Influence of binary, ternary and quaternary mixtures on oxidative stability and study of kinetics and thermodynamic parameters of the degradation process of soybean biodiesel

Currently, studies show that plant extracts may exert antioxidant action and have aroused great interest because they have the property of preventing or minimizing the oxidative stress caused by free radicals. This study concerning to the binary, ternary and quaternary mixtures of plant extracts and the natural antioxidant quercetin with soybean biodiesel in concentrations ranging from (500 to 3000 ppm), using the accelerated oxidation techniques (Rancimat and PetroOXY), where the temperatures varied from 90 degrees C to 140 degrees C. The stabilizing factor was studied and indicated the existence of an antioxidant effect and not an antagonistic effect. Among the extracts the highest quantitative levels of flavonoids and total carotenoids were obtained in the following decreasing sequence: oregano extract (OE) > basil extract (BAE) > bilberry extract (BE), while for antioxidant activity front the DPPH radical the decreasing sequence was: oregano extract (OE) > bilberry extract (BE) > basil extract (BAE). The activation energies of the antioxidant mixtures were obtained by the Arrhenius equation and followed the ascending order: OEQC (62.69 kJ/mol) < BOBE (65.01 kJ /mol) < BAEQC (77.15 kJ/mol) < BEQC (77.73 kJ/mol) < BOBEQC (81.73 kJ/mol) by the technique Rancimat and BEQC (40.90 kJ/mol) < OEQC (41.82 kJ/mol) < BAEQC (42.32 kJ/mol) < BOBEQC (44.48 kJ/mol) < BOBE (45.31 kJ/mol) by the technique PetroOXY. The thermodynamic parameters Delta H* and Delta S*, were obtained by the Eyring equation and the Delta G* by the fundamental equation of thermodynamics, being evident, by the results shown, the existence of an endothermic and non-spontaneous process. The shelf life of antioxidant mixtures with biodiesel followed the descending order: BAEQC > BEQC > OEQC > BOBEQC > BOBE and OEQC > BAEQC > BEQC > BOBEQC > BOBE in the Rancimat and PetroOXY method, respectively. Therefore, the use of mixtures of these natural antioxidants was very promising and of fundamental importance in the control of the oxidative process of soybean biodiesel.