Evaporation thermodynamics of the tetraoctylphosphonium bis(trifluoromethansulfonyl)imide([P8888]NTf2) and tetraoctylphosphonium nonafluorobutane-1-sulfonate ([P8888]NFBS) ionic liquids

The evaporation/thermaldecompositionbehaviour of twoionicliquids with phosphoniumcation (Phosphonium-IonicLiquids, PhosphoniumILs,), namelytetraoctylphosphonium bis(trifluoromethansulfonyl)imide ([P-8888] NTf2) and tetraoctylphosphonium perfluorobutylphosphonate ([P-8888]NFBS) wasstudied by thermogravi-metryand by the effusion-basedtechniques Knudsen Effusion Mass Spectrometry (KEMS) and Knudsen Effusioni Mass Loss (KEML). KEMS spectra show that no appreciable release of decomposition products occurs up to the highest temperature reached in the experiments (225 degrees Cfor [P-8888]NTf2 and 262 degrees C [P-8888]NFBS). Evaporation by loss of intact ion pair was detected at temperatures as low as 174 and 188 degrees C for the two compounds, much lower than the onset temperatures evaluated by the TG traces (383 degrees Cfor[P-8888]NTf2 and 407 degrees C [P-8888]NFBS). In the explored temperature intervals (185 degrees C - 242 degrees C for [P-8888]NTf2 and 180 degrees C - 230 degrees C for [P-8888]NFBS) the vapor pressures measured by KEML are very similar for the two compounds, falling in the ranges 1.8 10(-3)-0.19 Pa and 1.1 10(-3)-0.096 Pa. By monitoring the vapor pressure as a function of temperature, the following values of the evaporation enthalpies were determined at the average experimental temperature (227 degrees C):158.1 +/- 2.0 kJ/mol ([P-8888]NTf2) and 161.2 +/- 2.2 kJ/mol ([P-8888]NFBS), which confirm the very similar volatility of the two compounds under equilibrium conditions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study showed that two phosphonium cation-based ionic liquids did not release decomposition products at high temperatures, but evaporated at lower temperatures by losing intact ion pairs. The vapor pressures and enthalpies of evaporation of the two compounds were very similar under the same experimental conditions.