Thermal Properties
Thermal properties such as melting and crystallisation behaviour during the phase transition of the synthesized ionic liquids were determined by DSC thermo grams. The results obtained from the test is summarised in Fig. 3 and Table 1. Fig. 3 shows the DSC patterns of ionic liquids CTB-RA and BHI-RA. The CTB-RA exhibits the melting point at 2.6 °C during the heating cycle and crystallises at -7.8 °C during the cooling cycle. Table 1 summarises the Tm and Tctemperatures of fatty acid based IL’s. It is indicating that the significant differences in their Tm and Tc were observed. The difference in their Tm and Tc might be due to the difference in their cationic structure. Imidazolium cation containing IL showed lower Tm and Tc temperatures than all the quaternary ammonium salts. This is ascribed to the lower Vander Walls interaction of imidazolium cation due to sterically hindered structure. McFarlane et al ., stated that alkyl-imidazolium ions the geometric packing constrained the planar imidazolium ring, moreover, it has dangling alkyl chains, combined with the delocalization of the charge over the N–C–N moiety in the ring, with all of them serving to decrease the ion–ion interactions and lower the melting point (McFarlane et al., 2000). The higher Tm and Tc observed for CTB-RA is due to the high Vander Walls interaction between the methylene groups, as increasing the number of methylene groups increases Vander Walls interaction which leads to higher Tm and Tc.
Thermogravimetric analysis was carried out to find the thermal decomposition stability of synthesized ILs by determining the weight loss temperature. Fig. 4 shows the thermal decomposition patterns of the ILs under the nitrogen atmosphere. Thermal stability was measured in terms of onset decomposition temperature (Td(onset)) and maximum weight loss temperature (Td(max)). The fatty acid chain length has significant effect on the thermal degradation temperature of ILs. Exclusively, the longer chain fatty acid anion containing ILs decomposed at higher temperature due to increasing Vander Waals interaction (Gusain and Khatri, 2016). All the synthesized ILs were stable up to 196 °C. On further increase in temperature these ILs gradually decomposed in the temperature range of 197-258 °C. The variation was due to difference in constituted ion structure and interactive forces (Cao and Mu, 2014; Golets et al., 2016). Especially imidazolium ions provide better stability due to compact ring structure and high inter molecular interactions (Maton et al., 2013). All the four ILs exhibited high thermal stability hence, these synthesized ILs are effective lubricant additives for base oil operating at high temperature.