Ionic liquid anion effects in the extraction of metal ions by macrocyclic polyethers

• Cation extraction from acidic phases into several Cnmim+ ILs by DCH18C6 is examined.
• The balance between neutral complex extraction and ion exchange varies with IL anion.
• Hydrophobic IL anions favor metal ion partitioning via ion-exchange processes.
• IL anion effects on extraction efficiency/selectivity vary with aqueous acidity.

As part of a broader effort to determine the factors governing the relative contributions of ion-exchange processes and neutral complex/ion-pair partitioning in the extraction of metal ions into room-temperature ionic liquids by neutral extractants, the effect of ionic liquid (IL) anion on the extraction of alkali and alkaline earth cations by a crown ether from acidic nitrate and chloride media has been systematically examined. Results for a series of 1,3-dialkylimidazolium (Cnmim+)-based ILs incorporating hexafluorophosphate (PF6-), bis[(trifluoromethyl)sulfonyl] imide (Tf2N-Tf2N-), or bis[(perfluoroethyl)sulfonyl]imide (BETI−) as the counterion indicate that the choice of anion, in particular its hydrophobicity, can influence the balance among the various possible pathways for extraction, as well as both the extraction efficiency and selectivity. Overall, higher IL anion hydrophobicity is accompanied by a greater tendency toward ion exchange as the predominant mode of extraction. The effect upon extraction efficiency and selectivity is more complex, however, varying markedly with aqueous acidity.