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 ([C 4 mim][PF 6 ]) in the ideal gas state were calculated from molecular and spectral data. Because quantum chemical calculations demonstrated that K dis for [C 4 mim][PF 6 ] 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 kg 3 ·m 6 . The frequencies of normal vibrations were obtained from the experimental and calculated spectra. Rotation of CH 3 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 PF 6 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 [C 4 mim][PF 6 ] 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, Δ vap S, Δ vap 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.