Water-mediated ternary acid solvent system for fabricating surface-selective esterified and light-colored...

Lignin-containing cellulose nanofibrils (LCNF) offer distinct functional advantages for specific applications without requiring complete lignin removal, which alleviates both processing and economic constraints. However, the facile production of light-colored, high-performance LCNF remains challenging due to uncontrollable lignin-induced dark coloration, which substantially limits its applications in high-value domains. This study reports the production of surface esterified and light-colored LCNF using a ternary solvent system comprising lactic acid, l-cysteine, and water. The proposed approach effectively removes chromophoric lignin components while concurrently inducing cellulose surface esterification, improving fiber brightness without compromising thermal stability. Incorporating water into the solvent system reduces viscosity, improving process sustainability and promoting regioselective surface modification that predominantly targets the C6-OH. Subsequent mild mechanical fibrillation yielded LCNF with high stability (-33.7 mV) and fine diameter (4.0 ± 1.5 nm). A comprehensive analysis of the resulting nanofilms confirmed that surface esterification significantly enhances the mechanical properties, transparency, and water resistance of LCNF. The resulting films exhibit outstanding UV-blocking capabilities (∼99%, UPF > 40), tunable wettability (54.3-72.2°), and superior mechanical strength (115.1 MPa). This acidic ternary solvent system presents a promising approach for fabricating surface-esterified high-performance cellulose nanofibrils, thereby facilitating the high-value utilization of LCNF in food packaging, cosmetics, and medical dressings.