In situ Chelation of Monoazo Dyes in Human Hair Keratin Fibers Using Environmentally Benign Metal Ions

The coloration of human hair keratin fibers has long involved the oxidative coupling of primarily aromatic amines and phenols inside the fibers with the aid of harsh agents such as H₂O₂ and NH₄OH. Further, the traditional process has exposed millions of consumers and their hairstylists to toxic substances such as skin sensitizers. While alternative hair dyeing processes have been explored, they fail to be competitive with the traditional method, for reasons including impracticality and limited colors achievable. In the present study, we developed an approach to imparting color to human hair fibers that involves entrapping colorants inside hair fibers by forming chelated monoazo dyes in situ. Dyes employed were based on monoarylide, arylazopyrazolone, and arylazonaphthol families, which display yellow, orange, and magenta colors on dyed hair. The dyes were applied at 40 °C without the use of oxidants and alkali associated with current commercial hair dyes, with the best dye uptake observed when the arylazonaphthol dye was employed. The dyed hair fibers showed good durability to washing, and treatment of these fibers with Al³⁺ or Fe³⁺ ions at 40 °C led to the rapid in situ formation of 1:2 metal/dye structures. In addition, the dyed hair was soft, indicating that chelated dye occupies the interior of the fibers rather than the surface. Such an approach can be applied to the coloration of other materials, including textiles.