Temperature and Pressure Dependence of the Viscosity of the Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate

Abstract

New measurements have been made for the viscosity of the room-temperature ionic liquid 1-butyl- 3-methylimidazolium hexafluorophosphate ([BMIM]PF6) between (0 and 80) °C with a falling-body viscometer. High-pressure measurements were made at (25, 35, 50, 60, and 70) °C. The overall uncertainty is estimated at ± (2 to 3) %. Atmospheric pressure densities obtained with a vibrating tube densimeter are also reported for temperatures between (0 and 90) °C with an overall uncertainty estimated at ± 0.00005 g·cm-3. The viscosity behavior is qualitatively different from that of molecular liquids, with isotherms being best fitted as functions of the applied pressure rather than as functions of the molar volume. Modified Litovitz and Vogel−Fulcher−Tammann equations have been used to incorporate both the temperature and pressure dependence. Interestingly, the T0 parameter of the VFT equation appears to be independent of pressure within the state point range of the data, but the Angell strength parameter increases with increasing pressure.