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 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 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 ideal gas state was found from combination of the sublimation enthalpy and formation enthalpy of ionic liquid in the condensed state. The obtained value is in good agreement with those calculated by quantum chemical methods.
Keywords
ionic liquids; imidazolium; hexafluorophosphate; Knudsen effusion mass spectrometry; quantum chemical calculations; thermodynamics; evaporation, decomposition, vapor pressure
Subject
Chemistry and Materials Science, Physical Chemistry
Copyright:
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