Effect of pH on the phase separation in the ternary aqueous system containing the hydrophilic ionic liquid 1-butyl-3-methylimidazolium bromide and the kosmotropic salt potassium citrate at T = 298.15 K

The goal of this work is to study the effect of the aqueous medium pH on the two-phase equilibrium behaviour of the aqueous {1-butyl-3-methylimidazolium bromide ([C4mim]Br) + potassium citrate} system in order to obtain further information about the salting-out effect produced by the addition of a kosmotropic salt to an aqueous solution of a hydrophilic ionic liquid (IL). For this purpose the phase diagrams and the liquid–liquid equilibrium (LLE) data of the {[C4mim]Br + potassium citrate} aqueous two-phase system (ATPS) were determined experimentally at different pH values. Four medium pH values (5.00, 6.00, 7.00 and 8.00) were assayed at T = 298.15 K. The effect of pH on the ATPS-promoting capability of citrate anions in the aqueous [C4mim]Br solutions was explained based on the structural hydration Gibbs free energy (ΔGhyd) of the citrate anions at different pH. In the investigated system, the effect of the aqueous medium pH on the phase-forming capability was also evaluated in the shape of the salting-out coefficient (kS) obtained from fitting the tie-line data to a Setschenow-type equation. The mutual immiscibility for the [C4mim]Br and potassium citrate in water was observed to promote when the citrate ions have more negative ΔGhyd or higher kS value due to an increased medium pH value. Furthermore, the modified versions of the segment-based local composition NRTL and Wilson models were examined to correlate the phase behaviour of the investigated system at different pH values.

Research highlights
► We examine pH effect on the phase behaviour of {[C4mim]Br + potassium citrate + H2O}.
► We determine the phase diagrams at pH = (5.00, 6.00, 7.00 and 8.00) and T = 298.15 K.
► The results are used to study the salting-out effect in the investigated system.
► Increasing pH will promote mutual immiscibility of ionic liquid and salt in water.
► Modified NRTL and Wilson models are used to correlate the phase behaviour of system.