Mixing Schemes in Ionic Liquid−H2O Systems:  A Thermodynamic Study

We studied the hydration characteristics of room-temperature ionic liquids (IL). We experimentally determined the excess chemical potentials, , the excess partial molar enthalpies, , and the excess partial molar entropies in IL−H2O systems at 25 °C. The ionic liquids studied were 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and the iodide ([bmim]I). From these data, the excess (integral) molar enthalpy and entropy, and , and the IL−IL enthalpic interaction, , were calculated. Using these thermodynamic data, we deduced the mixing schemes, or the “solution structures”, of IL−H2O systems. At infinite dilution IL dissociates in H2O, but the subsequent hydration is much weaker than for NaCl. As the concentration of IL increases, [bmim]+ ions and the counteranions begin to attract each other up to a threshold mole fraction, x IL = 0.015 for [bmim]BF4 and 0.013 for [bmim]I. At still higher mole fractions, IL ions start to organize themselves, directly or in an H2O-mediated manner. Eventually for x IL > 0.5−0.6, IL molecules form clusters of their own kind, as in their pure states. We show that , a third derivative of G, provided finer details than and , second derivatives, which in turn gave more detailed information than and , first derivative quantities.