# Vapor Pressure of Ionic Liquids with a Common Tetrabutylammonium Cation and Three Different Anions
## 3 Results and Discussion
### Vapor Pressure Determination  
To obtain the vapor pressure of the samples, isothermal treatments at different temperatures were performed. Figure 4 reports the time evolution of the relative mass variation for the three samples at various temperatures within the liquidity range, that is, above T\({}_{\rm m}\) and below T\({}_{\rm d}\). As expected, for all samples, the mass loss increases as the temperature increases. For all samples and temperatures, the time dependence is well described by a straight line. A fit by a linear regression provided the k\({}_{\rm T}\) values (see lines in Figure 4) is used in Equation (1) to calculate the vapor pressure. To transform the k\({}_{\rm T}\) values into the absolute values of the vapor pressure, we used the calibration of our thermogravimetric  
Figure 3: TGA (**a**) and DTA (**b**) traces of the three investigated samples. The lowest values of the vapor pressure (Figure 5, Tables 3-5) are observed for TBA-NTF2; the values calculated for TBA-TFO are higher by no more than one order of magnitude, depending on the specific temperature. The highest p\({}_{\rm V}\) values are obtained for TBA-Br, which exceed those obtained for TBA-NTF2 and TBA-TFO by many orders of magnitude. The p\({}_{\rm V}\) values here obtained for TBA-NTF2 are close to those reported by [PERSON] et al. in a recent publication [31]. One can note in Figure 5 that the vapor pressure of TBA-NTF2 is similar to that previously measured for N-trimethyl-N-propylammonium-NTF2 (N1113-NTF2) [22], an ionic \begin{table}
\begin{tabular}{c c} \hline
**Temperature/\({}^{\circ}\)C** & **Vapor Pressure/Pa** \\ \hline
240 & (1.20 \(\pm\) 0.05) \\
260 & (2.82 \(\pm\) 0.06) \\
280 & (9.4 \(\pm\) 0.2) \\
300 & (2.79 \(\pm\) 0.08) \(\times\) 10 \\
320 & (1.28 \(\pm\) 0.05) \(\times\) 10\({}^{2}\) \\ \hline \end{tabular}
\end{table} 
Table 5: Calculated vapor pressure values of TBA-NTF2. Figure 5: Calculated vapor pressure of the three investigated samples compared to the values of N1113-NTF2 and N1114-FSI reported in [22]. \begin{table}
\begin{tabular}{c c} \hline
**Temperature/\({}^{\circ}\)C** & **Vapor Pressure/Pa** \\ \hline
200 & (3.02 \(\pm\) 0.06) \(\times\) 10\({}^{-1}\) \\
220 & (6.9 \(\pm\) 0.1) \(\times\) 10\({}^{-1}\) \\
240 & (3.1 \(\pm\) 0.1) \\
260 & (1.65 \(\pm\) 0.06) \(\times\) 10 \\
280 & (9.3 \(\pm\) 0.4) \(\times\) 10 \\
300 & (4.6 \(\pm\) 0.2) \(\times\) 10\({}^{2}\) \\ \hline \end{tabular}
\end{table} 
Table 4: Calculated vapor pressure values of TBA-TFO. liquid with the same anion and with a cation which is based on a quaternary ammonium with shorter alkyl chains compared to TBA-NTF2 (the structure of the cations is reported in Figure 1). This comparison is interesting because the previous literature pointed out that the vapor pressure of ionic liquids with the NTF2 anion and quaternary ammonium cations decreases as the length of the alkyl chain increases [11; 12; 13]. However, these results mainly considered cations in which three of the alkyl chains consisted of methyl groups, and only the length of the fourth alkyl chain was varied. In the presently investigated cation, all alkyl chains have a length of four carbon atoms. The vapor pressure of TBA-TFO is intermediate between the values of N1113-NTF2 and those of N-trimethyl-N-butylammonium -bis(fluorosulfonyl)imide (N1114-FSI) [22], which contains the FSI anion that seems to be particularly volatile. Ionic liquids or solids containing the TFO anion have been much less investigated than those containing halides or NTF2. Absolute vapor pressures and vaporization enthalpies of a series of ILs containing the imidazolium cation were measured by using a quartz crystal microbalance at 100 \({}^{\circ}\)C and they were found in the range of 0.01-0.1 Pa [37]. The measures were extended to 200 \({}^{\circ}\)C in refs. [38; 39]. TBA-Br is the more volatile sample, even though it was measured at much lower temperatures than the other two samples here investigated.