Aqueous biphasic systems comprising copolymers and cholinium-based salts or ionic liquids: insights on the mechanisms responsible for their creation

Abstract

Aqueous biphasic systems (ABS) formed by copolymers and ionic liquids (ILs) have demonstrated to be effective separation platforms, but there is still a gap on the complete understanding of the molecular-level mechanisms ruling the two-phase formation for this type of systems. This work addresses the determination of the liquid-liquid equilibrium of ABS composed of cholinium-based salts or cholinium-based ILs and the triblock copolymer Pluronic PE6200 (PL6200). It is demonstrated that PL6200 can form ABS with all investigated cholinium-based salts or ILs, contrarily to most poly(ethylene)glycol polymers, which is due to the presence of hydrophobic propylene oxide (PO) blocks. From the phase diagrams behavior and IL/salt anions properties, it is shown that the formation of ABS with cholinium-based salts is ruled by the anions polar surface and ability to be hydrated, whereas in systems comprising ILs van der Waals interactions between the copolymer and the IL cannot be discarded. The partition of a series of alkaloids in these systems, namely caffeine, nicotine, theophylline, and theobromine, was additionally appraised. It is shown that caffeine, theophylline, and theobromine preferentially migrate to the more hydrophobic PL6200-rich phase, and that their partition depends on the water content in the respective phase, being ruled by the phases’ hydrophobicity. On the other hand, nicotine, with the most prominent hydrophobic character amongst the studied alkaloids, preferentially migrates to the salt- or IL-rich phase, in which interactions occurring between this alkaloid and the IL/salt cannot be discarded. The ABS formed by cholinium dihydrogenphosphate is the most selective system identified to separate nicotine from the remaining alkaloids, giving some insights into their investigation as separation platforms for alkaloids from natural extracts.