Direct estimation of aromatic carbon (Ca) content of base oils by 1h-NMR spectroscopy.

A direct and fast method for the estimation of aromatic carbon content (Ca) in base oils produced through different processes has been developed based on H-1-NMR spectroscopy. The identity of the complex and overlapped H-1-NMR region of 2.0-3.5 ppm, which constitutes resonances from alpha-substituents (-CH, -CH2 and -CH3 groups), has been revealed by 2D HetCor spectral analyses. This information has facilitated the estimation of quaternary (Cq) and bridgehead (Cb) aromatic carbons from H-1-NMR spectrum using the derived equations. The complete and unambiguous assignment of the 1H-NMR spectrum led to the estimation of Ca content of base oils. The classical Brown-Ladner (BL) equation for Ca estimation has been modified by taking the actual contribution of -CH, CH2 and CH3 groups, instead of using average values. The proposed H-1-NMR, and the modified BL methods correlate very well (R-2 = 0.97) with the standard C-13-NMR method for Ca estimation.