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

Abstract

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 ([C 4mim]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 {[C 4mim]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 [C 4mim]Br solutions was explained based on the structural hydration Gibbs free energy (Δ G hyd) 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 ( k S) obtained from fitting the tie-line data to a Setschenow-type equation. The mutual immiscibility for the [C 4mim]Br and potassium citrate in water was observed to promote when the citrate ions have more negative Δ G hyd or higher k S 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.