Imidazolium-based Ionic Liquids as Adjuvants to Form Polyethylene Glycol with Salt Buffer Aqueous Biphasic Systems

Abstract

Aqueous biphasic systems (ABS) are biocompatible systems applied in the extraction of biomolecules. Despite the biocompatibility of polymers and, particularly polyethylene glycol (PEG), to form ABS, their limitation in terms of phase separation is recognized. A new approach was recently proposed based on the use of ionic liquids (ILs) as adjuvants in ABS, enlarging the polarity range of these systems. Up to now, the effects of ILs in PEG-salt ABS have been poorly described. To overcome this limitation, the phase diagrams of imidazolium-based ILs acting as adjuvants in ABS based in PEG with potassium salt buffers (pH = 7), that is potassium citrate (C6H5K3O7/C6H8O7) and potassium phosphate (K2HPO4/KH2PO4) buffers, are herein addressed. Imidazolium-based ILs were focused in this work, since they have been applied on the purification of several biomolecules with success, even as adjuvants or electrolytes. The phase diagrams were mapped out for PEG/salt ABS without adjuvants. In this work, systems composed of PEG (1000, 1500, 2000, 3350, 4000, 6000, and 8000) with potassium phosphate buffer and PEG (2000, 6000, 10 000, and 20 000) with potassium citrate buffer were tested. Moreover, the presence of 5 wt % of imidazolium-based ILs (varying the anion moiety) for the system PEG 1500 with potassium phosphate buffer was also investigated. Imidazolium-based ILs with different anions were tested to investigate a large range of polarities attributed to the adjuvant. Moreover, the effect of the adjuvant content (5, 10, and 20 wt %) in the PEG 2000 with potassium citrate buffer system was studied for two distinct ILs, namely [C4mim][CF3SO3] and [C4mim][(CH3O)2PO2], with lower and higher energy of intramolecular hydrogen bond, EHB, respectively, a parameter representing the ions' hydration. A correlation between the anion moiety of imidazolium-based IL and the ability to form two phases was observed, being this related to the ILs' anion EHB value. The concentration of the adjuvant confirmed the effects of enhancing or decreasing the ability to form two phases for ILs with lower and higher EHB value, respectively.