Toward the Elucidation of the Competing Role of Evaporation and Thermal Decomposition in Ionic Liquids: A Multitechnique Study of the Vaporization Behavior of 1-Butyl-3-methylimidazolium Hexafluorophosphate under Effusion Conditions

Abstract

The evaporation/decomposition behavior of the imidazolium ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF) was investigated in the overall temperature range 425-551 K by means of the molecular-effusion-based techniques Knudsen effusion mass loss (KEML) and Knudsen effusion mass spectrometry (KEMS), using effusion orifices of different size (from 0.2 to 3 mm in diameter). Specific effusion fluxes measured by KEML were found to depend markedly on the orifice size, suggesting the occurrence of a kinetically delayed evaporation/decomposition process. KEMS experiments revealed that other species are present in the vapor phase besides the intact ion pair BMImPF(g) produced by the simple evaporation BMImPF(l) = BMImPF(g), with relative abundances depending on the orifice size-the larger the orifice, the larger the contribution of the BMImPF(g) species. By combining KEML and KEMS results, the conclusion is drawn that in the investigated temperature range, when small effusion orifices are used, a significant part of the mass loss/volatility of BMImPF is due to molecular products formed by decomposition/dissociation processes rather than to evaporated intact ion pairs. Additional experiments performed by nonisothermal thermogravimetry-differential thermal analysis (TG-DTA) further support the evidence of simultaneous evaporation/decomposition, although the conventional decomposition temperature derived from TG curves is much higher than the temperatures covered in effusion experiments. Partial pressures of the BMImPF(g) species were derived from KEMS spectra and analyzed by second- and third-law methods giving a value of ΔH = 145.3 ± 2.9 kJ·mol for the standard evaporation enthalpy of BMImPF. A comparison is done with the behavior of the 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf) ionic liquid.

The Composition of Saturated Vapor over 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid: A Multi-Technique Study of the Vaporization Process

“…Formation of ethylimidazole from similar IL BMImPF 6 was observed by KEMS in Ref. [12]. Authors [12] noted the influence of orifice size on vapor composition: the larger the orifice, the larger the contribution of the BMImPF 6 vapor species.…”

“…Decomposition was marked for ILs with high electronegative anions (BF 4 − , PF 6 − , AsF 6 − , SCN − , etc.) as well [10][11][12]. The 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF 4 ) is a striking example of such a compound.…”

“…[12]. Authors [12] noted the influence of orifice size on vapor composition: the larger the orifice, the larger the contribution of the BMImPF 6 vapor species. The investigation of ILs with cyano-functionalized anions [11,30], which are close to the object under study, showed one more degradation route by the intrinsic cyclization (Figure 1d) of the butyl group on the C1 atom.…”

Insights into the levulinate-based ionic liquid class: synthesis, cellulose dissolution evaluation and ecotoxicity assessment

“…It is of primary importance to determine the safe and correct operating temperature range for each IL to understand its suitability for a given application. [35][36][37] Therefore, the thermal stabilities of the synthesized Lev ILs were investigated and compared, when possible, with their OAc analogues. It is wellknown that the experimental conditions (eg sample mass, heating rate, nature of the atmosphere and atmosphere flow rate) can affect the results obtained with this technique.…”

Experimental measurement and prediction of ionic liquid ionisation energies

“…14 Almost all E i values for ILs, E i (IL), have been measured on vaporised gas phase neutral ion pairs. [15][16][17][18][19][20][21][22][23][24][25][26] Whilst measuring E i is relatively facile in the gas phase, a major problem is that most ILs are very tricky to vaporise without significant thermal decomposition occurring/dominating, meaning many IL ion pairs cannot be easily studied in the vapour phase; [27][28][29][30][31][32] furthermore, vapour phase ion pairs do not have the complete solvation environment of the liquid phase. A major hurdle for measuring reliable, reproducible, and comparable binding energies (E B ) 14 and E i for liquid phase ILs is dealing with sample charging during XPS measurements, which is not understood.…”