On the thermodynamic properties and nanostructuration of the 1-alkyl-3-methylimidazolium trifluoromethanesulfonate series

Abstract

The volatility, heat capacity, and thermal behavior were used to explore the effect of alkyl size and anion nature in the thermodynamic properties of ionic liquids (ILs) by measuring the 1-alkyl-3-methylimidazolium trifluoromethanesulfonate series ([C_nC₁im][OTf], with n = 2, 4, 6, 8, 10, 12). The volatility of the ILs, assessed through the Knudsen effusion method coupled with quartz crystal microbalance (KEQCM), indicates that molar enthalpy and entropy of vaporization present a quasi-linear dependence with the alkyl chain length, while the molar Gibbs energy of vaporization is nearly constant until n = 6 and increases with alkyl chain length for n > 6. The [C_nC₁im][OTf] series was found to be less volatile than the [C_nC₁im][NTf₂] series. Their phase behavior was studied by means of DSC and a non-monotonous trend was found for the melting point, as well as for the standard molar enthalpy and entropy of fusion, along the number of carbon atoms on the alkyl side chain of the cation. The typical trendshift associated with the intensification of the nanostructuration of the ILs was clearly observed in the volumetric heat capacity, where it reaches a minimum at n ≈ 6 and becomes nearly constant for ILs with a longer alkyl chain. The high resolution and accurate measurement of thermodynamic properties can be used as a robust strategy for the analysis and interpretation of nanostructuration in the liquid phase, which is essential for the tuning of properties/functionalities, as well as for the deep molecular understanding of the ionic fluid behavior.