Thermally-induced phase separation of alkali lignin into mesoscale pigments for rapid mordant-free cotton dyeing.

Industrial lignin is a promising sustainable textile colorant, but its weak adhesion to fibers limits practical use. Here we report a metal-free dyeing strategy based on mesoscale lignin pigments (MLPs) formed through thermally induced phase separation. Alkali lignin treated in 50% (v/v) ethanol-water at 200 °C underwent partial depolymerization and, upon cooling, reorganized into irregular ~4 μm flocs driven by hydrophobic interactions and π-π stacking. The resulting metastable suspension mobilized ~52% of the initial lignin and remained dyeing-active for ~24 h, with coarser flocs settling out and the suspended population stayed centered at ~4 μm. Without mordants, the bath dyed cotton at room temperature in 10 min, yielding K/S values 54-200% higher than previous reports. Dye retention arose from long-range DLVO secondary-minimum capture, short-range hydrogen bonding and π-π stacking, and mesoscale mechanical interlocking, with compliant MLPs deforming onto fiber surfaces and lodging within inter-fiber voids. GC-MS identified syringaldehyde among the released phenolics, and control experiments implicated its role as an in situ interfacial promoter between MLPs and cellulose. Dyed fabrics showed UPF > 40 and a hydrophobic surface, while alkaline laundering fastness remains limited, defining the application as decorative and home textiles with infrequent washing.