Effects of ionicity and chain structure on the physicochemical properties of protic ionic liquids

The physicochemical properties of protic ionic liquids (PILs) determine their industrial applications. In this study, six PILs based on 1-vinylimidazole were synthesized, and their ionicities were determined by(1)H NMR method. The thermodynamic properties, viscosities, densities, and conductivities of these PILs were correlated with the ionicities and the chain lengths of the anions. The ionicity was found to depend on the acidity of the alkyl carboxylic acid. Since HAc has the strongest acidity among these acid precursors, Vim center dot HAc has the highest ionicity and thus the strongest Coulombic interactions among the six PILs. The other five PILs have similar ionicities but different chain lengths in their anions. So, we can separately identify the effects of Coulombic interactions and van der Waals forces on the physicochemical properties of PILs. From these trends, the viscosity and conductivity of PILs may be tuned by varying the acidity and alkyl chain length of the precursors.