Structure–Property Relationships in Ionic Liquids: A Study of the Influence of N(1) Ether and C(2) Methyl Substituents on the Vaporization Enthalpies of Imidazolium-Based Ionic Liquids

Abstract

In this work, the QCM and TGA methods were used concurrently to study the two alkoxy-substituted ionic liquid (IL) series: 1-[oligo(ethylene glycol)]-3-methylimidazolium bis(triflamide) ([P x mim][NTf 2 ]) and 1-[oligo(ethylene glycol)]-2,3-dimethylimidazolium bis(triflamide) ([P x mmim][NTf 2 ]). For comparison, enthalpies of vaporization measured at elevated temperatures were adjusted to the reference temperature 298 K and tested for consistency. It was found that the vaporization enthalpies of the alkoxy-substituted ILs are significantly lower than those of the analogous ILs with the alkyl-substituted cation. This is in contrast to molecular solvents, for which alkoxy groups are typically observed to increase vaporization enthalpy relative to those of the hydrocarbon analogues. Two useful group contributions for the quick estimation of vaporization enthalpies of various alkoxy-substituted IL cations (e.g., imidazolium, ammonium, pyridinium) are recommended based on the findings of this work.

Thermodynamics of Imidazolium‐Based Ionic Liquids Containing the Trifluoromethanesulfonate Anion

“…This way is even more simpler, but it has to be taken into account, that Eqs. (13) and (14) were parametrized only for the [C n mim][NTf 2 ] family. This means that coefficients of Eqs.…”

“…This means that coefficients of Eqs. (13) and (14) might be expected to be specific for each IL homologous series. But how large and how relevant are differences in coefficients of Eqs.…”

“…But how large and how relevant are differences in coefficients of Eqs. (13) and (14), as derived from different IL families? In order to answer this question, some additional information from our previous studies has been involved into interpretation.…”

“…As can be concluded from comparison of coefficients of Eqs. (13) and (15) as well as of coefficients of Eqs. (14) and (16), the differences are present, but they seem to be not dra-matically large.…”

“…These data are essential for the selection of the fluid with optimal properties for each system. With the aim of predicting thermodynamic properties of ILs, in the last years our research group has performed comprehensive studies on different ILs containing the imidazolium cation . In these studies, the length, branching, and functionalization of the alkyl chain attached to the N atom as well as the nature of the anion have been investigated in order to determine their effect on the thermodynamic properties of the ILs.…”