Influence of Structural Variation in Room-Temperature Ionic Liquids on the Selectivity and Efficiency of Competitive Alkali Metal Salt Extraction by a Crown Ether

An improved method for the preparation of l-alkyl-3methylimidazolium hexafluorophosphates provides a series of room-temperature ionic liquids (RTTLs) in which the 1-alkyl group is varied systematically from butyl to nonyl. For competitive solvent extraction of aqueous solutions of alkali metal chlorides with solutions of dicyclohexano-18-crown-6 (DC18C6) in these RTTLs, the extraction efficiency generally diminished as the length of the 1-alkyl group was increased. Under the same conditions) extraction of alkali metal chlorides into solutions ofDC18C6 in chloroform, nitrobenzene, and I-octanol was undetectable. The extraction selectivity order for DC18C6 in the KTTLs was K^+ > Rb^+ > Cs^+> Na^-> Li^+. As the alkyl group in the RTILwas elongated, the K^+/ Rb^+ and K^+/Cs selectivities exhibited general increases with the larger enhancement for the latter. For DC18C6 in l-oclyl-3-methylimidazolium hexafluorophosphate, the alkali metal cation extraction selectivity and efficiency were unaffected by variation of the aqueous-phase anion from chloride to nitrate to sulfate.