enthalpy–entropy compensation. Part 1. Stoicheiometric 1 : 1 complexation

Abstract

Quantitative solvent extractions were performed in the water-dichloromethane system at 10–25 °C to give the extraction equilibrium constants (K_ex) and the thermodynamic quantities (ΔH° and ΔS°) for the ion-pair extraction of aqueous alkali (Li⁺-Cs⁺), alkaline earth (Ca²⁺-Ba²⁺), and some heavy metal (Ag⁺, Tl⁺) picrates with 15-crown-5, benzo-15-crown-5, cis-cyclohexano-15-crown-5, 16-crown-5, 15-(2-methoxyethoxy)methyl-15-methyl-16-crown-5, 18-crown-6, dibenzo-18-crown-6, and dibenzo-24-crown-8. The cation-binding abilities and relative cation selectivities of the ligands are discussed from the thermodynamic point of view. Global treatment of the thermodynamic data obtained in the present and previous works reveals not only that, as is the case with the complexation in the homogeneous phase, the enthalpy-entropy compensation effect holds in the solvent extraction system, but also the ΔH°vs. TΔS° plot affords the characteristic slope (α) and intercept (TΔS_o) which are comparable to the corresponding values obtained for the homogeneous-phase complexation. This indicates that the conformational change and the extent of desolvation upon cation binding by crown ethers are quite analogous both in the homogeneous phase and in the solvent extraction beyond the apparent differences in the phase and the complexed species involved.