Acid precipitation pathways direct the morphology, colloidal stability, and functional performance of lignin nanoparticles for UV blocking composite films

Acid precipitation is a widely adopted method for lignin recovery from pulping black liquor, where the precipitation parameters determine the structural and physicochemical characteristics of the isolated lignin. Given the structural complexity and heterogeneity of lignin, the impact of acid type on its molecular structure and surface chemistry remains unclear. This study investigates how differences in protonation kinetics among various acids influence lignin particle morphology and their interfacial behavior in cellulose composite films. The results indicate that lignin precipitated using HCl forms spherical particles with size (190 ± 51.1 nm), good monodispersity (PDI = 0.072) and long-term aqueous stability. This is attributed to the strong acidity of HCl, which enables rapid H⁺ release and complete protonation of lignin molecules. Conversely, the weaker acidity of ethanoic acid (HAc) leads to a more gradual protonation, resulting in hierarchically structured particle aggregates with a broader size distribution (240 ± 111.6 nm, with many nanoparticles <50 nm). In addition, HCl-lignin and HAc-lignin composite films showed 38.4% and 39.3% increases in elongation at break over pure cellulose films, along with substantial UV-blocking efficiency enhancements of 540% and 745%.