A striking effect of ionic-liquid anions in the extraction of Sr2+ and Cs+ by dicyclohexano-18-crown-6

The nature of the ionic-liquid (IL) anion has been found to have a remarkable effect on the solvent extraction of Sr2+ and Cs+ by dicyclohexano-18-crown-6 dissolved in ionic liquids. In particular, the extraction efficiency increases with the hydrophobicity of the IL anion as reflected by the solubility in water of ILs having a common cation. Since a cation-exchange mechanism is operating in these systems, the influence of the IL anion is in large part attributable to an expected Le Chatelier effect in which a greater aqueous concentration of IL cation, obtained when using an IL anion of lower hydrophobicity, opposes cation exchange. This dependence is opposite to that found for IL cations, indicating a significant advantage of using ILs with hydrophobic anions for cation extraction. Furthermore, the extraction selectivity for Sr2+ over Na+, K+, and Cs+ can be significantly improved through the use of hydrophobic anions for the ILs containing 1-ethyl-3-methylimidazolium or 1-butyl-3-methylimidazolium cations.