Abstract
For 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and their cationic C2-methylated isomers, that is, [CnMIm]NTf2 and [Cn-1DMIm]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, [Cn-1DMIm]NTf 2 was found to possess less unoccupied (free) volume than [C nMIm]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 [Cn-1DMIm]NTf 2 relative to [CnMIm]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.
| Original language | English |
|---|---|
| Pages (from-to) | 2712-2718 |
| Number of pages | 7 |
| Journal | Journal of Physical Chemistry B |
| Volume | 118 |
| Issue number | 10 |
| DOIs | |
| Publication status | Published - Mar 13 2014 |
| Externally published | Yes |
ASJC Scopus Subject Areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry