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

Abstract

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.

Solvatochromic Study on Binary Solvent Mixtures with Ionic Liquids

“…A very similar behaviour of the solvatochromic shift of Reichardt's dye was obtained in case of [C 4 MIm][PF 6 ] addition to a water and ethanol mixture in the range of miscibility with the IL by Fletcher and Pandey [23]. In other work on binary IL/H 2 O mixtures [24], this was explained by salt dissolution at first and then hydration of ions at higher concentrations of IL. On dissolution of ILs into H 2 O at infinite dilution, IL ion pairs break away from their pure environment and settle in the water environment as hydrated ions.…”

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

“…1). Moreover, it has to be noticed that bulk solution behavior for water molecules around ions in both ILs (g i j (r) ≈ 1) can be observed at r ≥ The arrangement of the IL species can be studied throughG 22 . The values for EMIM/BF4 and BMIM/BF4 obtained by the direct and the inverse KB approach coincide and are largely negative, lying between ≈ −78 cm 3 /mol and ≈ −81 cm 3 /mol (EMIM/BF4) and ≈ −95 cm 3 /mol to ≈ −98 cm 3 /mol for BMIM/BF4.…”

“…As a prominent example, the influence of water significantly diminishes the broad electrochemical window known for pure ILs, 19 . In order to understand the profound hygroscopicity of ILs, several experiments and simulations were performed 9,13,17,[22][23][24][25][26][27] . An extensive discussion of the resulting effects can be found in recent reviews 16,28 .…”

Aggregation of Imidazolium Ionic Liquids in Molecular Liquids Studied by Small-Angle Neutron Scattering and NMR

“…[6][7][8][9][10] Based on thermodynamic parameters, such as excess chemical potentials and excess partial molar enthalpies and entropies, aggregations of 1-butyl-3-methlyimidazolium tetrafluoroborate (BMI + BF4 -) and BMI + iodide (BMI + I -) in aqueous solutions have been discussed. 6 This study revealed that the ILs form clusters in the range of IL mole fraction xIL > ~0. 5.…”