Marine-Biomass-Derived Melanin–Chitosan Composites as Natural Black Hair Colorants: Charge Reversal and Electrostatic Deposition Mechanism

Conventional oxidative hair dyes rely on aromatic amines, raising concerns about human health and environmental safety. This study reports a natural hair-coloring system using size-controlled ink particles (SIPs, ~170 nm in diameter) from cuttlefish ink and chitosan. Because both SIPs and hair surfaces carry negative charges near neutral pH, original SIPs exhibited poor deposition onto hair. Polyelectrolyte complexation with chitosan reversed the SIP surface charge under acidic conditions (maximum ζ ≈ +41 mV at pH 2.4), enabling electrostatic deposition onto hair fibers. Dynamic light scattering (DLS) revealed pH-responsive aggregation at pH 1.6–1.8 and redispersion at pH 2.8–4.3, while ultraviolet–visible (UV–Vis) spectra confirmed that the broadband absorption of melanin was preserved, consistent with predominantly noncovalent interactions. Scanning electron microscopy (SEM) showed a particle-based composite coating on hair fibers. An optimal SIP:chitosan weight ratio of 10:1 at pH ~4.7 yielded the darkest and most uniform coloration (L* = 32.89, ΔE*ab = 55.89) without metallic mordants, achieving darker coloration than representative plant-based natural colorants reported in the literature. These results demonstrate a marine-biomass-derived approach to natural black hair coloration with strong darkening performance.