Ionic Liquid (IL) Cation and Anion Structural Effects on Metal Ion Extraction into Quaternary Ammonium-based ILs

Numerous factors have previously been shown to influence the mode of extraction of alkali and alkaline earth cations from an acidic aqueous phase into 1, 3-dialkylimidazolium-based ionic liquids (ILs) by a crown ether, among them the hydrophobicity of both the IL anion and cation. To determine if this observation is “generic” and thus, could provide the basis for guidelines for the rational design of ILs to be used as solvents in metal ion extraction, other families of ILs must be studied. A series of quaternary ammonium-based ILs have therefore been examined as solvents for the extraction of various metal ions from acidic nitrate- and chloride-containing aqueous phases by dicyclohexano-18-crown-6 (DCH18C6). Although the overall metal ion extraction behavior in these systems is similar to that observed for 1, 3-dialkylimidazolium-based ILs, significant differences in metal ion separation factors (e.g., α_Sr/Na) are observed under certain conditions, differences that may be sufficient to influence the choice of IL in separations applications.