Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Hexyl-3-methylimidazolium Tetrafluoroborate and 1-Ethyl- and 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amides

Abstract

The viscosities of the ionic liquids 1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM]BF4, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [EMIM][Tf2N], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [HMIM][Tf2N], have been measured at pressures up to 400 MPa and between 278 and 363 K for [HMIM]BF4, 960 mPa·s maximum viscosity, between 273 and 363 K for [HMIM][Tf2N], 731 mPa·s maximum viscosity, and between 268 and 363 K for [EMIM][Tf2N], 478 mPa·s maximum viscosity, with a falling-body viscometer. The expanded relative uncertainty is estimated at 100·Ur(η) = 2.5. Modified Litovitz and Vogel–Fulcher–Tammann (VFT) equations are used to represent the temperature and pressure dependence. Comparison is made with other 1-alkyl-3-methylimidazolium ionic liquids measured in this laboratory using density scaling. The values of the scaling factor, γ < 3, are consistent with an interplay between the repulsive van der Waals interactions between the ions, which yield high γ, and the Coulombic attractions of oppositely charged ions that act to decrease it. The Angell equation relating the strength factor D, the VFT parameter T0, and the glass temperature Tg is confirmed and extended to estimate the pressure dependence of Tg for 1-alkyl-3-methylimidazolium ionic liquids. Densities between 273 and 363 K at atmospheric pressure are also reported for [HMIM]BF4.