Influence of structural variation in room-temperature ionic liquids on theselectivity and efficiency of competitive alkali metal salt extraction by a crown ether

An improved method for the preparation of 1-alkyl-3-methylimidazolium hexaf luorophosphates provides a series of room-temperature ionic liquids (RTILs) in which the 1-alkyl group is varied systematically from butyl to nonyl. F or competitive solvent extraction of aqueous solutions of alkali metal chlo rides with solutions of dicyclohexano-18-crown-6 (DC18C6) in these RTILs, t he 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 of DC18C6 in chloroform, nitrobenzene, and 1-octan ol was undetectable. The extraction selectivity order for DC18C6 in the RTI Ls was K+ > Rb+ > Cs+ > Na+ > Li+. As the alkyl group in the RTIL was elong ated, the K+/ Rb+ and K+/Cs+ selectivities exhibited general increases with the larger enhancement for the latter. For DC18C6 in 1-octyl-3-methylimida zolium hexafluorophosphate, the alkali metal cation extraction selectivity and efficiency were unaffected by variation of the aqueous-phase anion from chloride to nitrate to sulfate.