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The speed of sound in p-xylene has been measured at temperatures ranging from (306 to 447) K and at pressures from just above saturation to 66 MPa. Measurements were performed using a new double-path pulse-echo instrument, fabricated from Invar 36, that was designed for ease of assembly and calibration as well as robust operation. The cell’s path length was calibrated with water at a single state point against the IAPWS-95 equation of state, with path length corrections for temperature and pressure calculated using material-property data. Validation measurements on water over the range of experimental conditions investigated resulted in deviations from IAPWS-95 smaller than the equation’s relative uncertainty of 0.1 %. The expanded relative uncertainty of the measurements over the reported ranges of pressure and temperature varied from (0.023 to 0.104) % at 95 % confidence. The measured data for p-xylene were compared with the Helmholtz equation of state (EOS) of Zhou et al., which is stated to have a relative uncertainty in sound-speed of 0.3 % in the liquid region. Relative deviations between experiment and the EOS of up to 1 % were observed, especially at high temperatures and low pressures, indicating that the current Helmholtz model should be revised using the new experimental data. Additionally, density, isobaric specific heat capacity, and other thermodynamic properties of p-xylene were derived from the speed-of-sound data by thermodynamic integration; these results expand upon the available literature data and are generally in good agreement with the current Helmholtz EOS. The relative expanded uncertainties for liquid density and isobaric specific heat capacity in this work are estimated to be 0.2 % and 1 %, respectively, equivalent to the uncertainty of the EOS.

The series of N-alkylpyridinium bis(trifluoromethylsulfonyl)imide, [Cnpy][NTf2], where n = 2, 3, 4, 6, and 8 have been synthesized and the effect of alkyl chain length in cation on their thermophysical properties have been studied and compared with those observed in analogous imidazolium-based ILs. To drive this comparison, the speed of sound, density, isobaric heat capacity, electric conductivity, TGA for all [Cnpy][NTf2], as well as, refractive index of [Cnpy][NTf2] (n = 3, 6) and surface tension of [Cnpy][NTf2] (n = 2, 4, 6, 8) were determined as a function of temperature at atmospheric pressure. From these experimental data, the isentropic compressibility, isobaric thermal expansion, isothermal compressibility, isochoric heat capacity, internal pressure, as well as, the surface entropy and surface enthalpy were calculated. The electric conductivity together with the literature viscosity data are used to obtain the Walden plot for the investigated ionic liquids. The minimum of the speed of sound dependence on the alkyl chain length in the cation is observed for [C6py][NTf2]. The isentropic compressibility and the isothermal compressibility of [Cnpy][NTf2] increase with increasing alkyl chain length in the cation and the slope of this dependence is changed around n = 6. These effects were more pronounced than that for imidazolium-based ionic liquids with [NTf2]− anion. Finally, COSMOthermX software was used to further evaluate its predictive capability by comparing calculated density and isobaric heat capacity as a function of the IL structure and temperature with experimental data for the [Cnpy][NTf2] (n = 2, 3, 4, 6, 8) series. This analysis shows a very good agreement in the case of the Cp with an average absolute relative deviation close to 1.4% while a deviation close to 0.72% is observed in the case of the density of the [Cnpy][NTf2].