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.