*2.2. TGA and DSC Analysis*

Thermogravimetric analyzer (TGA) was used to study the thermal stability of the PILs. Table 1 shows the thermal stability data while Figure 1 displays the TGA profiles of the synthesized PILs. It could be observed from the data that the thermal stability of PILs is in the range of 416 to 437 K. For a common cation, lengthening the alkyl chain branch in the anion caused an increment in the thermal decomposition (Td) of the PIL. This could be evidenced by the relatively high Td of [EHA][C7] and [BEHA][C7] as compared to the others. A similar observation was reported by Bhattacharyya et al. in which the thermal stability of an amino acid ionic liquid with longer alkyl chain attached to the nitrogen, [N1,1,14,2O12][Lys], has a higher thermal stability than another amino acid ionic liquid, [N1,1,6,2O12][Lys], with a relatively shorter alkyl chain branch [40]. According to Keshapolla et al. and Bandres et al., the relationship between high thermal stability and long alkyl chain attached to the ionic liquid could be attributed to the presence of strong intermolecular and intramolecular forces in the alkyl chain [41,42]. In addition, it was observed that the Td values of PILs with a common cation are relatively close to one another and a similar observation was recorded by Cai et al., involving a series of ionic liquids namely triethanolamine methanesulfonate [TEA][mesy], triethanolamine trifluoromethanesulfonate [TEA][OTf] and triethanolamine benzenesulfonate [TEA][Bsa] [43]. On the other hand, the thermal stability of the ammonium-based PILs synthesized in this work is relatively low when compared to other types of ionic liquids. For instances, the thermal stability of an imidazolium-based ionic liquid (1-butylimidazolium dicyanamide, [BMIM] [DCA]) and a phosphonium-based ionic liquid, (phosphonium bis-dicarbollylcobalt (III) [PC6C6C6C14][CoCB]) are greater than 300 ◦C [35,44,45]. Xu and Cheng have summarized that the thermal stability of imidazolium ionic liquids was improved by increasing the degree of substitution of hydrogen by alkyl groups on the imidazolium ring [46].


**Table 1.** Thermal decomposition, Td; glass transition, Tg; melting point, Tm.

**Figure 1.** Thermal decomposition curves of (**a**) [EHA][C5], [EHA][C6], [EHA][C7], and (**b**) [BEHA][C5], [BEHA][C6], [BEHA][C7] at heating rate of 10 ◦C.min<sup>−</sup>1.

The phase transitions which are glass transition temperature (Tg), and melting point (Tm) of the ammonium-based PILs were investigated by using a Differential Scanning Calorimeter (DSC) from −150 ◦C to 50 ◦C and the results are tabulated in Table 1. This temperature range was chosen based on the fact that many ILs exhibit glass transition at low temperatures even beyond −100 ◦C [47]. Apart from providing the fundamental information, the study of phase transition of PILs at this condition is crucial due to demand in other technological areas with extreme environments. For example, in space-related applications, ILs is potentially being used as hypergolic fluids in orbiting satellites, manned spacecraft and deep-space probes [48]. Figure 2 shows the examples of DSC curves for the ammonium-based PILs synthesized in this study. Data show that all PILs possess a glass transition temperature (Tg) ranging from −98.37 ◦C to −90.89 ◦C, which indicates that all PILs experience the flow of heat from amorphous glass to liquid state [19]. As Tg represents the cohesive energy of the sample, PILs that exhibit Tg values have low cohesive energy that could contribute to advantageous physiochemical properties such as low viscosity and high ionic conductivity [47]. A similar trend of marginal difference in the Tg values for the ammonium-based PILs was also observed and discussed by other researchers employing ammonium-based ionic liquids as well [47]. In contrast, only ammonium-based PILs with [BEHA] cation exhibited a melting temperature (Tm) in which all Tm are in the range of −68.34 ◦C to −66.69 ◦C. Only a minimal increment in the Tm values was observed when the alkyl chain of anion increases [C5] to [C7]. Primarily, the Tm of PIL is dependent on the crystal lattice strength in the PIL. The low Tm of the PIL could be related to the low crystal lattice energy due to poor packing efficiency in the crystal lattice of PIL itself [43,49]. The data obtained in this work suggests that [BEHA][C7] has a better packing of the counterions in its structure than that of [BEHA][C5] and [BEHA][C6].

**Figure 2.** Differential scanning calorimetry (DSC) curves of (a) [EHA][C7], and (b) [BEHA][C6] at a heating rate of 10 ◦C min<sup>−</sup>1.
