Thermodynamic study of the aqueous two-phase systems of 1-butyl-3-methylimidazolium tetrafluoroborate and sodium dodecylbenzenesulfonate

A ternary phase diagram of the system of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4), sodium dodecylbenzenesulfonate (SDBS) and water using ocular estimate and turbidity method was drawn based on experimental data at 298.2 K. We got a narrow aqueous two-phase region near the side of water-ionic liquid (IL). A series of samples with different compositions around the phase diagram were chosen. The enthalpy change of these systems after injecting triply distilled water into these samples has been studied by using isothermal titration microcalorimetry (ITC). It is found that with increasing the water content there are weak endothermic and exothermic processes appearing, separately, which matches perfectly with the location of the formation and disappearance of the aqueous two-phase in the ternary phase diagram. This suggests the ITC is a feasible and effective method for the thermodynamic study of aqueous two-phase systems (ATPS) and was first reported. The experimental results show that the formation of the ATPS is endothermic (ΔH > 0), but the process of disappearing is exothermic (ΔH < 0). Due to the formation of aqueous two-phase is a thermodynamic spontaneous process under the isothermal constant pressure, we can judge that the formation of this ATPS is an entropy driven course and the vanishing is a synergy driven process by entropy and enthalpy.