Comparison between Cycloalkyl- and n-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis(trifluoromethanesulfonyl)amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective n-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by C-13 NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents.