Computational and Experimental Studies of Biolubricant Stability Derived from Oleic Acid

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In the present work, the stability of six biolubricant compounds, i.e. Acetal, Ketal, D[4.4], D[4.5], Cyclic-6, and Cyclic-7, was evaluated both theoretically and experimentally. These compounds were prepared from oleic acid through hydroxylation and esterification reactions. The computational study of the compounds was conducted by using the Density Functional Theory (DFT) method at B3LYP level of theory and 6-31 G (d,p) basis set. The theoretical stability was reflected from the internal energy value of the hydrolysis reaction of the biolubricant compounds to form the 9,10-dihydroxystearic acid. The order of stability is given as follows: Cyclic-6 (-3.458 kJ/mol) > Acetal (-3.446 kJ/mol) > Cyclic-7 (-3.364 kJ/mol) > D[4.5] (-3.343 kJ/mol) > D[4.4] (-3.261 kJ/mol) > Ketal (-3.058 kJ/mol). On the other hand, the experimental stability of the biolubricant compounds was measured using the American Society for Testing and Material (ASTM) standard method for total acid number (TAN) and total base number (TBN). It was found that the Cyclic-6 derivative yielded the lowest TAN (1.37 mg KOH/g) and TBN (3.53 mg KOH/g) values compared to the other biolubricant compounds. Meanwhile, the Cyclic-6 also showed the lowest internal energy value (-3.458 kJ/mol) from the computational study due to the high stability of six-membered ring. These results reveal that the experimental TAN and TBN values could be predicted from the theoretical internal energy value, i.e. TAN (mg KOH/g) = 35.183 (DE) – 123.02 (R2 = 0.9226) and TBN (mg KOH/g) = 105.71 (DE) – 369.72 (R2 = 0.8946), which is remarkable.