Structure–Property Relationship for 1‑Isopropyl-3-methylimidazolium- and 1-tert-Butyl-3-methylimidazolium-Based Ionic Liquids: Thermal Properties, Densities, Viscosities, and Quantum Chemical Calculations

We systematically analyzed the thermal phase behaviors (phase change and thermal decomposition temperatures), densities, and viscosities of 1-isopropyl-3-methylimidazolium ([iC3mim]+)- and 1-tert-butyl-3-methylimidazolium ([tC4mim]+)-based ionic liquids (ILs) with [I]−, [PF6]−, bis­(fluorosulfonyl)­imide, bis­(trifluoromethylsulfonyl)­imide, and bis­(pentafluoroethylsulfonyl)­imide counterions. The results were compared to those for ILs based on the corresponding linear alkyl imidazolium cations (1-methyl-3-propylimidazolium and 1-butyl-3-methylimidazolium). Our results revealed that the melting points of the branched ILs are always higher than those of the linear ILs. The densities of [tC4mim]­X are higher than those of the linear ILs, which is somewhat counterintuitive when considering previously reported molecular liquid data. Using the van der Waals volume data estimated in this work, the fractional free volumes of the ILs were estimated. This revealed that the linear and branched ILs have largely similar void space values in the liquid state. The viscosities of [tC4mim]­X are higher than those of the linear ILs, while those of [iC3mim]­X are comparable. To better understand the structure–property relationship for the branched ILs, density functional theory calculations were performed for the ions in the gas phase. The structure–property relationship of the branched ILs is discussed in terms of bulkiness, conformational entropy, structural symmetry, and cation–anion interaction changes.