Evaluation of the impact of phosphate salts on the formation of ionic-liquid-based aqueous biphasic systems

The present study aims at evaluating the capability of phosphate-based salts, whose anions can coexist in water depending on the media pH, to promote aqueous biphasic systems (ABS) formation with 1-butyl-3-methylimidazolium-based ionic liquids, as well as to infer on the influence of the ionic liquid anion in the overall process of liquid–liquid demixing. In this context, novel phase diagrams of ABS composed of several imidazolium-based ionic liquids and three phosphate salts and a mixture of salts (K3PO4, K2HPO4, K2HPO4 + KH2PO4, and KH2PO4) were determined by the cloud point titration method at 298 K and atmospheric pressure. The corresponding tie-line compositions, tie-line lengths, and pH values of the coexisting phases were also determined. The ionic liquids ability to promote ABS is related with the hydrogen-bond basicity of the composing anion – the lower it is the higher the ability of the ionic fluid to undergo liquid–liquid demixing. Moreover, similar patterns on the ionic liquids sequence were observed with the different phosphate salts. The phosphate anion charge plays a determinant role in the formation of ABS. The two-phase formation aptitude (with a similar ionic liquid) decreases in the rank: K3PO4 > K2HPO4 > K2HPO4 + KH2PO4 > KH2PO4. Yet, besides the charge of the phosphate anion, the pH and ionic strength of the aqueous media also influence the phase separation ability.

► Ternary phase diagrams for imidazolium-based ionic liquids, orthophosphate salts, and water were determined.
► The effect of the ionic liquid anion on the two-phase formation was evaluated.
► The influence of the phosphate salt on the phase splitting was analysed.
► Trends for the ionic liquids to undergo liquid−liquid demixing are presented.
► The pH and ionic strength effects on phase separation are discussed.