Comparative study of effect of alkyl chain length on thermophysical characteristics of five N-alkylpyridinium bis(trifluoromethylsulfonyl)imides with selected imidazolium-based ionic liquids

Abstract

The series of N-alkylpyridinium bis(trifluoromethylsulfonyl)imide, [Cnpy][NTf2], where n = 2, 3, 4, 6, and 8 have been synthesized and the effect of alkyl chain length in cation on their thermophysical properties have been studied and compared with those observed in analogous imidazolium-based ILs. To drive this comparison, the speed of sound, density, isobaric heat capacity, electric conductivity, TGA for all [Cnpy][NTf2], as well as, refractive index of [Cnpy][NTf2] (n = 3, 6) and surface tension of [Cnpy][NTf2] (n = 2, 4, 6, 8) were determined as a function of temperature at atmospheric pressure. From these experimental data, the isentropic compressibility, isobaric thermal expansion, isothermal compressibility, isochoric heat capacity, internal pressure, as well as, the surface entropy and surface enthalpy were calculated. The electric conductivity together with the literature viscosity data are used to obtain the Walden plot for the investigated ionic liquids. The minimum of the speed of sound dependence on the alkyl chain length in the cation is observed for [C6py][NTf2]. The isentropic compressibility and the isothermal compressibility of [Cnpy][NTf2] increase with increasing alkyl chain length in the cation and the slope of this dependence is changed around n = 6. These effects were more pronounced than that for imidazolium-based ionic liquids with [NTf2]− anion. Finally, COSMOthermX software was used to further evaluate its predictive capability by comparing calculated density and isobaric heat capacity as a function of the IL structure and temperature with experimental data for the [Cnpy][NTf2] (n = 2, 3, 4, 6, 8) series. This analysis shows a very good agreement in the case of the Cp with an average absolute relative deviation close to 1.4% while a deviation close to 0.72% is observed in the case of the density of the [Cnpy][NTf2].