Ionic Liquid Based Aqueous Biphasic Systems with Controlled pH: The Ionic Liquid Cation Effect

This work addresses the evaluation of the ability of ionic liquid cations on the formation of aqueous biphasic systems (ABS's), with K2HPO4 or a mixture of inorganic salts, K2HPO4/KH2PO4, aiming at controlling the pH values of the coexisting aqueous phases. Using chloride-based ionic liquids, the effects of the cation core, the length of the alkyl side chain, and the positional isomerism on ABS formation ability were investigated. All binodal curves were determined by the cloud-point titration method at 298 K. From the obtained phase diagrams it is shown that the biphasic area increases with the cation side chain length, from ethyl to hexyl chains, although for longer chains, an inversion on the binodal curves sequence appears due to the self-aggregation of the longer chain ionic liquids in aqueous solutions. The influence of the cation core and the positional isomerism of the ionic liquids on their ability to form ABS closely correlates with the ionic fluid affinity for water.