Density, Viscosity, and Refractive Index of 1-Alkyl-2-methylpyrazolium Methyl Sulfate and Bis(trifluoromethylsulfonyl)imide Ionic Liquids

In this study, six pure 1-alkyl-2-methylpyrazolium-based ionic liquids (ILs) with methyl sulfate and bis(trifluoromethylsulfonyl)imide anions ([Cnmpz][MeSO4] and [Cnmpz][NTf2], n = 2, 3, and 4) were prepared and characterized. The density, viscosity, and refractive index of these pure ILs were measured from 298.15 to 353.15 K at 101.3 kPa, which fitted well with the corresponding empirical equations. Moreover, the values of the isobaric thermal expansion coefficient were also obtained. The results of experiments and density functional theory calculations indicate that the adjacent positions of two nitrogen atoms in pyrazolium cations lead to the close packing of ions, strong cation–anion interaction, and high molar polarizability per unit volume. The effects of temperature and N-alkyl chain length on the density, viscosity, and refractive index of these ILs are similar to those for imidazolium-based salts. Compared with [Cnmpz][MeSO4], [Cnmpz][NTf2] exhibits a higher density, lower viscosity, and lower refractive index under the same conditions. Moreover, the correlation between density and refractive index was studied, and a new empirical equation was proposed to express this correlation at different temperatures. It is expected that this work will enable deeper insights into the properties, structure, and structure–property relationship of pure pyrazolium-based ILs.