Electrochemistry of ionophore-coordinated Cs and Sr ions in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid

Abstract

The selective extraction of Cs + and Sr 2+ from aqueous solutions by using the ionophores calix[4]arene-bis( tert-octylbenzo-crown-6) (BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6), respectively, was demonstrated in the hydrophobic, room-temperature ionic liquid (RTIL), tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Tf 2N −). The electrochemistry of Cs + coordinated by BOBCalixC6 and Sr 2+ coordinated by DCH18C6 was examined at a mercury film electrode (MFE) in this ionic liquid by using cyclic staircase voltammetry, sampled current voltammetry at a rotating electrode, and chronoamperometry. Both BOBCalixC6·2Cs + and DCH18C6·Sr 2+ exhibit well-defined reduction waves at approximately −2.4 and −2.9 V versus the ferrocene/ferrocenium (Fc/Fc +) couple, respectively, in which the coordinated ions are reduced to their respective amalgams, permitting the recycling of the ionophores. The diffusion coefficients of BOBCalixC6·2Cs + and DCH18C6·Sr 2+ are (2.7 ± 0.1) × 10 −9 and (2.1 ± 0.1) × 10 −9 cm 2 s −1, respectively, at 30 °C. The coulometric efficiency for the reduction and stripping of Cs at mercury pool electrodes was about 90% and was independent of the deposition time, whereas the efficiency for Sr was slightly less than 90% at short times and decreased with the deposition time, probably due to the formation of a passive layer of Sr(Tf 2N) 2.