Liquid-liquid equilibria of aqueous biphasic systems of ionic liquids and dipotassium hydrogen phosphate at different temperatures: Experimental study and thermodynamic modeling

The phase diagrams of the {1-butyl-3-methylimidazolium nitrate ([Bmim][NO3]) / 1-hexyl-3-methylimidazolium nitrate ([Hmim][NO3]) + dipotassium hydrogen phosphate (K2HPO4)}c aqueous biphasic systems (ABS) and the binodal curves of {1-octyl-3-methylimidazolium nitrate ([Omim][NO3]) + K2HPO4} ABS have been determined experimentally at T = (288.15, 298.15, and 308.15) K. The Merchuk equation with three dependent-temperature adjustable parameters was used for reproducing and predicting the binodal curves. The effect of the alkyl chain length of ionic liquids (ILs) was studied. It was found that the two-phase formation in the investigated ABS is decreased with decreasing the alkyl chain length of IL in the order: [Hmim][NO3] > [Omim][NO3] > [Bmim][NO3]. Moreover, the effect of temperature on the binodal curves and tie-lines was also discussed. It was shown that the biphasic region expanded with a decrease in temperature, whereas the absolute value of slope of the tie-lines slightly decreased with an increase in temperature. Finally, the liquid-liquid equilibrium (LLE) data for the studied systems are correlated by using the NRTL thermodynamic model, and good agreement was obtained between the correlation and the experimental data.