Thermodynamic Properties of 1-Butyl-3-methylimidazolium Hexafluorophosphate in the Ideal Gas State

Abstract

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 Kdis 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 CH3N 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°, Cp, 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 Psat 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 Psat for [C4mim][PF6] in the temperature range (433 to 522) K, but no reproducible values of Psat 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 ( Cp, ΔvapS, ΔvapH) and vapor pressure (Psat) were calculated. At room temperature, the calculated Psat was found to be 10-10 Pa, a value that is much smaller than the lower detection limit for effusion measurements.