Viscosity of Imidazolium-Based Ionic Liquids at Elevated Pressures: Cation and Anion Effects

The viscosity of imidazolium-based ionic liquids with four different cations and three different anions was measured to pressures of 126 MPa and at three temperatures (298.15 K, 323.15 K, and 343.15 K). The high-pressure viscosity of 1-ethyl-3-methylimidazolium ([EMIm]), 1-n-hexyl-3-methylimidazolium ([HMIm]), and 1-n-decyl-3-methylimidazolium ([DMIm]) cations with a common anion, bis(trifluoromethylsulfonyl)imide ([Tf₂N]), was measured to determine the alkyl-chain length effect of the cation. An increase in the alkyl-chain length increased the viscosity at elevated pressures. [DMIm] exhibited a larger nonlinear increase with pressure over the shorter alkyl substituents. Anion effects were investigated with [HMIm] as a common cation and anions of [Tf₂N], hexafluorophosphate ([PF₆]), and tetrafluoroborate ([BF₄]). [HMIm][PF₆], with the highest viscosity, demonstrated a very nonlinear pressure dependence even at relatively moderate pressures (to 30 MPa), similar to the results for [BMIm][PF₆]. A combined Litovitz and Tait equation was utilized to describe the viscosity of the ionic liquids with pressure and temperature and demonstrated good correlation with the experimental data.