Photochemical behavior of colloidal lignin particles under controlled UV exposure: Balancing self-stabilization and degradation

Colloidal Lignin Particles (CLPs), with their polyphenolic structure, are promising sustainable alternatives to chemical UV filters. This study investigates the photochemical behavior of CLPs under ultraviolet irradiation synthetized from five different technical raw lignins (Alkali, Organosolv, two Enzymatic Hydrolyzed and Softwood Kraft Lignin) via solvent-shift procedure. The suspensions were irradiated using a self-developed UV-pen set-up and a commercially available UV chamber, enabling controlled UV exposure over time. Variations in the physicochemical properties of irradiated colloidal lignin particles were characterized by Multi-Angle Dynamic Light Scattering (MADLS), Fourier Transform Infrared Spectroscopy (FTIR) coupled with chemometrics, UV-Vis spectroscopy, and High-Performance Size Exclusion Chromatography (HP-SEC). CLPs exhibited comparable responses after 14 days of UV exposure, with alterations in particle chemistry, particularly in the formation of new functional groups and molecular rearrangement. Moreover, the photochemical stability of CLPs was found to be highly dependent on both their concentration and the nature of the dispersion medium (aqueous, hexane-based, and solid-state). At higher concentrations (1.8 mg/mL), CLPs demonstrated effective self-stabilization, while lower concentrations led to rapid degradation, resulting in the disappearance of particles and the formation of low-molecular-weight fragments. Additionally, Minimal photochemical changes were observed in non-aqueous or solid-state environments due to restricted molecular mobility. These findings provide valuable insights into the degradation pathways and stability across different environmental contexts, which can contribute to understanding the potential environmental impact of CLPs in aquatic ecosystems.