Sergey P. Verevkin

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(n)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.

A new method for the determination of vaporization enthalpies of ionic liquids at low temperatures.

A new method for the determination of vaporization enthalpies of extremely low volatile ILs has been developed using a newly constructed quartz crystal microbalance (QCM) vacuum setup. Because of the very high sensitivity of the QCM it has been possible to reduce the average temperature of the vaporization studies by approximately 100 K in comparison to other conventional techniques. The physical basis of the evaluation procedure has been developed and test measurements have been performed with the common ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C(2)mim][NTf(2)] extending the range of measuring vaporization enthalpies down to 363 K. The results obtained for [C(2)mim][NTf(2)] have been tested for thermodynamic consistency by comparison with data already available at higher temperatures. Comparison of the temperature-dependent vaporization enthalpy data taken from the literature show only acceptable agreement with the heat capacity difference of -40 J K(-1) mol(-1). The method developed in this work opens also a new way to obtain reliable values of vaporization enthalpies of thermally unstable ionic liquids.

Pyrrolidinium-Based Ionic Liquids. 1-Butyl-1-methyl Pyrrolidinium Dicyanoamide: Thermochemical Measurement, Mass Spectrometry, and ab Initio Calculations

The standard molar enthalpy of formation of the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide has been determined at 298 K by means of combustion calorimetry, while the enthalpy of vaporization and the mass spectrum of the vapor (ion pairs) have been determined by temperature-programmed desorption and line of sight mass spectrometry. Ab initio calculations for 1-butyl-1-methylpyrrolidinium dicyanamide have been performed using the G3MP2 and CBS-QB3 theory, and the results from homodesmic reactions are in excellent agreement with the experiments.

Predicting Enthalpy of Vaporization of Ionic Liquids: A Simple Rule for a Complex Property

Ionic liquids (ILs) have been suggested as replacement solvents in reactions and separations since they have negligible vapor pressure and thus would reduce the fugitive emissions that are common when organic solvents are used in these applications. This lack of volatility has been assumed to be common to all ionic liquids that do not undergo thermal decomposition, but recent experiments have challenged this assumption. The first quantitative data on vapor pressures of 1-alkyl-3-methylimidazolium bis(trifloromethylsulfonyl)imide ILs [CnMIM][NTf2] (length of alkyl chain: n= 2, 4, 6, or 8) were measured by well-established effusion techniques. The temperature dependence of the vapor pressures allowed determination of their molar enthalpies of vaporization (see Table 1). Qualitatively, the possibility of distilling a number of pure ILs at 300 8C has been demonstrated by Earle et al. Later, the relative volatilities of a variety of mixtures of common aprotic ionic liquids were studied in a glass sublimation apparatus at approximately 473 K by Widegren et al. These findings have shown that not all thermally stable ILs have negligible vapor pressure, and therefore the vapor pressure of all new ILs should be checked experimentally. For this reason, the scientific community has been faced with the continuously increasing challenge to measure or predict the vapor pressure and vaporization enthalpies of ILs. In practice researchers are confronted with two main problems. At room temperature the low vapor pressures of ILs are practically not measurable, whereas at high temperatures some of themmay decompose by processes such as transfer of an alkyl group or, in the case of protic ionic liquids, through deprotonation. To date, only a few experimental studies on vapor pressures and vaporization enthalpies of ILs have become available, (see Table 1), and rapid progress to address this paucity of data is hardly to be expected, due to the timeconsuming nature of these experiments. Recently, a valuable procedure was developed to obtain vaporization enthalpies of ILs by using a combination of traditional combustion calorimetry with modern high-level ab initio calculations. For this purpose, a thermodynamic relationship [Eq. (1)] was used to obtain the molar enthalpy of vaporization of 1-butyl3-methylimidazolium dicyanamide [C4MIM][N(CN)2] (see Table 1).

Ionic Liquids. Combination of Combustion Calorimetry with High-Level Quantum Chemical Calculations for Deriving Vaporization Enthalpies

In this work, the molar enthalpies of formation of the ionic liquids [C2MIM][NO3] and [C4MIM][NO3] were measured by means of combustion calorimetry. The molar enthalpy of fusion of [C2MIM][NO3] was measured using differential scanning calorimetry. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3MP2 theory. We have used a combination of traditional combustion calorimetry with modern high-level ab initio calculations in order to obtain the molar enthalpies of vaporization of a series of the ionic liquids under study.

Thermodynamic analysis of strain in the five-membered oxygen and nitrogen heterocyclic compounds.

Cyclopentane is conventionally strained. Replacement of a carbon atom by a heteroatom obviously impacts angular strain in the five-membered ring compounds. Changes of strains in the five-membered cycles are also caused by a double bond or atttached benzene rings. We studied the thermochemical properties of Indane, 2,3-dihydrobenzofuran, indoline, N-methyl-indoline, carbazole, and N-ethyl-carbazole to obtain a better quantitative understanding of the energetics associated with these compounds containing five-membered ring units. We used combustion calorimetry, transpiration method, and high-level first-principles calculations to derive gaseous enthalpies of formation of the five-membered heterocyclic compounds. Our new values together with the selected values for parent heterocyclic compounds, available from the literature, were used for calculation of the strain energies H(S) of five-membered C-, N-, and O-containing cycles. Quantitative analysis of the resulting stabilization or destabilization of a molecule due to interaction of benzene rings with the heteroatom has been performed.

Enthalpies of Solution of Organic Solutes in the Ionic Liquid 1-Methyl-3-ethyl-imidazolium Bis-(trifluoromethyl-sulfonyl) Amide

Enthalpies of solution of six organic solutes in the ionic liquid 1-methyl-3-ethyl-imidazolium bis-(trifluoromethyl-sulfonyl) amide have been measured at 25°C at low concentrations using titration calorimetry. Results at infinite dilution are compared with data obtained indirectly from activity coefficients at infinite dilution. Thermodynamic consistency has been confirmed within the experimental error of both methods.

Thermodynamics of ionic liquids precursors: 1-methylimidazole.

The standard molar enthalpy of formation in the liquid state for 1-methylimidazole (MeIm) was obtained from combustion calorimetry. The enthalpy of vaporization of the compound was derived from the temperature dependence of the vapor pressure measured by the transpiration method. Additionally, the enthalpy of vaporization for MeIm was measured directly using Calvet-type calorimetry. In order to verify the experimental data, first-principles calculations of MeIm were performed. The enthalpy of formation evaluated at the G3MP2 level of theory is in excellent agreement with the experimental value. The heat capacity and parameters of fusion of MeIm were measured in the temperature range (5 to 370) K using adiabatic calorimetry. The thermodynamic functions for the compound in the crystal and liquid states were calculated from these data. Based on the experimental spectroscopic data and the results of quantum-chemical calculations, the ideal-gas properties for MeIm were calculated by methods of statistical thermodynamics.

Imidazolium-based ionic liquids. 1-methyl imidazolium nitrate: thermochemical measurements and ab initio calculations.

In this work data of the molar enthalpies of formation of the ionic liquid 1-methylimidazolium nitrate [H-MIM][NO3] was measured by means of combustion calorimetry. The molar enthalpy of fusion of [H-MIM][NO3] was measured using DSC. Experiments to vaporize the ionic liquid into vacuum or nitrogen stream in order to obtain vaporization enthalpy have been performed. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3MP2 theory. The combination of traditional combustion calorimertry with modern high-level ab initio calculations allow the determination of the molar enthalpy of vaporization of the ionic liquid under study. The ab initio calculations indicate that [H-MIM][NO3] is most probably separated into the neutral species methyl-imidazole and HNO3 in the gaseous phase at conditions of the vaporization experiments.

Vaporization and Formation Enthalpies of 1-Alkyl-3-methylimidazolium Tricyanomethanides

Thermochemical studies of the ionic liquids 1-ethyl-3-methylimidazolium tricyanomethanide [C(2)MIM][C(CN)(3)] and 1-butyl-3-methylimidazolium tricyanomethanide [C(4)MIM][C(CN)(3)] have been performed in this work. Vaporization enthalpies have been obtained using a recently developed quartz crystal microbalance (QCM) technique. The molar enthalpies of formation of these ionic liquids in the liquid state were measured by means of combustion calorimetry. A combination of the results obtained from QCM and combustion calorimetry lead to values of gaseous molar enthalpies of formation of [C(n)MIM][C(CN)(3)]. First-principles calculations of the enthalpies of formation in the gaseous phase for the ionic liquids [C(n)MIM][C(CN)(3)] have been performed using the CBS-QB3 and G3MP2 theory and have been compared with the experimental data. Furthermore, experimental results of enthalpies of formation of imidazolium-based ionic liquids with the cation [C(n)MIM] (where n = 2 and 4) and anions [N(CN)(2)], [NO(3)], and [C(CN)(3)] available in the literature have been collected and checked for consistency using a group additivity procedure. It has been found that the enthalpies of formation of these ionic liquids roughly obey group additivity rules.