Alkali metal ions transfer across the water/1,2-dichloroethane interface facilitated by a series of crown ethers

Abstract

The facilitated transfer of alkali metal ions (Li⁺ and Na⁺) across the water/1,2-dichloroethane (W/1,2-DCE) interface was studied by using a series of crown ethers as ionophores: 4′-ethynylbenzo-15-crown-5-ether (L1), 3′,6′-diethynylbenzo-15-crown-5-ether (L2) and 4′,5′-diethynylbenzo-15-crown-5-ether (L3). Cyclic voltammetry was employed to study the electrochemical behaviour of the facilitated ion transfer across the W/1,2-DCE interface supported at the tip of a micropipette. The diffusion coefficients of the ionophores in the 1,2-DCE phase were determined, while the metal–ligand complexes formed by these ions with all the ionophores were obtained to be in a 1 : 1 stoichiometric ratio. The association constants, log β°, for complexes LiL1⁺, LiL2⁺, LiL3⁺, NaL1⁺, NaL2⁺ and NaL3⁺ were calculated to be 3.3, 4.2, 4.0, 2.1, 3.5 and 2.2, respectively. The theoretical calculations have shown that the conjugated constituent groups on the benzene ring have an essential effect on the spatial structures of the crown ether rings, which determine the supramolecular interaction between the ions and ionophores.