Vibrational spectroscopy studies of the role of water in metal ion extraction into room temperature ionic liquids

Abstract

In efforts to evaluate the suitability of ionic liquids (ILs) as solvents for the extraction of fission products, two series of room temperature ILs, one comprising a diakylimidazolium cation and the other containing an analogous (1-hydroxyalkyl)-functionalized imidazolium cation, have been prepared as their bis[(trifluoromethyl)sulfonyl]imide salts. The extraction of Sr(II) from aqueous nitric acid solutions by dicyclohexano-18-crown-6 into these ILs has been measured. A linear correlation has been observed between the extent of Sr(II) extraction and the equilibrium water content of the IL phases. Although a precedence has been established similarly with n-alcohol diluents, the relationship in extraction systems employing IL diluents has yet to be adequately studied.

To investigate how water may be functioning in these biphasic systems, vibrational spectroscopy has been employed. A challenge to this analysis has been to differentiate between the vibrational modes of water and those of the of IL diluents. Raman spectroscopy offers the possibility to distinguish between -OH vibrational modes from solvent moieties and those in a different state, such as molecular water. Along with the near-IR and mid-IR ranges, Raman spectroscopy is being utilized the interrogate states of molecular water and other species that may be involved in metal ion partitioning. With the ultimate goal of elucidating the interactions between IL constituent ions, metal ion-crown ether complexes, co-extracted ions (e.g., nitrate) and water, these studies represent a principal step in gaining a better understanding of partitioning mechanisms in IL-based separation media.