Determination of kinetic and thermodynamic parameters of the B100 biodiesel oxidation process in mixtures with natural antioxidants

• Natural antioxidant extracts were effective in retarding the oxidation of biodiesel.
• The thermodynamic data served to evaluate the efficiency of natural antioxidants.
• The simplex-centroid design was suitable as optimization tool.

Biodiesel samples containing rosemary, oregano and basil extracts, free of alcohol, were submitted to oxidative stability tests at temperatures of 110, 115, 120 and 125 °C. The rate constants and enthalpy (ΔH‡), entropy (ΔS‡) and the Gibbs free energy (ΔG‡) in the activated state were determined from the experimentally obtained induction periods for evaluation of efficiency of natural antioxidants. A simplex-centroid mixture design was applied and the predictive equations were optimised simultaneously resulting in values of 85.89 kJ mol− 1 for ΔH‡, − 32.25 J K− 1 mol− 1 for ΔS‡ and 98.48 kJ mol− 1 K− 1 for ΔG‡ at ratios of 50% rosemary extract, 25% oregano and 25% basil. The results (ΔH‡ > 0; ΔS‡ < 0 and ΔG‡ > 0) showed that, in the presence of natural antioxidants, the oxidation reaction was endothermic and not spontaneous.