title: Free volume model for the unexpected effect of C2-methylation on the properties of imidazolium ionic liquids.
abstract: 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.
authors.affiliation: School of Chemical and Biomedical Engineering, Nanyang Technological University , 62 Nanyang Drive, Singapore 637459.
authors.forename: Zheng Jian
authors.initials: ZJ
authors.lastname: Chen
authors.forename: Jong-Min
authors.initials: JM
authors.lastname: Lee
mesh_terms: D000838:Anions / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D004553:Electric Conductivity; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D007536:Isomerism; D008745:Methylation; D008956:Models, Chemical; D012031:Refractometry; D013500:Surface Tension; D013696:Temperature; D014783:Viscosity
chemical_list: D000838:Anions; D002412:Cations; D007093:Imidazoles; D052578:Ionic Liquids; D002244:Carbon