Structural variations in room-temperature ionic liquids: Influence on metal ion partitioning modes and extraction selectivity

Studies of the partitioning behavior of alkali and alkaline earth cations in systems comprising a nitric acid solution and an immiscible ionic liquid (IL) to which a crown ether has been added have established trends in the predominant mode by which these cations are transferred between phases. For a series of 1,3-dialkylimidazolium-based ILs, it is shown that the relative importance of cation exchange and neutral complex extraction to the overall ion transfer process can vary considerably with the hydrophobicity of the IL cation, the concentration of the aqueous acid, and the metal ion extracted. The overall trends observed display a simple correlation with charge density within each group of cations. Coupled with existing knowledge of ion partitioning into ILs, these results provide a framework by which variations in the extraction behavior of metal ions with changing conditions can be rationalized and the unique extraction selectivity exhibited by these systems can be exploited to develop improved systems for metal ion separations.

► Extraction of alkali and alkaline earth ions into CnmimTf2N ILs by DCH18C6 is examined.
► A three-path model describes extraction of these ions from acidic nitrate media.
► Neutral complex extraction or one or both of two types of ion exchange is possible.
► The nature of the IL, the metal ion, and aqueous acidity determine the primary path.
► The Sr/Na and Sr/Ca extraction selectivity is improved in C10mimTf2N vs. 1-octanol.