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

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_<i>x</i>mim]­[NTf₂]) and 1-[oligo­(ethylene glycol)]-2,3-dimethylimidazolium bis­(triflamide) ([P_<i>x</i>mmim]­[NTf₂]). 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 PF₆ Anions

Imidazolium-based ionic liquids (ILs) with PF₆^– anions are considered as low-cost solvents for separation processes, but they exhibit restricted thermal stabilities. Reliable measurements of vaporization thermodynamics by conventional methods have failed. In this work, we applied a quartz-crystal microbalance method to determine for the first time the absolute vapor pressures for the [C<i>_n</i>mim]­[PF₆] family, with <i>n</i> = 2, 4, 6, 8, and 10, in the temperature range 403–461 K. An absence of decomposition of ILs in experimental conditions was determined by the attenuated total reflection-infrared spectroscopy. The consistency of the experimental results within the homologous series was established through enthalpy and entropy analyses of the liquid and gas phases as well as by molecular dynamics simulations

Applications of Correlation Gas Chromatography and Transpiration Studies for the Evaluation of the Vaporization and Sublimation Enthalpies of Some Perfluorinated Hydrocarbons

The fusion, vaporization, and sublimation enthalpies of a series of perfluorinated alkanes have been measured by combining measurements obtained by differential scanning calorimetry (DSC), transpiration, and correlation-gas chromatography and comparing the results to available data in the literature. Additionally, experiments are reported to provide a guide in identifying appropriate structural features of fluorinated compounds suitable for use as standards in correlation gas chromatography measurements. Fusion enthalpies and fusion temperatures by DSC for the following compounds were measured (in kJ·mol^–1; K): decafluorobiphenyl (20.5 ± 0.6, 339.6), perfluorododecane (24.2 ± 0.6, 346.6), perfluorotridecane (27.9 ± 0.4, 361.7), perfluorotetradecane (31.5 ± 0.3, 375.6), perfluoropentadecane (35.1 ± 0.2, 388.1), perfluorohexadecane (38.7 ± 0.1, 399.7), perfluoroeicosane (50.3 ± 0.3, 436.2), and perfluorotetracosane (63.2 ± 0.6, 461.1). Sublimation enthalpies for the following compounds were measured by transpiration (in kJ·mol^–1; <i>T</i> = 298.15 K): perfluorododecane (85.8 ± 0.6), perfluorotridecane (94.3 ± 0.5), perfluorotetradecane (102.4 ± 1.0), and perfluoropentadecane (109.4 ± 0.4). Vaporization and sublimation enthalpies, respectively, were also evaluated for the following compounds (kJ·mol^–1; <i>T</i>/K = 298.15): perfluorohexadecane (88.6 ± 4.0, 117.6 ± 4.9), perfluoroeicosane (113.7 ± 7.4, 148 ± 8.8), and perfluorotetracosane (141.4 ± 2.2, 168.3 ± 11.1). The measured vaporization enthalpies of the perfluorinated alkanes behave linearly as a function of the number of CF₂ groups similar to what is observed with n-alkanes. Correlation-gas chromatography experiments confirmed previous findings that hydrocarbons can be used as standards for compounds containing a few fluorine atoms but otherwise standards need to be chosen with similar fluorine substitution and functionality

Making Sense of Enthalpy of Vaporization Trends for Ionic Liquids: New Experimental and Simulation Data Show a Simple Linear Relationship and Help Reconcile Previous Data

Vaporization enthalpy of an ionic liquid (IL) is a key physical property for applications of ILs as thermofluids and also is useful in developing liquid state theories and validating intermolecular potential functions used in molecular modeling of these liquids. Compilation of the data for a homologous series of 1-alkyl-3-methylimidazolium bis­(trifluoromethane-sulfonyl)­imide ([C_<i>n</i>mim]­[NTf₂]) ILs has revealed an embarrassing disarray of literature results. New experimental data, based on the concurring results from quartz crystal microbalance, thermogravimetric analyses, and molecular dynamics simulation have revealed a clear linear dependence of IL vaporization enthalpies on the chain length of the alkyl group on the cation. Ambiguity of the procedure for extrapolation of vaporization enthalpies to the reference temperature 298 K was found to be a major source of the discrepancies among previous data sets. Two simple methods for temperature adjustment of vaporization enthalpies have been suggested. Resulting vaporization enthalpies obey group additivity, although the values of the additivity parameters for ILs are different from those for molecular compounds