Molecular interactions in aqueous biphasic systems composed of polyethylene glycol and crystalline vs. liquid cholinium-based salts

The relative ability of cholinium-([Ch]⁺)-based salts, including ionic liquids (ILs), to form biocompatible aqueous biphasic systems (ABS) with polyethylene glycols (PEGs) was deeply scrutinized in this work. Aqueous solutions of low molecular weight PEG polymers (400, 600, and 1000 g mol ^-1) and [Ch]⁺ salts of chloride, acetate, bicarbonate, glycolate, lactate, dihydrogenphosphate, dihydrogencitrate, and bitartrate can undergo liquid-liquid demixing at certain concentrations of the phase-forming components and at several temperatures. Cholinium butanoate and propanoate were also studied; however, these long alkyl side chain ILs are not able to promote an immiscibility region with PEG aqueous solutions. The ternary liquid-liquid phase diagrams, binary water activities, PEG-salt and salt-H₂O solubility data, and binary and ternary excess enthalpies estimated by COSMO-RS (COnductor-like Screening MOdel for Realistic Solvation) were used to obtain new insights into the molecular-level mechanisms responsible for phase separation. Instead of the expected and commonly reported salting-out phenomenon induced by the [Ch]⁺ salts over the polymer, the formation of PEG-[Ch] ⁺ salt ABS was revealed to be an end result of a more intricate molecular scenario. The multifaceted approach employed here reveals that the ability to promote an ABS is quite different for the higher melting salts vs. the lower melting or liquid ILs. In the latter systems, the ABS formation seems to be controlled by the interplay of the relative strengths of the ion-ion, ion-water, ion-PEG, and water-PEG interactions, with a significant contribution from specific hydrogen-bonding between the IL anion and the PEG hydroxyl groups.