The current study focuses on 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4], and 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. The objective is to study the influence of pressure as well as that of the anion on several properties of this type of ionic liquid. The speed of sound and densities in pure ionic liquids (ILs) as a function of temperature and pressure have been determined. Several other thermodynamic properties such as compressibilities, expansivities, and heat capacities have been obtained. To the best of our knowledge, this research comprises both the first speed of sound data and the first evaluation of heat capacities at high pressures for ILs. Speed of sound measurements have been carried out in broad ranges of temperature (283 < T/K < 323) and pressure (0.1 < p/MPa < 150), sometimes inside the metastable liquid region using a nonintrusive microcell. The T−P melting line of [bmim][PF6] has also been determined by an acoustic method. Density measurements have been performed for broad ranges of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 60) using a vibrating tube densimeter. The pressure dependence of the heat capacities, which is generally mild, is found to be highly dependent on the curvature of the temperature dependence of the density.
De Azevedo, R. G., Esperança, J. M. S. S., Najdanovic-Visak, V., Visak, Z. P., Guedes, H. J. R., Ponte, M. N. D., & Rebelo, L. P. N. (2005). Thermophysical and thermodynamic properties of 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazoliuin hexafluorophosphate over an extended pressure range. Journal of Chemical & Engineering Data, 50(3), 997-1008. https://doi.org/10.1021/je049534w