Evaluation of the Toxicological and Color Properties of Anionic Hydrophobic Monoazo Dyes for Sustainable Human Hair Coloration

The design of hair dyes having environmentally benign characteristics in addition to very good durability to washing and UV light is of importance to overcome the limitations of conventional permanent hair dyes in the marketplace. As a step toward meeting this vision, we synthesized anionic azo dyes having a built-in C4, C8, or C12 alkyl chain and evaluated their hair dyeing and toxicological properties. As anticipated, the dyes increased in hydrophobicity (SlogP) with the increase in alkyl chain length. Based on prior technology, it is known that certain anionic textile dyes possessing a C12 group exhibit high affinity for wool fibers during rigorous washing in the commercial milling process. In the present study, the dye containing the C8 chain displayed the greatest uptake and the most promise as a potential permanent hair dye. Further, dyes containing a C4 or C8 carbon chain were found to be nontoxic <= 100 mg L-1 using the crustaceans Daphnia similis and Parhyale hawaiensis as test organisms. The dye containing a C12 carbon chain provided an EC50 of 57.4 mg L-1 using D. similis, but it was nontoxic <= 100 mg L-1 to P. hawaiensis. All dyes were nontoxic using the algae Raphidocelis subcapitata, and none of the dyes exhibited mutagenicity toward the strains used.