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.

Heat capacities of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 6 mim][NTf 2 ] have been measured with a new adiabatic calorimeter in a range of temperatures of (5 to 370) K. The substance was found to form glass with T g ) 184.3 K and ∆ gl l C s,m ) (170.7 ( 2.8) J‚K -1 ‚mol -1 . At different modes of crystallization [C 6 mim][NTf 2 ] forms at least three crystals, the heat capacities of which differ up to 2 % in the temperature range of (220 to 250) K but coincide within 0.1 % in the temperature range of (80 to 220) K. Triple-point temperature for all the crystals is T 0 ) (272.03 ( 0.01) K, but the enthalpies of fusion are somewhat different. Thermodynamic properties for [C 6 mim][NTf 2 ] are calculated based on the obtained data.

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