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 knowledge of thermodynamic high-pressure speed of sound in ionic liquids (ILs) is a crucial way either to study the nature of the molecular interactions, structure and packing effects or to determine other key thermodynamic properties of ILs essential for their applications in any chemical and industrial processes. Herein, we report the speed of sound as a function temperature at pressures up to 101 MPa in four ultrapure ILs: 1-propyl-3-methylimidazolium bis-[(trifluoromethyl)sulfonyl]imide, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, 1-pentyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, and 1-hexyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, taking into consideration their relaxation behavior. Additionally, to further improve the reliability of the speed of sound results, the density, isentropic compressibility, and isobaric heat capacity as a function of temperature and pressure are calculated using an acoustic method.

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The components of biodiesel derived from coconut oil or babassu oil such as ethyl caprylate and ethyl caprate are studied. The experimental speeds of sound in ethyl caprylate were reported at p = (0.1 to 101.32) MPa and at T = (292.90 to 318.32) K and in ethyl caprate at p = (0.1 to 101.32) MPa and at T = (292.90 to 318.28) K together with the experimental densities at 0.1 MPa for ethyl caprylate at T = (278.15 to 358.15) K and for ethyl caprate at T = (283.15 to 353.14) K as well as heat capacities were reported at 0.1 MPa for ethyl caprylate at T = (285.01 to 343.78) K and for ethyl caprate at T = (286.16 to 341.11) K. The density, adiabatic and isothermal compressibilities, heat capacity, isobaric thermal expansion, and internal pressure at elevated pressures and temperatures were calculated from the experimental results in the way described by Davis and Gordon (J. Chem. Phys. 1967Phys. , 46, 2650Phys. −2660 with the aim of studying the effect of changing of the carbon chain length from ethyl caprylate to ethyl caprate on pressure and temperature dependence of above-mentioned properties. Moreover, densities of ethyl caprylate and ethyl caprate were compared with densities of ethyl laurate and ethyl myristate to improve intersection of their isotherms.

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