Vapor pressures for a series of 1-n-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (alkyl = ethyl, butyl, hexyl, and octyl) ionic liquids (ILs) were measured by the integral effusion Knudsen method. Thermodynamic parameters of vaporization for ILs were calculated from these data. The absence of decomposition of ILs during the vaporization process was proved by IR spectroscopy. Enthalpies of vaporization of ILs were correlated with molar volumes and surface tensions of the compounds.

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Ionic liquids are attracting growing interest as alternatives to conventional molecular solvents. Experimental values of vapor pressure, enthalpy of vaporization, and enthalpy of formation of ionic liquids are the key thermodynamic quantities, which are required for the validation and development of the molecular modeling and ab initio methods toward this new class of solvents. In this work, the molar enthalpy of formation of the liquid 1-butyl-3-methylimidazolium dicyanamide, 206.2 +/- 2.5 kJ.mol-1, was measured by means of combustion calorimetry. The molar enthalpy of vaporization of 1-butyl-3-methylimidazolium dicyanamide, 157.2 +/- 1.1 kJ.mol-1, was obtained from the temperature dependence of the vapor pressure measured using the transpiration method. The latter method has been checked with measurements of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, where data are available from the effusion technique. The first experimental determination of the gaseous enthalpy of formation of the ionic liquid 1-butyl-3-methylimidazolium dicyanamide, 363.4 +/- 2.7 kJ.mol-1, from thermochemical measurements (combustion and transpiration) is presented. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for 1-butyl-3-methylimidazolium dicyanamide using the G3MP2 theory. Excellent agreement with experimental results has been observed. The method developed opens a new way to obtain thermodynamic properties of ionic liquids which have not been available so far.

Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. The purpose of the present paper is to answer three questions: Do hydrogen bonds exist in these Coulomb fluids? To what extent do hydrogen bonds contribute to the overall interaction between anions and cations? And finally, are hydrogen bonds important for the physical properties of ionic liquids? All these questions are addressed by using a suitable combination of experimental and theoretical methods including newly synthesized imidazolium-based ionic liquids, far infrared spectroscopy, terahertz spectroscopy, DFT calculations, differential scanning calorimetry (DSC), viscometry and quartz-crystal-microbalance measurements. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. This is demonstrated for the case of melting points, viscosities and enthalpies of vaporization. As a consequence, a variety of important properties can be tuned towards a larger working temperature range, finally expanding the range of potential applications.

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.

Activity coefficients at infinite dilution γ i ∞ of 19 alkanes, alkenes, and alkylbenzenes in the ionic liquid 4-methyl-n-butylpyridinium tetrafluoroborate (C 10 H 16 BF 4 N) were determined by gas chromatography using the ionic liquid as stationary phase. The measurements were carried out at different temperatures between 313.1 K and 363.1 K. From the temperature dependence of the limiting activity coefficients partial molar excess enthalpies at infinite dilution H i E,∞ of the organic solutes in the ionic liquids have been derived.

Experimental vapor pressures, vaporization, fusion, and sublimation enthalpies of a number of bromo-and iodo-substituted methylbenzenes have been studied by transpiration method in order to evaluate a series of experimental measurements that appear to be internally self-consistent. The compounds studied in this regard include bromobenzene, iodobenzene, 1-bromo-2-methylbenzene, 1-bromo-3-methylbenzene, 1-bromo-4-methylbenzene, 1-iodo-2-methylbenzene, 1-iodo-3-methylbenzene, 1-iodo-4-methylbenzene, 1-bromo-2,6-dimethylbenzene, 1-iodo-2,6-dimethylbenzene, and 1-iodo-2,4-dimethylbenzene. Gas-phase enthalpies of formation of halogen-substituted methylbenzenes were calculated by using quantum-chemical methods. Simple group-additivity procedures were developed for estimation of vaporization enthalpies and gas-phase and liquid-phase enthalpies of formation of halogensubstituted methylbenzenes.

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