Ultrasonic microreactor-mediated fabrication of stable W₁/O/W₂ double emulsions for efficient vitamin C encapsulation

Water-in-oil-in-water (W₁/O/W₂) emulsions are promising carriers for the encapsulation and controlled release of bioactive compounds. However, achieving long-term stability of W₁/O/W₂ emulsions remains a significant challenge. In this study, an ultrasonic microreactor (USMR) was utilized for the fabrication of stable W₁/O/W₂ emulsions, achieving the simultaneous encapsulation of vitamin C (VC) in the internal W₁ phase and vitamin E (VE) in the intermediate oil phase. The precisely controlled acoustic cavitation within the USMR facilitates the uniform fragmentation of the primary W₁/O droplets to the nanoscale (∼100 nm), which provides a robust foundation for the subsequent encapsulation into the micron-sized W₁/O/W₂ system. The resulting W₁/O/W₂ emulsions displayed a uniform micrometer-scale droplet size, with an average diameter of ∼ 1.1-2.5 μm, depending on ultrasonic power and flow rate. Furthermore, the addition of konjac glucomannan (KGM) induced a weak gel-like network of the inner aqueous phase, providing additional stabilization. The encapsulation efficiency of VC reached up to 80%. The double emulsions exhibited excellent physical stability, showing negligible phase separation after centrifugation for 20 min, and minimal droplet size variation during accelerated thermal storage at 60°C for 12 h. Long-term storage tests further demonstrated stable droplet size and morphology for up to 35 days at room temperature. By encapsulating VE within the oil phase to protect VC, the freshly prepared emulsions exhibited nearly 100% DPPH and ABTS radical-scavenging activity. Overall, the USMR provides an efficient and controllable approach for continuously preparing highly stable double emulsions, promising for cosmetics and related functional delivery systems.