Fabrication and Optimization of Pickering Emulsion Stabilized by Lignin Nanoparticles for Curcumin Encapsulation

To develop reversible pH-responsive emulsifiers of natural origin, alkali lignin (AL) was used to develop oil-in-water Pickering emulsions. AL was first modified to synthesize quaternized alkali lignin (QAL), which displayed pH-responsive properties and demonstrated solubility in both acidic and alkaline solutions. In contrast, QAL exhibited insolubility and formed particles in neutral solutions, thereby making it a suitable candidate for utilization as an emulsifier in doubly pH-responsive Pickering emulsions. At pH 5–9, the emulsions were stable. Above or below this pH range, the system demulsifies, resulting in a reversible Pickering emulsifier with two pH-controlled transitions. On the basis of this pH-dependent behavior, lignin-based Pickering emulsions (LPE) could be subjected to several cycles of emulsification–demulsification by alternating the pH of the aqueous phase between basic and acidic, while the droplet size and storage stability were maintained. Curcumin was used as a drug model to study the loading/release behavior of LPE, finding that 50.08% of curcumin could be encapsulated in LPE. The in vitro release of curcumin was pH-dependent. In addition, LPE exhibited an outstanding protective effect against the ultraviolet-induced degradation of curcumin.