Effects of Methylating Imidazolium-Based Ionic Liquids on Viscosity: New Insights from the Compensated Arrhenius Formalism.

Methylation of the C(2) carbon on imidazolium-based room temperature ionic liquids (RTILs) causes an unexpected increase in viscosity when paired with the anion bis(trifluoromethylsulfonamide) [Tf 2 N] - , but the viscosity decreases when the methylated imidazolium is paired with a tetracyanoborate [B(CN) 4 ] - anion. This paper investigates these different observations in viscosity using the compensated Arrhenius formalism (CAF) for fluidity (inverse viscosity), which assumes fluidity to be a thermally activated process. CAF activation energies are determined for imidazolium [Tf 2 N] - and methylated imidazolium [Tf 2 N] - and compared to imidazolium [B(CN) 4 ] - and methylated imidazolium [B(CN) 4 ] - . The results show that the activation energy increases with methylation for [Tf 2 N] - , but it decreases with methylation for [B(CN) 4 ] - . The CAF results also yield information concerning the entropy of activation, which are compared for the two systems.