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, (∂2ρ/∂2)n, were estimated using the present high-pressure ρ-T measurements and they were pretty low (0.78×10-7to 1.50×10-7m3kg-1K-1), indicating that the heat capacity of [OMIM][PF6] very weakly depends on pressure, since (∂CP/∂P)T≈≈ (∂2∂ρT2)n. 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.