Chromatographic determination of Gibbs free energy of transfer for crown ether and monodispersed polyoxyethylene complexes with alkali cations from methanol to acetonitrile

Abstract

Several chromatographic methods are cooperatively employed to determine the Gibbs free energies of transfer (ΔG_tr^°) for alkali cation complexes with crown ethers and monodispersed polyoxyethylene (POE) oligomers having 6–13 oxyethylene units. In all cases, polyether complexation is enhanced in acetonitrile (AN) relative to methanol (MeOH). This enhancement is most pronounced for Li⁺ complexation, and becomes marginal with increasing the sizes of alkali cations. The enhancement of crown ether complexation in AN dominantly comes from the smaller energy loss in the desolvation process of a cation in this solvent. The preferable free energies of transfer for POE complexes from MeOH to AN is another important factor in the enhancement of POE complexation in AN. The comparison of thermodynamic data with conductivity data imply that there are structural differences between POE complexes in MeOH and in AN, and that strong coordination occurs between POE and a metal ion in AN.