Solvent Water Content as a Factor in the Design of Metal Ion Extraction Systems Employing Ionic Liquids

Abstract

ABSTRACTIn the extraction of strontium nitrate from aqueous nitric acid solutions into a series of hydrophobic N-(1-hydroxyalkyl)-N’-alkylimidazolium bis[(trifluoromethyl)sulfonyl]imides by dicyclohexano-18-crown-6, a correlation has been demonstrated between the conditional extraction constant of strontium (K’ex,Sr) and the concentration of water in the equilibrated ionic liquid (IL) phase. Despite the absence of a hydroxyl group on the IL cation, essentially the same relationship is observed for the analogous N,N’-dialkylimidazolium ILs. In both instances, the correlation is attributed to the coextraction of the hydrated nitrate ion and its solvation by the hydrated diluent. Transitions of water in the attenuated total reflectance-infrared spectra of the ILs are consistent with the existence of multiple states of water. The extraction efficiency for a given water content is greater for the ILs examined than for conventional solvents (i.e., n-alcohols). When coupled with existing knowledge of the factors influencing cation partitioning into ILs, these results provide insight into the IL structural characteristics required to effect efficient metal ion extraction from acidic media.