FeCl3-catalyzed acetic acid pretreatment for surface-acetylated cellulose nanofibrils enabling Pickering emulsions and tunable optical composite films

The development of green and sustainable strategies to produce cellulose nanofibrils (CNFs) with tailored surface chemistry for interfacial stabilization and advanced functional composites is of significant importance. In this study, acetylated CNFs were fabricated through FeCl₃-catalyzed acetic acid (AA) pretreatment followed by high-pressure homogenization, enabling simultaneous hydrolysis and surface acetylation. The optimized CNFs exhibited a narrow diameter distribution, enhanced thermal stability, and increased acetyl group content, resulting in improved amphiphilicity and interfacial activity. The modified CNFs effectively stabilized oil-in-water Pickering emulsions over a wide range of oil contents with excellent long-term stability. Incorporation of the CNFs-stabilized emulsions into a waterborne polyurethane (WPU) matrix yielded flexible composite films with high transparency (>88%) and tunable haze (up to ∼82%). The close refractive index matching among CNFs, WPU, and oil droplets, together with controlled microscale scattering, enabled the decoupling of transparency and haze. This work establishes a sustainable catalytic route for producing surface-functionalized CNFs for interfacial engineering and light-management applications.