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Densities, isobaric thermal expansivities, and excess enthalpies were determined for the binary systems formed by ionic liquids with a common anion, 1-hexyl-3-methylimidazolium tetrafluoroborate + 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-hexyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium tetrafluoroborate, and by those with a common cation, 1-butyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium methylsulfate and 1-butyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium hexafluorophosphate. Volumetric properties were obtained by means of a vibrating tube densimeter, whereas Calvet microcalorimetry was used to determine the excess enthalpies. Due to the high viscosity of the samples, nonstandard experimental techniques had to be used to obtain the results. The experimental data are interpreted on the basis of the chemical structure of the ionic liquids.

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Isobaric thermal expansivity α p as a function of temperature and pressure was determined by means of a calorimetric method for a set of ionic liquids with a common cation within the temperature and pressure intervals (278.15 to 348.15) K and (5 to 50) MPa. The selected common cation was 1-butyl-3-methylimidazolium, whereas the chosen anions were tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, hexafluoroantimonate, trifluoromethanesulfonate, and methylsulfate. The results are compared with available literature data, obtained from density measurements. An unusual, negative dependence of α p against temperature was found for all liquids over the whole temperature and pressure interval. The results are discussed in terms of the differences in the chemical nature of the anions.

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