# Vapor Pressure of Ionic Liquids with a Common Tetrabutylammonium Cation and Three Different Anions
## 3 Results and Discussion
### Calculation of the Vaporization Enthalpy  
Figure 6 displays the natural logarithm of the calculated p\({}_{\rm v}\) of the three investigated samples as a function of the inverse of the absolute temperature. The slope of the graph is the mean enthalpy of vaporization in the investigated temperature range, \(\Delta\)H\({}_{\rm vap}\). From a linear fit of the data, we obtained the vaporization enthalpy, as reported in Table 6. The uncertainties were calculated from the fit. TBA-Br and TBA-TFO display the highest values that are of the order of \(\approx\)170 kJ mol\({}^{-1}\), while \(\Delta\)H\({}_{\rm vap}\) for TBA-NTF2 is \(\approx\)145 kJ mol\({}^{-1}\). All these values are similar to those of most ionic liquids reported in the literature until now [11; 12; 13; 15; 21; 22], which span the range between 120 and 200 kJ mol\({}^{-1}\). In particular, \(\Delta\)H\({}_{\rm vap}\) here derived for TBA-NTF2 is consistent with the value of 144.2 kJ mol\({}^{-1}\) reported by [PERSON] et al. for the same compound [31]. The other two ionic solids were not previously investigated in terms of their vaporization enthalpy. It can be noted that \(\Delta\)H\({}_{\rm vap}\) of a series of ionic liquids with the Br anion and imidazolium-based cations increased with the length of the alkyl chain, varying from 136.5 \(\pm\) 1.5 kJ mol\({}^{-1}\) for C\({}_{2}\)C\({}_{1}\)Im-Br to 160.8 \(\pm\) 1.2 kJ mol\({}^{-1}\) for C\({}_{10}\)C\({}_{1}\)Im-Br and 229 \(\pm\) 9 kJ mol\({}^{-1}\) for C\({}_{18}\)C\({}_{1}\)Im-Br [42]. The same phenomenology was reported for the series of ionic liquids with the trifluoromethanesulfonate anion and imidazolium cations with various lengths of the alkyl chain [37; 38; 39]. In this case, \(\Delta\)H\({}_{\rm vap}\) increased from 126.4 \(\pm\) 1.0 kJ mol\({}^{-1}\) for C\({}_{2}\)C\({}_{1}\)Im-TFO to 149.5 \(\pm\) 1.0 kJ mol\({}^{-1}\) for C\({}_{10}\)C\({}_{1}\)Im-TFO and 161.8 \(\pm\) 1.0 kJ mol\({}^{-1}\) for C\({}_{14}\)C\({}_{1}\)Im-TFO [37]. For the presently investigated TFO-containing sample, the vaporization enthalpy is similar to that of the samples with the longest alkyl chain reported in Ref. [37].  
The present results can be useful for a comprehensive understanding of the tetrabuty-lammonium salts also in view of their applications. In fact, in the last few years, many uses of these compounds have been proposed, especially in the field of energy storage. As an example, TBATFO has been used as an additive to extend the anodic stability in magnesium batteries [43], to form a stable solid-electrolyte interphase in lithium-sulfur batteries [44], and in the electrolytes of sodium-oxygen batteries [45]. Moreover, tetrabuty-lammonium bromide was used for the production of high-quality perovskite films with enhanced crystallinity and a smooth surface for applications in solar cells [46; 47].  
From a conceptual point of view, the determination of the vapor pressure provides a valuable contribution in several fields. Indeed, a relation between the vaporization enthalpy and the viscosity of ionic liquids based on [PERSON]'s theory has been proposed for selected ionic liquids [48]. Moreover, vaporization enthalpy is useful to derive the sublimation enthalpy and the cohesive properties, thanks to the complementarity with DFT calculations [49]. Finally, some preliminary vapor pressure values should be included in the PC-SAFT parametrization of ionic liquids [50].