Temperature Dependence and Structural Influence on the Thermophysical Properties of Eleven Commercial Ionic Liquids

Density, speed of sound, refractive index, and viscosity of the commercial ionic liquids 1-ethyl-3-methylpyridinium bis­(trifluoromethylsulfonyl)­imide, [EMpy]­[NTf2], 1-propyl-3-methylpyridinium bis­(trifluoromethylsulfonyl)­imide, [PMpy]­[NTf2], 1-butyl-3-methylpyridinium bis­(trifluoromethylsulfonyl)­imide, [BMpy]­[NTf2], 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide, [EMim]­[NTf2], 1-hexyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide, [HMim]­[NTf2], 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide, [BMpyr]­[NTf2], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMpyr]­[TFO], 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMim]­[TFO], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, [HMim]­[TFO], 1-butyl-3-methylimidazolium dicyanamide, [BMim]­[N­(CN)2], and 1-hexyl-3-methylimidazolium dicyanamide, [HMim]­[N­(CN)2] were measured as a function of temperature at atmospheric pressure. The density, speed of sound, and refractive index data were properly fitted to a linear equation, while viscosity data were fitted to Arrhenius-like law, Vogel–Fulcher–Tamman (VFT), a modified VFT, Litovitz, and fluidity equations. Besides, from the experimental density values, the thermal expansion coefficient, α, was calculated. Moreover, the experimental data were used to analyze the effect of temperature, the role of the alky chain length of the cation, and the influence of the nature of the ions (cation and anion) on the physical properties. Finally, an exhaustive comparison with available literature data of the studied ionic liquids was also carried out.