Free volume model for the unexpected effect of C2-methylation on the properties of imidazolium ionic liquids

For 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and their cationic C2-methylated isomers, that is, [C(n)MIm]NTf2 and [C(n-1)DMIm]NTf2 (n = 3-8), the latter always has much higher viscosities (∼1.66 times at 25 °C) than the former. This finding is unexpected and contrary to the loss of the predominant hydrogen bonding between the cation and anion, caused by the C2-methylation in the imidazolium ionic liquids. In the present study, we propose a free volume model to explain this mysterious phenomenon. For the isomeric pairs, [C(n-1)DMIm]NTf2 was found to possess less unoccupied (free) volume than [C(n)MIm]NTf2 by ∼4.5 mL mol(-1) determined by both Fürth's surface tension and molar volume methods, and the free volume difference can be quantitatively correlated with the C2-methylation effect on viscosity. Generally, the less free volume in [C(n-1)DMIm]NTf2 relative to [C(n)MIm]NTf2 reduces the number of the hole carriers for molecular transport with a resulting increase in viscosity. Besides, the free volume model can also be applied to account for the C2-methylation effect on other properties, such as conductivity, surface tension, density, and refractive index.