Density, refractive index and volumetric properties of water-ionic liquid binary systems with imidazolium-based cations and tetrafluoroborate, triflate and octylsulphate anions at T=293 to 343 K and p=0.1 MPa

Abstract

The density and refractive index of ionic liquids (ILs) + water binary mixtures were determined as a function of temperature (from 293.15 to 343.15 K) at atmospheric pressure over the whole composition range in which the mixtures were miscible. To carry out a systematic study, all the ILs selected are imidazolium-based ILs with a different number of carbons in the alkyl chain of the cation and also different anions (tetrafluoroborate, triflate and octylsulphate). Specifically, the studied ILs were 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4], 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4], 1-hexyl-3-methylimidazolium tetrafluoroborate [hmim][BF4], 1-methyl-3-octylimidazolium tetrafluoroborate [omim][BF4], 1-ethyl-3-methylimidazolium triflate [emim][TfO], 1-butyl-3-methylimidazolium triflate [bmim][TfO] and 1-butyl-3-methylimidazolium octylsulphate [bmim][OcSO4]. The excess molar volumes and the deviation in the molar refraction of the binary mixtures were calculated for a better understanding of the interactions that take place between the components and successfully correlated by the Redlich-Kister empirical correlations. Bahe-Varela model, which has a more physical meaning, was also used to successfully correlate the excess molar volume values. Volumetric properties, such as apparent molar volumes, partial molar volumes, isobaric thermal expansion coefficients, partial molar volumes at infinite dilution and excess partial molar volumes at infinite dilution were also calculated in order to obtain information about the influence of composition and temperature on the thermodynamic behavior of the selected ILs and water in the mixture. The results are discussed in order to understand the formation of hydrogen bonds between components of the mixture and the possible packing effects that take place in the mixing process. Density and refractive index experimental data were correlated by the Lorentz-Lorenz, Wiener, Dale-Gladstone and Eykman equations to determine the relationship between both parameters, and a good agreement between the experimental and calculated refractive index values was obtained.