Salt-Free and Water-Saving Dyeing of Cotton with Curcumin: High Color Depth and Multifunctionality

Conventional dyeing of cotton with hydrophobic curcumin is inefficient, typically requiring massive amounts of salt and water. This study introduces a paradigm-shifting, salt-free, and water-saving process by employing an innovative ethanol–water solvent system, precise acidic pH control (5.0), and tannic acid as a biobased molecular bridge. Crucially, in a direct comparison within the same ethanol–water system, our approach achieved a high color strength (K/S of 18.0) comparable to that achieved using a high salt load (K/S of 17.5), effectively eliminating the environmental burden of salt without sacrificing performance. Density functional theory calculations quantitatively validated tannic acid’s role as a bifunctional bridge, enhancing the curcumin–cellulose binding energy from 31.27 to 75.31 kcal/mol. The dyed cotton exhibited excellent washing fastness (rating 4), potent antibacterial activity (94.5% reduction against Staphylococcus aureus), and intelligent pH-responsive discoloration. This work establishes a sustainable, high-performance dyeing route by designing a coherent system in which solvent, pH, and a biobased fixative act in concert to replace conventional industrial chemicals.