Quantifying Individual Partitioning Modes in the Extraction of Alkaline Earth Cations by Crown Ethers into Room-Temperature Ionic Liquids (RTILs)

Alkali and alkaline earth cation extraction from an acidic aqueous phase into 1,3-dialkylimidazolium-, quaternary ammonium-, and pyridinium-based RTILs by a crown ether (CE) has previously been shown to involve neutral complex/ion pair extraction (NC/IPE) and two forms of ion-exchange (IX): exchange of the cationic metal-CE complex formed for an IL cation (IX-1) and exchange of the metal ion for the hydronium ion present in a CE-H3O+ complex formed during acid preconditioning of the IL phase (IX-2). Previously, the relative contributions of these modes have been estimated only qualitatively, by interpreting the shape and direction of the dependence of DM values on aqueous acidity. Here, the first quantitative treatment of these extraction systems is described. Specifically, the extraction of Sr2+ from nitric acid by dicyclohexano-18-crown-6 (DCH18C6) into two 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imides (Cnmim+Tf2N−, with n = 5 or n = 10) is examined. For C5mim+Tf2N−, IX-1 is shown to be the dominant pathway for extraction at low acidity, gradually giving way to IX-2 and ultimately to NC/IPE as the acidity rises. In contrast, for the more hydrophobic C10-IL, IX-1 is of little importance, while NC/IPE predominates at modest acidities, with its contribution to the overall extraction process exceeding 80% under certain conditions.