Vaporization of the prototypical ionic liquid BMImNTf2 under equilibrium conditions: a multitechnique study

Abstract

The vaporization behaviour and thermodynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2) were studied by combining the Knudsen Effusion Mass Loss (KEML) and Knudsen Effusion Mass Spectrometry (KEMS) techniques. KEML studies were carried out in a large temperature range (398-567) K by using effusion orifices with 0.3, 1, and 3 mm diameters. The vapor pressures so measured revealed no kinetically hindered vaporization effects and provided second-law vaporization enthalpies at the mean experimental temperatures in close agreement with literature. By exploiting the large temperature range covered, the heat capacity change associated with vaporization was estimated, resulting in a value of -66.8 J K(-1) mol(-1), much lower than that predicted from calorimetric measurements on the liquid phase and theoretical calculations on the gas phase. The conversion of the high temperature vaporization enthalpy to 298 K was discussed and the value Δ(l)(g)H(m)(298 K) = (128.6 ± 1.3) kJ mol(-1) assessed on the basis of data from literature and present work. Vapor pressure data were also processed by the third-law procedure using different estimations for the auxiliary thermal functions, and a Δ(l)(g)H(m)(298 K) consistent with the assessed value was obtained, although the overall agreement is sensitive to the accuracy of heat capacity data. KEMS measurements were carried out in the lower temperature range (393-467) K and showed that the largely prevailing ion species is BMIm(+), supporting the common view of BMImNTf2 vaporizing as individual, neutral ion pairs also under equilibrium conditions. By monitoring the mass spectrometric signal of this ion as a function of temperature, a second-law Δ(l)(g)H(m)(298 K) of 129.4 ± 7.3 kJ mol(-1) was obtained, well consistent with KEML and literature results. Finally, by combining KEML and KEMS measurements, the electron impact ionization cross section of BMIm(+) was estimated.

Vapor Pressure Mapping of Ionic Liquids and Low-Volatility Fluids Using Graded Isothermal Thermogravimetric Analysis

“…Several of these literature values are plotted with the P vap values determined here to facilitate a more thorough assessment of our data ( Figure S6A). The plot suggests that our vapor pressure results are in reasonably good agreement with the literature data, in particular with Brunetti et al [40] and Rocha et al [26]. However, the P vap value obtained by Aschenbrenner et al [29] for [C 4 mIm][Tf 2 N] at 393.15 K (1.2 Pa) is several orders of magnitude greater than our P vap under similar conditions (2.029 × 10 −3 Pa), despite using a similar TGA instrument.…”

“…As seen in (Table S3, entries 8 and 9, respectively) have been studied using a variety of techniques and reported [10,11,26,40]. Several of these literature values are plotted with the P vap values determined here to facilitate a more thorough assessment of our data ( Figure S6A).…”

On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites

“…A graphite resistor is the heating element of a quartz tube containing the Knudsen cell. The effusion source, whose mass is monitored, was specifically modified in our laboratory29 in order to allow an optimal temperature measurement and to maximize the uniformity of the sample temperature. Briefly, both the Knudsen cell and a Pt100 platinum resistance thermometer are inserted into a capped copper cylinder; so that the temperature of the molecular source is directly measured instead of the usual “dummy” cell placed in the isothermal section of the furnace.…”

Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

“…In the KEMS technique the effusing vapors are analyzed with a mass spectrometer 32 . Details of the apparatus used here can be found in refs 33 , 34 . Briefly, it is based on a single focusing 90° magnetic sector mass spectrometer.…”