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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In this work, we discussed and compared new acoustic (speed of sound), transport (viscosity), surface (surface tension and contact angle on stainless steel and glass), and thermophysical properties with special regard for compressibilities and isobaric thermal expansion of 1,3-diethylimidazolium ethyl sulfate ([C2C2im]­[EtSO4]) with those of 1-ethyl-3-methylimidazolium ethyl sulfate ([C2C1im]­[EtSO4]), the reference mineral, synthetic, and biodegradable oils as well as hydraulic oils for use as hydraulic fluids. The refractive indices, NMR spectra, and cytotoxicities of [C2C1im]­[EtSO4] and [C2C2im]­[EtSO4] were also investigated. [C2C1im]­[EtSO4] and [C2C2im]­[EtSO4] have low isothermal compressibility and isobaric thermal expansion values, which are weakly dependent on the pressure and temperature. The viscosity, surface tension, and contact angle of [C2C2im]­[EtSO4] are more similar to that of commercial hydraulic oils than [C2C1im]­[EtSO4]. The investigated ILs have a 20 times lower toxicity on normal fibroblast from human new-born skin than that of bis­(trifluoromethylsulfonyl)­imide-based ILs. The presented results show that [C2C2im]­[EtSO4] can potentially be used as a hydraulic fluid in the same way as [C2C1im]­[EtSO4].