Comparison of Electronic and Physicochemical Properties between Imidazolium-Based and Pyridinium-Based Ionic Liquids

To compare 1-butyl-3-methylimidazolium ([BMIM]⁺)- and 1-butyl-3-methylpyridinium ([BMPy]⁺)-based ionic liquids (ILs) and investigate the influence of intramolecular and intermolecular interactions on physicochemical properties, a systematic study was performed on the electronic structures and physicochemical properties of [BMIM]⁺ tetrafluoroborate ([BMIM][BF₄]), [BMIM]⁺ hexafluorophosphate ([BMIM][PF₆]), [BMIM]⁺ hydrogen sulfate ([BMIM][HSO₄]), [BMIM]⁺ methylsulfate ([BMIM][MSO₄]), [BMIM]⁺ ethylsulfate ([BMIM][ESO₄]), [BMPy]⁺ tetrafluoroborate ([BMPy][BF₄]), [BMPy]⁺ hexafluorophosphate ([BMPy][PF₆]), [BMPy]⁺ hydrogen sulfate ([BMPy][HSO₄]), [BMPy]⁺ methylsulfate ([BMPy][MSO₄]), and [BMPy]⁺ ethylsulfate ([BMPy][ESO₄]) using density functional theory and molecular dynamics simulation. The results reveal that aggregation behavior exists in [HSO₄]^-- and [ESO₄]^--based ILs, and the differences between their densities and self-diffusion coefficients are smaller when there is an aggregation effect in ILs. A dimer is formed by two strong hydrogen bonds between two [HSO₄]^- anions in [HSO₄]-based ILs, and the existence of hydrogen bonds in ILs increases density and decreases the self-diffusion coefficient. The intermolecular interaction strength of [BMIM]⁺-based ILs is stronger than that of [BMPy]⁺-based ILs.