## 3 Results and Discussion
### Thermal Properties  
The thermal stability of the two synthesized samples was investigated by means of thermogravimetric measurements. For comparison, the TGA curves of the ionic liquids containing the same N1114\({}^{+}\) cation and the FSI\({}^{-}\) or TFSI\({}^{-}\) anions or the N122(2O1)\({}^{+}\) cation and the TFSI anion are also displayed. The thermal stability of the FTSI-based ILs (Figure 2) is similar to that of the FSI-based ionic liquid and is much lower than that of the ILs containing the TFSI anion [29]. In fact, one observes a mass reduction of about 2% (1%) around 381 (371) \({}^{\circ}\)C in N1114-TFSI, \(\approx\)373 (366) \({}^{\circ}\)C in N122(2O1)-TFSI, \(\approx\)315 (301) \({}^{\circ}\)C in N1114-TFSI, about 304 (288) \({}^{\circ}\)C in N1114-FSI and \(\approx\)298 (278) \({}^{\circ}\)C for N122(2O1)-FTSI. The lower thermal stability of the FTSI and FSI ILs is due to the presence in both anions of at least one SF bond, which is much weaker than the CF\({}_{3}\) bonds present in TFSI. Moreover, both ILs based on the N122(2O1)\({}^{+}\) cation have lower decomposition temperatures than those of the materials with the N1114\({}^{+}\) cations. The values of the thermal decomposition of the presently investigated ILs based on FTSI are close to those previously reported for PYR13-FTSI, PYR14-FTSI and PYR12O2-FTSI ranging between 252 and 285 \({}^{\circ}\)C for a 1% mass loss [8]. [PERSON] et al. reported that the main decomposition of pure PYR14-FTSI takes place at 305 \({}^{\circ}\)C [9], whereas [PERSON] [38] showed that \(N\)-isopropyl-\(N\)-methylpyrrolidinium-FTSI decomposed above 289 \({}^{\circ}\)C. On the other hand, [PERSON] [17] reported a much lower decomposition temperature of -218 \({}^{\circ}\)C for \(N\)-\(N\)-diethyl-\(N\)-methyl-\(N\)-(2-methoxyethyl)ammonium (fluorosulfonyl)(trifluoromenthanesulfonyl)imide. Differently from the other ILs samples, N122(2O1)-FTSI shows three decomposition steps, likely due to formation of less volatile decomposition products.  
The possible occurrence of low temperature phase transition was studied by means of differential scanning calorimetry. Figure 3 reports the DSC curves measured on heating between \(-\)85 and + 50 \({}^{\circ}\)C. Neither liquids display marks of melting or glass transition in this temperature range, nor any sign of crystallization on cooling (data not shown). Most likely, this behavior is ascribable to the asymmetric configuration of the FTSI\({}^{-}\) anion [22], which (analogously to that observed in other asymmetric anion ILs [35]) hinders the formation of the IL ion lattice and, consequently, prevents the crystallization process (or lowers the melting temperature). It is plausible that both ILs undergo a glass transition at temperatures lower than the minimum here investigated. This result is in line with the presence of glass transitions in similar ionic liquids based on the FTSI anion. Indeed, [PERSON] reported a glass transition temperature between \(-\)103 and \(-\)107 \({}^{\circ}\)C for PYR13-FTSI, PYR14-FTSI and PYR12O2-FTSI [8]; [PERSON] indicated a T\({}_{\text{g}}\) = \(-\)103 \({}^{\circ}\)C for PYR14-FTSI [9], whereas [PERSON] showed that DEME-FTSI has a glass transition at \(-\)107.8 \({}^{\circ}\)C [17]. On the other hand, [PERSON] reported a melting transition (25 \({}^{\circ}\)C) and two solid-solid phase transitions in \(N\),\(N\)-Diethylpyrrolidininium(fluorosulfonyl)(trifluoromenthanesulfonyl)imide, an IL which possesses a symmetric cation [39]. \(N\)-isopropyl-\(N\)-methylpyrrolidinium FTSI was reported to melt at 25 \({}^{\circ}\)C [38]. [PERSON] suggested that a crucial role in the high melting temperature of this IL was played by the branching of the cation alkyl chain [38].  
Figure 2: TGA curves of the investigated FTSI-based ionic liquids in comparison with ILs containing the same cations and the FSI or TFSI anions.