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.

Thermodynamic properties of an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) in the ideal gas state were calculated from molecular and spectral data. Because quantum chemical calculations demonstrated that K dis for [C4mim][PF6] did not exceed 10-11 at temperatures below 1000 K, the gas was assumed to consist of ion pairs. The product of the principal moments of inertia was found to be 16.49 × 10-132 kg3·m6. The frequencies of normal vibrations were obtained from the experimental and calculated spectra. Rotation of CH3N was assumed to be free. The parameters for all alkyl tops were taken to be close to those in alkanes. The parameters for Bu- and PF6 were calculated ab initio. The calculated thermodynamic functions of the ideal gas (S°, C p, and −(G° − H°(0))/T) were (657.4, 297.0, and 480.3) J·K-1·mol-1, respectively, at 298 K and were (843.1, 424.4, and 252.8) J·K-1·mol-1, respectively, at 500 K. Experiments were performed to better characterize the thermal stability and vapor pressure P sat of this substance. DSC experiments were carried out in a temperature range from (303 to 523) K and suggest that the substance starts to decompose at temperatures greater than 473 K. Knudsen effusion experiments were attempted to measure P sat for [C4mim][PF6] in the temperature range (433 to 522) K, but no reproducible values of P sat were obtained. By combining a published value of the cohesive energy density, measured heat capacities, and thermodynamic properties in the ideal gas state, thermodynamic properties of vaporization ( C p, Δvap S, Δvap H) and vapor pressure (P sat) were calculated. At room temperature, the calculated P sat was found to be 10-10 Pa, a value that is much smaller than the lower detection limit for effusion measurements.

Thermodynamic functions for 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) are reported in a range of temperatures from (5 to 550) K, based on new measurements by calorimetry. Heat capacities of the crystal, glass, and liquid phases for [C4mim][PF6] were measured with a pair of calorimeters. A vacuum-jacketed adiabatic calorimeter was used at temperatures between (5 and 310) K, and a heat bridge-scanning calorimeter was used from (300 to 550) K. With the adiabatic calorimeter, the fusion T fus = 283.51 K,   = 19.60 kJ·mol-1, and the glass transition T g = 190.6 K were observed. The [C4mim][PF6] test sample was determined to have a mole fraction purity of 0.9956 by a fractional melting analysis. Densities of the liquid were measured in a range of temperatures from (298 to 353) K with a pycnometer equipped with a capillary neck. An unexpected endothermal transition, with a very small enthalpy change of 0.25 J·g-1 (0.071 kJ·mol-1), was observed in a range of temperatures from (394 to 412) K. Heat capacity jumps were determined at the glass transition C s = 81.6 J·K-1·mol-1 and fusion C s = 44.8 J·K-1·mol-1, and the observed entropy change at fusion is S (283.51 K) = 69.23 J·K-1·mol-1.

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