A ternary mechanism for the facilitated transfer of metal ions into room-temperature ionic liquids (RTILs): implications for the “greenness” of RTILs as extraction solvents

Abstract

Partitioning of sodium ions between aqueous nitrate media and 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imides (C_nmim⁺Tf₂N⁻) in the presence of dicyclohexano-18-crown-6 is shown to take place via as many as three pathways: conventional nitrato complex extraction and/or either or both of two ion-exchange processes, the relative importance of which is determined by aqueous acidity and the hydrophobicity of the ionic liquid cation. Contrary to expectations, increasing the alkyl chain length of the IL cation (from C₅mim⁺ to C₁₀mim⁺) is insufficient to eliminate the possibility of ion exchange as a mode of metal ion partitioning between the two phases, an observation with negative implications for the utility of ILs as environmentally benign extraction solvents.