Ionic liquid (IL) cation and anion structural effects on the extraction of metal ions into N-alkylpyridinium-based ILs by a macrocyclic polyether

Abstract

ABSTRACT Previous studies of the effect of IL cation/anion hydrophobicity and the nature of the aqueous phase anion on the partitioning of alkali and alkaline earth cations between an acidic aqueous phase and 1,3-dialkylimidazolium-based or quaternary ammonium-based ILs in the presence of a crown ether have revealed significant similarities between the behavior of the two IL families. To further explore the generality of these observations and their potential application in the rational design of ILs for use as replacements for the molecular diluents typically employed as extraction solvents, the extraction of several metal ions (Na+, Sr2+ and Ba2+) from nitric and hydrochloric acid solutions by dicyclohexano-18-crown-6 (DCH18C6) into a series of N-alkylpyridinium-based ILs (Cn-pyr+ ILs) has been studied. Although much of the extraction behavior observed in these systems is consistent with that seen for the 1,3-dialkylimidazolium and quaternary ammonium-based ILs, certain observations, in particular the effect of IL cation hydrophobicity on divalent metal ion extraction from HNO3 by N-alkylpyridinium-ILs bearing long (dodecyl- and tetradecyl-) alkyl chains, cannot be fully explained by the established three-path model for metal ion partitioning. Instead a complete description of metal ion extraction in these systems requires consideration of the aggregation of the IL cation in the aqueous phase. The effects of this aggregation on the efficiency and selectivity of metal ion extraction into Cn-pyr+ ILs by the crown ether are systematically explored.