High-pressure speed of sound and related thermodynamic properties of N-alkylpyridinium bis(trifluoromethylsulfonyl)imides

The speed of sound was measured in three N-alkylpyridinium bis(trifluoro-methylsulfonyl)imides containing cations with ethyl, butyl and hexyl chains at pressures up to 101 MPa over a temperature range from 293.15 to 323.15 K. This paper is the first reported direct measurement of the speed of sound in N-hexylpyridinium bis(trifluoro-methylsulfonyl)imide as a function of pressure and temperature at two frequencies (1.95 and 6.25 MHz), demonstrating the absence of ultrasonic velocity dispersion under experimental conditions. An acoustic method was used to determine the high-pressure density, isentropic compressibility, isothermal compressibility, isobaric thermal expansion, isobaric and isochoric heat capacities, and internal pressure. For the studied homologous series, the dependence of the speed of sound on the alkyl chain length of the pyridinium cation exhibits a minimum at both atmospheric and high pressures. With increasing pressure, the minimum becomes shallower and is shifted towards homologues with shorter carbon chains in the cation.