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 (PhosphoniumIonicLiquids, PhosphoniumILs,), namelytetraoctylphosphonium bis(trifluoromethansulfonyl)imide ([P]NTf) and tetraoctylphosphonium perfluorobutylphosphonate ([P]NFBS) wasstudied by thermogravimetryand 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 °Cfor [P]NTf and 262 °C [P]NFBS). Evaporation by loss of intact ion pair was detected at temperatures as low as 174 and 188 °C for the two compounds, much lower than the onset temperatures evaluated by the TG traces (383 °Cfor[P]NTf and 407 °C [P]NFBS). In the explored temperature intervals (185 °C - 242 °C for [P]NTf and 180 °C - 230 °C for [P]NFBS) the vapor pressures measured by KEML are very similar for the two compounds, falling in the ranges 1.8 10-0.19 Pa and 1.1 10-0.096 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 °C):158.1 ± 2.0 kJ/mol ([P]NTf) and 161.2 ± 2.2 kJ/mol ([P]NFBS), which confirm the very similar volatility of the two compounds under equilibrium conditions.