Comparative study of effect of alkyl chain length on thermophysical characteristics of five N-alkylpyridinium bis(trifluoromethylsulfonyl)imides with selected imidazolium-based ionic liquids
Résumé
The series of N-alkylpyridiniumbis(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], aswell 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].