Acoustic and volumetric properties of triflate-based ionic liquids at high pressures.

We report new experimental data of the ultrasonic measurements in three ionic liquids with triflate anion: 1-ethylpyridinium triflate, 1-butyl-3-methylimidazolium triflate, diethylmethylammonium triflate. The speed of sound was determined in the temperature range of (297 – 374) K and the pressure range of (0. 1 – 196. 2) MPa. In addition, the respective densities in this range were found using the acoustic route. For the first two substances, the effect of pressure-induced solidification is detected. It is shown that the high-pressure data on the sound velocity can be accurately predicted by applying the pressure fluctuation theory-based model using parameters, which requires only the data determined at the ambient atmospheric pressure. In turn, the density can be predicted using the Fluctuation Theory-based Equation of State (FT-EoS), which includes the isothermal nonlinearity parameter, which is argued as related to the frequency of interparticle oscillations. This interdependence is confirmed by the comparative analysis with the respective peak in the low-frequency Raman–Kerr spectrum. • The speed of sound is measured and the density is determined to 196. 1 MPa. • Pressure-induced freezing of ionic liquids at macroscopic volumes is detected. • Interplay of thermodynamics and microscopic interparticle oscillations is discussed. [ABSTRACT FROM AUTHOR]