Ultrasound absorption spectra of four 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide were determined as a function of the alkyl chain length on the cation from 1-propyl-to 1-hexyl-from 293.15 to 323.15 K at ambient pressure. Herein, the ultrasound absorption measurements were carried out using a standard pulse technique within a frequency range from 10 to 300 MHz. Additionally the speed of sound, density and viscosity have been measured. The presence of strong dissipative processes during the ultrasound wave propagation was found experimentally, i.e. relaxation processes in the megahertz range were observed for all compounds over the whole temperature range. The relaxation spectra (both relaxation amplitude and relaxation frequency) were shown to be dependent on the alkyl side chain length of the 1-alkyl-3-methylimidazolium ring. In most cases, a single Debye model described the absorption spectra very well. However, a comparison of the determined spectra with the spectra of a few other imidazolium-based ionic liquids reported in the literature (in part recalculated in this work) shows that the complexity of the spectra increases rapidly with the elongation of the alkyl chain length on the cation.This complexity indicates that both the volume viscosity and the shear viscosity are involved in relaxation processes even in relatively low frequency ranges. As a consequence, the sound velocity dispersion is present at relatively low megahertz frequencies.

The speeds of sound in heptan-1-ol, octan-1-ol, and nonan-1-ol at pressures up to 101 MPa and in decan-1-ol at pressures up to 76 MPa have been measured within the temperature range of (293 to 318) K. The densities have been measured in the same temperature range under atmospheric pressure. The densities, isobaric heat capacities, isentropic and isothermal compressibilities, isobaric thermal expansions, and internal pressures as functions of temperature and pressure have been calculated using the experimental results and the literature isobaric heat capacities for the atmospheric pressure. A modified method, based on the suggestion of Davis and Gordon (J. Chem. Phys. 1967, 46, 2650−2660), has been applied. The effect of pressure and temperature on the isothermal compressibility, isobaric thermal expansion, isobaric heat capacity, and internal pressure is discussed.

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The isobaric and isochoric heat capacities of seven 1-alkyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imides, two 1-alkyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imides, and two bis­(1-alkyl-3-methylimidazolium) tetrathiocyanatocobaltates were determined at atmospheric pressure in the temperature range from 293.15 to 323.15 K. The isobaric heat capacities were determined by means of differential scanning calorimetry, whereas isochoric heat capacities were determined along with isothermal compressibilities indirectly by means of the acoustic method from the speed of sound and density measurements. Based on the experimental data, as expected, the isobaric heat capacity increases linearly with increasing alkyl chain length in the cation of 1-alkyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imides and no odd and even carbon number effect is observed. After critical comparison of the obtained data with the available literature data, the most reliable values are recommended. It has been also shown that, although the COSMOthermX calculations underestimated the isobaric heat capacity values whatever the temperature and the ionic liquid structure, the approach used during this work may be applied to estimate physical properties of non-single-charged ions as well. Additionally, based on the speeds of sound the thermal conductivities were calculated using a modified Bridgman relation.

Ionic liquids are viewed as green media for many engineering applications and exhibit exceptional properties, including negligible vapor pressure, null flammability, wide liquid range, and high thermal and chemical stabilities. We present new thermophysical properties of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides ([C n C 1 pyr][NTf 2 ] with n = 3, 4) for future application them as heat-transfer media. The speed of sound was measured at pressures up to 100 MPa and at temperatures from 293 K to 318 K. The pρT, pC p T data, and derived thermophysical properties were determined using the acoustic method. TGA of [C n C 1 pyr][NTf 2 ] and cytotoxicity of [C n C 1 pyr][NTf 2 ] and their imidazolium counterparts ([C n C 1 im][NTf 2 ]) are investigated. The physicochemical properties of [C n C 1 pyr][NTf 2 ] are compared with those of [C n C 1 im][NTf 2 ] and commercial heat-transfer fluids (Therminol VP-1, Therminol 66, Marlotherm SH). [C 3 C 1 pyr][NTf 2 ] and [C 4 C 1 pyr][NTf 2 ] have a wide liquid range of ∼480 K and high decomposition onset temperatures of 771 and 776 K, respectively. [C n C 1 pyr][NTf 2 ] exhibit high energy storage density of ∼1.98 MJ m −3 K −1 , which is slightly dependent on temperature and pressure. The thermal conductivity of [C n C 1 pyr][NTf 2 ] is comparable to that of commercial heat-transfer fluids. [C n C 1 pyr][NTf 2 ] have lower toxicity for normal human dermal fibroblast cells than [C n C 1 im][NTf 2 ]. Thus, [C n C 1 pyr][NTf 2 ] are promising heat-transfer fluid candidates.