Dual mode of extraction for Cs+ and Na+ ions with dicyclohexano-18-crown-6 and bis(2-propyloxy)calix[4]crown-6 in ionic liquids: density functional theoretical investigation

Abstract

The unusually high selectivity of the Cs⁺ ion over the Na⁺ ion with bis(2-propyloxy)calix[4]crown-6 (BPC6) compared to dicyclohexano-18-crown-6 (DCH18C6) has been investigated using generalized gradient approximated (GGA) BP86, hybrid B3LYP and meta hybrid TPSSH density functionals, employing split valence plus polarization (SVP) and triple zeta valence plus polarization (TZVP) basis sets in conjunction with the COSMO (conductor like screening model) solvation approach. The calculated theoretical selectivity of the Cs⁺ ion over the Na⁺ ion was found to be in accordance with the experimental selectivity obtained using solvent extraction experiments in ionic liquids (IL) and octanol. The distribution constant of the Cs⁺ ion, D_Cs with DCH18C6 in the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTF2N) IL phase was found to be significantly large than that in octanol. The experimentally measured D_Cs value was found to be very large compared to the value of D_Na in the IL phase. The presence of the BMIM cation in the recorded UV-visible spectra of the raffinate phase with and without DCH18C6 indicates the BMIM cation exchange with Cs⁺ and Na⁺ ions, thus supporting the dual mode of extraction. The dual mode of metal ion extraction observed in the experimental study was complemented by density functional theoretical study. The calculated free energy of extraction, ΔG_ext, for the metal ion was found to be higher in IL compared to octanol. Further, preferential selectivity of the Cs⁺ ion over the Na⁺ ion was established from the free energy difference, ΔΔG_ext, between the two competing metal ions. The unusually high selectivity of Cs⁺ over the Na⁺ ion by BPC6 in IL compared to DCH18C6 is also demonstrated by the free energy difference, ΔΔΔG_ext, between the two competing ligands which was shown to be free from the complicated metal ion solvation energy.