Ionic-Liquid-Based Aqueous Biphasic Systems with Controlled pH : The Ionic Liquid Anion Effect

This work addresses the effect of different anions in imidazoliumbased ionic liquids (ILs) through the formation of aqueous biphasic systems (ABS) using a phosphate buffer solution (KH2PO4/K2HPO4 mixture of salts) aiming at controlling the pH value of the coexisting aqueous phases. Twelve ABS were investigated and the corresponding solubility curves were determined by the cloud point titration method at 298 K. In general, the IL anions aptitude to induce the formation of ABS increases with the decrease of the ability of the ions to form hydration complexes, i.e., with the decrease on the hydrogen-bond basicity of individual fluids. This trend is in close agreement to that observed in ABS with different inorganic salts, showing that the ILs rank is not dependent on the saltingout species used. ■ INTRODUCTION Liquid−liquid extraction approaches that make use of aqueous biphasic systems (ABS) are advantageous choices for the development of more environmentally benign and biocompatible separation processes, due to the use of nonvolatile compounds and a dominant aqueous media. ABS are typically formed by the concomitant addition of aqueous solutions of two-water-soluble polymers or by the addition of a salting-out inorganic salt to a polymer aqueous solution. However, polymer−polymer ABS have a relatively high cost, are highly viscous, and require longer periods for phase separation. For electrolyte−polymer systems, the high ionic strength of the aqueous media strongly limits their applications in the biotechnological field. In the past decade, ionic liquids (ILs) have shown great potential as possible replacements for conventional organic solvents, and also as phase tunable additives. The large number of potential anion/cation combinations allows the fine-tuning of the ILs’ physical properties, and the design of ILs with an adjustable solubility in water and controllable biocompatibility. Those factors are crucial in the ABS formation ability and on the use of ILs in the biotechnological field as separation/purification agents. Since the first work reporting on ABS composed of ILs and inorganic salts, by Rogers and co-workers, the number of publications dealing with IL-based ABS (IL-ABS) has been rising in the past few years. In most of these works, the number of ILs studied is indeed quite limited, with the study of the inorganic salt effect as the major source of interest. Concerning the employed ILs, most works have focused on imidazoliumbased ILs and reported the effect of the cation side alkyl chain length, with the chloride and bromide counterions. Additional works have addressed a wide range of IL cations and anions conjugated with the kosmotropic salt K3PO4, 21−24,30−32 and more recently, with Na2SO4 and K2HPO4/KH2PO4. 12,33 The K3PO4 is the inorganic salt most studied as an IL-ABS promoter, followed by hydrogenophosphates, carbonates, citrates, or other compounds (weaker salting-out species), such as carbohydrates and aminoacids. Nevertheless, in most of these systems, there is not an adequate control of the pH value of the aqueous phases which is a crucial issue regarding the separation/extraction of particular biomolecules. Only recently, the inorganic phosphate buffer solution composed of K2HPO4/KH2PO4 has found some interest among researchers within the topic of biomolecules purification. This work, and its companion article in ref 39, are focused on the capability of forming IL-ABS making use of a phosphate buffer (PB), K2HPO4/KH2PO4 mixture. The major advantage of this salt, over other salting-out inducing salts, relays on the possibility of controlling the pH values of the aqueous phases, what is essential when dealing with biomolecules. Imidazolium-based ILs combined with a wide series of hydrophilic anions were investigated, and the effect of the IL anion on the formation of ABS is here presented and discussed. Received: October 6, 2011 Accepted: January 9, 2012 Published: February 2, 2012 Article

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