Effect of aqueous phase anion on the mode of facilitated ion transfer into room-temperature ionic liquids

Measurements of the partitioning of various alkali and alkaline earth cations between solutions of hydrochloric acid and a series of 1,3-dialkylimidazolium-based ionic liquids (ILs) to which a crown ether has been added have revealed substantial differences in extraction behavior versus both conventional molecular solvents (e.g., 1-octanol) under the same conditions and the same ILs when nitric acid solutions are employed as the aqueous phase. These results can be rationalized by application of a three-path model for metal ion partitioning into ILs in the presence of a neutral extractant. Additionally, the results point to a significant role for anion hydration energy in determining the balance amongst the possible modes of partitioning and strongly suggest that ion exchange involving the cationic metal complex and the cationic constituent of the ionic liquid constitutes the “default” route for metal ion extraction in IL systems incorporating a neutral extractant.