High-temperature and high-pressure (p, ρ, T) measurements and derived thermodynamic properties of 1-octyl-3-methylimidazolium hexafluorophosphate

Abstract

Densities of ionic liquid (IL) 1-octyl-3-methylimidazolium hexaflu­oro­phosphate [OMIM][PF6] at high temperatures and high pressures were mea­sured. The measurements were made along 10 isotherms over a temperature range T = 278.15 to 413.15 K, at pressures up to 140 MPa by means of an Anton Paar DMA HPM vib­ration tube densimeter (VTD). The combined exp­anded relative uncertainties of the density, pressure and temperature measure­ments at the 95 % confidence level with a coverage factor of k = 2 are esti­mated to be 0.03 to 0.08 % (depending on temperature and pressure ranges), 0.1 %, and 0.015 K, respectively. We have critically assessed all of the rep­orted high-pressure densities for [OMIM][PF6], together with the presented results, in order to carefully select primary data for development of a reference wide-ranging equation of state. Values of ρ–T isobars curvatures, (¶2r/¶T2)п, were estimated using the present high-pressure ρ‑T measurements and they were pretty low (0.78´10-7 to 1.50·´10-7 m3 kg-1 K-1), indicat­ing that the heat capacity of [OMIM][PF6] very weakly depends on pressure, since (¶CP/¶P)T ≈ ≈ (¶2r/¶T2)п. Density data were fitted to the modified Tammann–Tait equation and the multiparametric polynomial-type equation of state (EOS) for the IL was developed using the measured high-pressure and high-temperature (p, ρ, T) data. This EOS, together with our previous measured heat capacity data at atmospheric pressure, was used to calculate high-pressure and high-tempe­ra­ture derived thermodynamic properties, such as isothermal compressibility, isentropic compressibility, isobaric thermal expansion coefficient, heat capa­cities, etc.