Controlling the Formation of Ionic-Liquid-based Aqueous Biphasic Systems by Changing the Hydrogen-Bonding Ability of Polyethylene Glycol End Groups.

The formation of aqueous biphasic systems (ABS) when mixing aqueous solutions of polyethylene glycol (PEG) and an ionic liquid (IL) can be controlled by modifying the hydrogen-bond-donating/-accepting ability of the polymer end groups. It is shown that the miscibility/immiscibility in these systems stems from both the solvation of the ether groups in the oxygen chain and the ability of the PEG terminal groups to preferably hydrogen bond with water or the anion of the salt. The removal of even one hydrogen bond in PEG can noticeably affect the phase behavior, especially in the region of the phase diagram in which all the ethylene oxide (EO) units of the polymeric chain are completely solvated. In this region, removing or weakening the hydrogen-bond-donating ability of PEG results in greater immiscibility, and thus, in a higher ability to form ABS, as a result of the much weaker interactions between the IL anion and the PEG end groups.

Effect of polyvalent ions in the formation of ionic-liquid-based aqueous biphasic systems.

Two main approaches were combined aiming at evaluating the impact of polyvalent salt cations on the formation of ionic-liquid-based aqueous biphasic systems (ABS): (i) experimental determination of a large array of ternary phase diagrams composed of sodium-, magnesium-, and aluminum-based salts with 1-butyl-3-methylimidazolium-based ionic liquids and (ii) determination of the ions speciation in each of these systems. The results here reported show, for the first time, that the phase behavior of ionic-liquid-based aqueous biphasic systems is not only dominated by the ability of the strong and "free" salting-out ions to interact with water creating thus hydration complexes but also a result of the interactions occurring between the different ions and, particularly, on their speciation in aqueous solutions. The gathered data indicate that the higher the salt ion valence, the more complex is its speciation with a number of different species in aqueous media present. Further results based on NMR spectroscopy and a proper analysis of the pH influence clearly demonstrated the impact of ion speciation through the phase separation and ABS formation.