Facilitated Transfer of Alkali-Metal Cations by Dibenzo-18-crown-6 across the Electrochemically Polarized Interface between an Aqueous Solution and a Hydrophobic Room-Temperature Ionic Liquid

Abstract

The facilitated transfer of alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) by dibenzo-18-crown-6 (DB18C6) across the electrochemically polarizable interface between an aqueous solution (W) and a hydrophobic ionic liquid, N-octadecylisoquinolinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([C₁₈Iq][TFPB]), has been studied using cyclic voltammetry at the interface formed at the tip of a micropipet. In cyclic voltammograms (CVs), the current due to the facilitated transfer of the cations by DB18C6 from W to [C₁₈Iq][TFPB] can be measured within the polarized potential window of the [C₁₈Iq][TFPB]|W interface. The stoichiometry of the complexes in [C₁₈Iq][TFPB] for Li⁺, Na⁺, K⁺, and Rb⁺ are found to be 1:1 while for the Cs⁺ transfer both 1:1 and 1:2 complexes are likely to be formed. The formation constants of the 1:1 complexes for Li⁺, Na⁺, K⁺, and Rb⁺ in [C₁₈Iq][TFPB], , evaluated from CVs are log = 5.0, 7.0, 8.2, and 7.3, respectively. The value for K⁺ is 1 order of magnitude greater than that for Na⁺. This higher selectivity of DB18C6 to K⁺ over Na⁺ in [C₁₈Iq][TFPB] compared with that in molecular solvents suggests that the RTIL provides a unique solvation environment for the complexations of DB18C6 with the ions.