Isobaric and Isochoric Heat Capacities as Well as Isentropic and Isothermal Compressibilities of Di- and Trisubstituted Imidazolium-Based Ionic Liquids as a Function of Temperature

The isobaric and isochoric heat capacities of six disubstituted imidazolium-based ionic liquids with different anions and two trisubstituted imidazolium-based bis­(trifluoromethylsulfonyl)­imides were determined at atmospheric pressure in the temperature range from 293.15 to 323.15 K. The isobaric heat capacities were determined using differential scanning calorimetry. The isochoric heat capacities were determined indirectly by means of the acoustic method from the speed of sound and density measurements. Also, other connected with the speed of sound quantities such as isentropic and isothermal compressibilities as well as internal pressures were determined. The highest compressibilities show both trisubstituted imidazolium-based bis­(trifluoromethylsulfonyl)­imides whereas 1-ethyl-3-methylimidazolium thiocyanate shows the lowest compressibility. The critical comparison of the isobaric heat capacity data with the available literature data makes possible a recommendation of the most reliable heat capacity values. Analyzed predictive capability of two heat capacity models based on the group contribution method is poor in the case of the [C­(CN)₃]⁻ anion. Additionally, based on the speeds of sound, the thermal conductivities were calculated using a modified Bridgman relation and compared with values estimated by a group contribution method. Here, the worst results are obtained in the case of [SCN]⁻ anion