Search Results (4)

The processes of the sublimation and thermal decomposition of the 1-ethyl-3-methylimidazolium hexafluorophosphate ionic liquid (EMImPF₆) were studied by a complex approach including Knudsen effusion mass spectrometry, IR and NMR spectroscopy, and quantum chemical calculations. It was established that the vapor over the liquid phase primarily consists of decomposition products under equilibrium conditions. Otherwise, the neutral ion pairs are the only vapor components under Langmuir conditions. To identify the nature of the decomposition products, an experiment on the distillation of the ionic liquid was performed and the collected distillate was analyzed. It was revealed by the IR and NMR spectroscopy that EMImPF₆ decomposes to substituted imidazole-2-ylidene (C₆N₂H₁₀PF₅) and HF. The measured vapor pressure of C₆N₂H₁₀PF₅ reveals a very low activity of the decomposition products (<10⁻⁴) in the liquid phase. The absence of a significant accumulation of decomposition products in the condensed phase makes it possible to determine the enthalpy of sublimation of the ionic liquid assuming its unchanged activity. The thermodynamics of the EMImPF₆ sublimation was studied by Knudsen effusion mass spectrometry. The formation enthalpy of EMImPF₆ in the ideal gas state was found from a combination of the sublimation enthalpy and formation enthalpy of the ionic liquid in the condensed state. The obtained value is in good agreement with those calculated by quantum chemical methods. Full article

The evaporation/decomposition behavior of the ionic liquid 1-butyl-3-methylimidazolium chloride (BMImCl) was studied with various techniques, such as thermogravimetry (TG), Knudsen effusion mass loss (KEML), and Knudsen effusion mass spectrometry (KEMS), in order to investigate the competition between the simple evaporation of the liquid as gaseous ion pairs (NIP: neutral ion pair) and the thermal decomposition releasing volatile species. TG/DSC experiments were carried out from 293 to 823 K under both He and N₂ flowing atmospheres on BMImCl as well as on BMImNTf₂ (NTf₂: bis(trifluoromethylsulfonyl)imide). Both ionic liquids were found undergoing a single step of mass loss in the temperature range investigated. However, while the BMImNTf₂ mass loss was found to occur in different temperature ranges, depending on the inert gas used, the TG curves of BMImCl under helium and nitrogen flow were practically superimposable, thus suggesting the occurrence of thermal decomposition. Furthermore, KEML experiments on BMImCl (in the range between 398 and 481 K) indicated a clear dependence of the unit area mass loss rate on the effusion hole diameter, an effect not observed for the ILs with NTf₂ anion. Finally, KEMS measurements in the 416–474 K range allowed us to identify the most abundant species in the vapor phase, which resulted in methyl chloride, butylimidazole, butyl chloride, and methylimidazole, which most probably formed from the decomposition of the liquid. Full article

A multi-technique approach based on Knudsen effusion mass spectrometry, gas phase chromatography, mass spectrometry, NMR and IR spectroscopy, thermal analysis, and quantum-chemical calculations was used to study the evaporation of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF₄). The saturated vapor over BMImBF₄ was shown to have a complex composition which consisted of the neutral ion pairs (NIPs) [BMIm⁺][BF₄⁻], imidazole-2-ylidene C₈N₂H₁₄BF₃, 1-methylimidazole C₄N₂H₆, 1-butene C₄H₈, hydrogen fluoride HF, and boron trifluoride BF₃. The vapor composition strongly depends on the evaporation conditions, shifting from congruent evaporation in the form of NIP under Langmuir conditions (open surface) to primary evaporation in the form of decomposition products under equilibrium conditions (Knudsen cell). Decomposition into imidazole-2-ylidene and HF is preferred. The vapor composition of BMImBF₄ is temperature-depended as well: the fraction ratio of [BMIm⁺][BF₄⁻] NIPs to decomposition products decreased by about a factor of three in the temperature range from 450 K to 510 K. Full article

Benzaldehydes are components of atmospheric aerosol that are poorly represented in current vapour pressure predictive techniques. In this study the solid state ($P_{\mathrm{S}}^{\mathrm{sat}}$) and sub-cooled liquid saturation vapour pressures ($P_{\mathrm{L}}^{\mathrm{sat}}$) were measured over a range of temperatures (298–328 K) for a chemically diverse group of benzaldehydes. The selected benzaldehydes allowed for the effects of varied geometric isomers and functionalities on saturation vapour pressure ($P^{\mathrm{sat}}$) to be probed. $P_{\mathrm{S}}^{\mathrm{sat}}$ was measured using Knudsen effusion mass spectrometry (KEMS) and $P_{\mathrm{L}}^{\mathrm{sat}}$ was obtained via a sub-cooled correction utilising experimental enthalpy of fusion and melting point values measured using differential scanning calorimetry (DSC). The strength of the hydrogen bond (H-bond) was the most important factor for determining $P_{\mathrm{L}}^{\mathrm{sat}}$ when a H-bond was present and the polarisability of the compound was the most important factor when a H-bond was not present. Typically compounds capable of hydrogen bonding had $P_{\mathrm{L}}^{\mathrm{sat}}$ 1 to 2 orders of magnitude lower than those that could not H-bond. The $P_{\mathrm{L}}^{\mathrm{sat}}$ were compared to estimated values using three different predictive techniques (Nannoolal et al. vapour pressure method, Myrdal and Yalkowsky method, and SIMPOL). The Nannoolal et al. vapour pressure method and the Myrdal and Yalkowsky method require the use of a boiling point method to predict $P^{\mathrm{sat}}$. For the compounds in this study the Nannoolal et al. boiling point method showed the best performance. All three predictive techniques showed less than an order of magnitude error in $P_{\mathrm{L}}^{\mathrm{sat}}$ on average, however more significant errors were within these methods. Such errors will have important implications for studies trying to ascertain the role of these compounds on aerosol growth and human health impacts. SIMPOL predicted $P_{\mathrm{L}}^{\mathrm{sat}}$ the closest to the experimentally determined values. Full article