LCST-Type Phase Behavior of Aqueous Biphasic Systems Composed of Phosphonium-Based Ionic Liquids and Potassium Phosphate

Although aqueous biphasic systems (ABS) composed of phosphonium-based ionic liquids (ILs) have demonstrated superior performance as viable media for biocompatible extraction processes, the formation of ABS with tetrabutylphosphonium trifluoroacetate ([P₄₄₄₄]­CF₃COO) and tributyloctylphosphonium bromide ([P₄₄₄₈]­Br) were seldom investigated. To evaluate the hydrophilicity of [P₄₄₄₄]­CF₃COO and [P₄₄₄₈]­Br, the solubility curves for the mixture of the ILs and water were determined. The influence of temperature on the phase behavior for the [P₄₄₄₈]­Br + K₃PO₄ + H₂O and [P₄₄₄₄] CF₃COO + K₃PO₄ + H₂O systems were also obtained. Results show that both [P₄₄₄₈]Br + H₂O and [P₄₄₄₄]­CF₃COO + H₂O binary systems undergo lower critical solution temperature-type phase transition, and the ABS composed of [P₄₄₄₄]­CF₃COO and [P₄₄₄₈]Br can be formed with K₃PO₄ at a wide range of temperatures. Moreover, the ABS are highly temperature dependent, and biphasic region expands with an increase in temperature. On the other hand, the capability of the ILs to induce ABS follows the tread [P₄₄₄₈]Br > [P₄₄₄₄]­CF₃COO. This suggests that the lower cation hydrophilicity of ILs results in greater biphasic area in the ABS formed by phosphonium-based ILs