Talanta 95 (2012) 25–30 Contents lists available at SciVerse ScienceDirect Talanta j ourna l ho me page: www.elsevier.com/locate/talanta Effect of aqueous phase anion on the mode of facilitated ion transfer into room-temperature ionic liquids Sarah L. Garvey, Cory A. Hawkins, Mark L. Dietz ∗ Department of Chemistry and Biochemistry, University of Wisconsin – Milwaukee, Milwaukee, WI 53211, United States a r t i c l e i n f o Article history: Received 1 February 2012 Received in revised form 21 March 2012 Accepted 22 March 2012 Available online 17 April 2012 Keywords: Extraction Ionic liquids Alkali cations Alkaline earth cations Crown ethers a b s t r a c t 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 solu- tions 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Liquid–liquid extraction (LLE) represents one of the most well established methods of separation, a result of its relative simplic- ity, ease of use, and versatility. As applied to metal ions, LLE has proven itself to be a flexible and powerful approach to achieving separations, both on the analytical and process scale [1]. Despite its many virtues, however, as ordinarily practiced the technique also suffers from a variety of limitations, most notably the fre- quent need for toxic (e.g., chlorinated hydrocarbons) or volatile (e.g., paraffinic hydrocarbons) organic solvents. This drawback has led to widespread interest in alternative approaches to extrac- tion employing more environmentally benign solvents, including supercritical fluids [2] and, more recently, bio-based diluents [3] and ionic liquids (ILs) [4–6]. Work in this laboratory has con- cerned the application of the latter solvents, particularly those that are liquid at room temperature (RTILs), in the extraction of metal ions from the highly acidic aqueous phases encountered in such applications as nuclear fuel reprocessing [7] or analytical- scale radionuclide separation and preconcentration for subsequent determination [8]. In the course of these studies, it has become apparent that the process by which a metal ion is extracted into an ionic liquid can be significantly more complex than that observed with conventional (i.e., molecular) organic solvents [9–12]. This is particularly true, it has been found, when neutral extractants ∗ Corresponding author. Tel.: +1 414 229 1748; fax: +1 414 229 5530. E-mail address: dietzm@uwm.edu (M.L. Dietz). are employed. Prior work indicates that in systems employing a crown ether as the extractant, for example, partitioning can follow any one or more of three distinct pathways: extraction of a neu- tral complex, in which the metal ion-crown ether complex initially formed is paired with an aqueous phase anion to yield a neutral species, which then partitions into the IL phase; formation of a cationic metal-crown ether complex, followed by its exchange for the cationic component of the IL; and formation of an acid-crown ether complex, followed by exchange of an aqueous phase metal ion for the complexed proton (or hydronium ion) [9–11]. Despite progress in elucidating the factors governing the balance among these pathways, these factors remain incompletely understood. It is now well known that certain characteristics of an ionic liq- uid strongly influence the mode of metal ion partitioning into it. For example, the use of ILs incorporating a hydrophobic cation has been shown to favor the extraction of neutral complexes, while more hydrophilic cations favor ion-exchange processes [10]. Other recent results suggest that the nature of the IL anion is similarly influential in determining the preferred mode of extrac- tion [13]. To date, the effect of aqueous phase composition on the “balance of pathways” has received scant attention, despite its obvious importance. That is, because of their utility in a wide variety of metal ion separations involving extraction into conven- tional organic solvents, nearly all studies of metal ion extraction into ILs have employed acidic nitrate-containing aqueous phases. Chloride-containing aqueous phases, however, are also of signifi- cance in numerous conventional metal ion extraction systems. For example, a chloride-based analog of the well-known TRUEX Process for nuclear waste treatment has been described [14]. 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