Double nano-emulsions for stabilizing Vitamin C and enhancing antioxidant capacity with macadamia oil and tea tree essential oil

Vitamin C, a potent antioxidant with broad therapeutic applications, is limited by rapid degradation under environmental stressors, which compromises its stability and bioactivity. This study addresses these limitations by formulating a double nano-emulsion (W/O/W) system incorporating macadamia oil and tea tree oil, using homogenization and phase inversion temperature (PIT) techniques. Comprehensive physicochemical characterization, including droplet size, polydispersity index (PDI), zeta potential, turbidity, Fourier transform infrared spectroscopy (FTIR), and SEM, was conducted alongside stability assessments under varying pH, temperature, and storage conditions. The optimized nano-emulsions exhibited nanoscale droplet sizes (10–40 ​nm), low PDI values (indicating high uniformity), and robust stability. Interestingly, the formulation with 2% W/O loading, with a particle size of 11.57 ​nm and a PDI of 0.04, demonstrated an antioxidant capacity of 4622.62 ​μg ascorbic acid equivalents (AA)/g, which was significantly higher (p ​< ​0.05) compared to both natural oils (macadamia oil: 20.91 ​μg AA/g, tea tree oil: 16.86 ​μg AA/g) and a 10% Vitamin C aqueous solution (592.94 ​μg AA/g). FTIR analysis confirmed the molecular integrity of Vitamin C and its successful encapsulation with macadamia and tea tree oils, while SEM images revealed uniformly spherical and well-dispersed droplets. Moreover, the formulation retained its structural integrity and antioxidant functionality under diverse pH and thermal conditions. These findings underscore the potential of double nano-emulsion systems to overcome the stability challenges of Vitamin C, offering a promising approach to enhance its bioavailability and therapeutic performance in pharmaceutical and cosmetic applications.