The recent development of hydrophobic deep eutectic solvents (HDESs) has led to growing interest in their potential utility as environmentally-benign replacements for conventional organic media in a host of applications, among them metal ion separations by liquid-liquid extraction.  To evaluate these novel solvents in this application, a systematic examination of the extraction of selected alkali and alkaline earth cations into representative HDESs from aqueous solution by a macrocyclic polyether (<i>i.e</i>., a crown ether) has been undertaken.  Comparison of the results to those obtained for various oxygenated, aliphatic solvents (<i>n</i>-alcohols) and a series of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs) under the same conditions indicate that despite frequent suggestions that some HDESs resemble ILs, metal ion distribution in HDES-aqueous systems more closely mimics that seen for the alcohols. Metal ion partitioning in these systems appears less dependent on the water content of the organic phase and on structural variations in the solvent than is the case for either alcohols or ionic liquids, however. The implications of these results for the design and application of HDES-based extraction systems for metal ions are described.

Systematic evaluation of hydrophobic deep-melting eutectics as alternative solvents for the extraction of metal ions from aqueous solution

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