Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Methyl-3-octylimidazolium Hexafluorophosphate and 1-Methyl-3-octylimidazolium Tetrafluoroborate

Abstract

The viscosities of the ionic liquids 1-methyl-3-octylimidazolium hexafluorophosphate, [OMIM]PF6, and 1-methyl-3-octylimidazolium tetrafluoroborate [OMIM]BF4, have been measured between (0 and 80) °C and at pressures to 176 MPa ([OMIM]PF6) and 224 MPa ([OMIM]BF4) with a falling-body viscometer. The overall uncertainty is estimated at ± (2 to 3) %. Modified Litovitz and Vogel−Fulcher−Tammann equations are used to represent the temperature and pressure dependence. The T0 parameter for the latter is found to be consistent with Angell's relationship linking T0 and the strength parameter D with the glass temperature Tg. Densities between (0 and 90) °C at atmospheric pressure are also reported for both substances. These have an overall uncertainty estimated at ± 0.00005 g·cm-3. The in-built viscosity correction for the Anton-Paar DMA5000 densimeter was confirmed using high-viscosity reference standards to an upper limit of 16 Pa·s.