# Vapor Pressure of Ionic Liquids with a Common Tetrabutylammonium Cation and Three Different Anions
###### Abstract  
The vapor pressures, p\({}_{\rm v}\), of three ionic compounds used as starting materials of deep eutectic systems, namely, tetrabutylammonium bromide (TBA-Br), tetrabutylammonium trifluoromethanesulfonate (TBA-TFO), and tetrabutylammonium bis(trifluoromethaneesulfonyl)imide (TBA-NTF2), were measured using isothermal thermogravimetry. TBA-Br displays large values of p\({}_{\rm v}\), reaching \(\approx\)700 Pa at 170 \({}^{\circ}\)C. TBA-NTF2 is the less volatile liquid, with a vapor pressure of \(\approx\)1 Pa at 240 \({}^{\circ}\)C, while TBA-TFO has a slightly higher value of p\({}_{\rm v}\), of about 3 Pa at the same temperature. The values of p\({}_{\rm v}\) for the NTF2-containing ionic compound are comparable to those of ionic liquids containing the same anion. The obtained mean vaporization enthalpy, \(\Delta\)H\({}_{\rm vap}\), for TBA-Br and TBA-TFO (\(\approx\)170 kJ mol\({}^{-1}\)) is higher than for TBA-NTF2 (\(\approx\)145 kJ mol\({}^{-1}\)). The obtained vaporization enthalpy values fall within the typical range observed for ionic liquids. ionic liquids; vapor pressure; thermal analysis; tetrabutylammonium; bromide; trifluoromethanesulfonate; bis(trifluormethanesulfonyl)imide  
2025 12 2025 20 Volatility is strictly linked to the vapor pressure, p\({}_{\rm v}\), of liquids, which is certainly an important physical quantity to be considered for applications. In the case of ILs, in 2005, [PERSON] et al. first measured the vapor pressure of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI-NTF2) in the temperature range 185-250 \({}^{\circ}\)C [10]. This work was an important milestone, as no quantification of the vapor pressure of ionic liquids was available before, and ILs were generically considered to be non-volatile. The paper by [PERSON] et al. opened the way to a large investigation of the vapor pressure of ILs in the subsequent years. A series of 1-alkyl-3-methylimidazolium NTF2 ILs were investigated, and it was pointed out that p\({}_{\rm v}\) increases as the length of the alkyl chain decreases [11; 12; 13]. It was also shown that the vapor pressure of the series of 1-alkyl-3-methylimidazolium hexafluorophosphate ILs with various lengths of the alkyl chain is lower than in the corresponding ILs containing NTF2 as the anion [11]. ILs with other anions were investigated, and it was shown that p\({}_{\rm v}\) decreases in the order Br (bromide) > di-cyanamide > tetrafluoroborate > NTF2 > hexafluorophosphate > trifluoromethanesulfonate (TFO) [14]. More recently, dication ionic liquids (DILs) were also studied concerning their vapor pressure [15; 16]. DILs are ILs in which a single cation has a double positive charge and is coordinated to two anions. Usually, DILs have higher melting points of monocation ILs, but they are more thermally stable [15]. Meanwhile, studies on the vapor pressure of ILs were also extended to liquids containing less usual anions, such as tetrafluoroborate [17], methanesulfonate [18], alanine or threonine [19], taurine [20], or nonafluorobutane-1-sulfonate [21]. An important thermodynamic quantity is vaporization enthalpy, which can be obtained from the temperature dependence of the vaporization pressure. The typical values of the vaporization enthalpy of ionic liquids range between 100 and 200 kJ mol\({}^{-1}\)[11; 12; 13; 15; 22]. In the last few years, to extend the liquid range of solvents to even lower temperatures and in search of more eco-friendly materials, eutectic systems were explored. These systems are particularly intriguing since one can obtain a liquid phase from solid components by mixing them. In some cases, a depression of the melting point of the mixtures with respect to the ideal thermodynamic behavior was observed and, in these cases, the obtained mixtures are indicated as Deep Eutectic Systems (DESs) [23]. DESs are classified according to the chemical structure of their precursor components and fall into different categories.