Synthesis, In Vitro Antioxidant Activity, and Physicochemical Stability of Antioxidant-Containing Nanospheres and Their Effects on Biodiesel Oxidation Stability

Biodiesel is highly susceptible to oxidation due to its chemical structure, which necessitates antioxidants. This study developed a controlled-release system using poly-ε-caprolactone (PCL) nanospheres containing tert-butyl-hydroquinone (TBHQ) and ascorbic acid (AA) to provide long-term protection against environmental degradation. Consequently, the physicochemical stability and antioxidant activity of these systems were monitored over 150 days. The results demonstrated the successful production of stable nanospheres (polydispersity index < 0.3) that effectively shielded the compounds. TBHQ-loaded nanospheres retained over 80% antioxidant activity, while AA-loaded versions maintained more than 50% activity during the 150-day study at 30 °C. Furthermore, the antioxidant-loaded nanospheres were tested in biodiesel samples. Unexpectedly, adding these nanospheres reduced the induction time compared to pure biofuel. This reduction may be attributed to a hypothesis that nanometric surface imperfections on the spheres serve as active sites which potentially promote radical oxidation reactions. However, further surface analyses are necessary to confirm this hypothesis. Ultimately, these results suggest that PCL nanospheres excel as long-term storage carriers, and that their morphological interaction with the biodiesel matrix requires further investigation to fully understand the observed effects on induction time.