Synthesis and Characterization
Ricinoleic acid ((9Z, 12R)-12-hydroxy-9-octadecenoic acid) is the most abundant fatty acid in castor oil and it provides better lubricity performance due to the presence of hydroxy functionality (Drown et al., 2009; Madankar et al., 2013). Four ricinoleate anion based ILs with different types of bromine salts (TBA-Br, TPA-Br, CTB-Br and BHI-Br) were synthesized using a green synthetic route and the chemical structures of synthesized ILs is given in Fig. 1. The synthesized ricinoleate anion-based ILs were characterised by 1H,13C NMR and FTIR spectroscopy. The characteristic peak appeared at 3330-3356 cm-1 in the FTIR spectrum (Fig. 2) is due to hydroxy functional group. The signals observed at 3008-3004 cm-1 are due to unsaturation and the peaks appeared in the range of 1713-1698 cm-1 and 1470-1451 cm-1 correspond to C=O, COO stretching vibrations respectively which confirm the presence of carboxylate group. Strong vibrational bands appeared at 2929-2854 cm-1corresponding to asymmetric and symmetric C–H frequencies of the alkyl chain. The peaks present at 1470-1386 cm-1 are attributed to the methylene and methyl bending modes in the cationic alkyl chains.
The peaks observed at 0.87-0.88 ppm in 1H NMR spectra are ascribed to terminal methyl protons of fatty acid anion and the corresponding carbons in the 13C NMR spectra given signals at 13.4-14.1 ppm. The methylene groups in long alkyl chain of fatty acid anion exhibited proton and carbon peaks in the range of 1.26-1.73 and 15.7-31.8 ppm respectively. The methylene protons adjacent to the electronegative carboxylic group (COO‾) showed downfield signals near 2.20-2.48 ppm as triplet peak. Moreover, the signals observed at 178.8-183.3 ppm and 35.8-37.8 ppm in 13C spectra are due to the COO‾ and neighbouring CH2 carbons respectively. The unsaturated protons and the protons adjacent to unsaturation shows chemical shifts at 5.36-5.55, 2.01-2.21 ppm and their carbon signals appeared at 124.9-125.9 and 29.6-29.9 ppm respectively. The more prominent peaks appeared at 7.18-7.28 ppm and ∼121 ppm in1H and 13C spectra indicate the presence of aromatic protons and carbons for BHI-RA ionic liquid. The protons adjacent to the cationic centred nitrogen suffer from severe deshielding effect, these proton signals appeared at 3.27-3.39 ppm. Further moving away from the nitrogen centre the chemical shift values shifted to upfield (Gusain et al., 2016).