Experimental and theoretical study on the extraction of keratin from human hair using protic ionic liquids

Keratin, the naturally derived biomaterials have been developed and widely applied in many different fields. Among the resources containing keratin, human hair is one of the most abundant natural fibers rich in keratin and one of the most abundant waste materials produced by humans. It is critical for both value-added human hair utilization and environmental protection if suitable solvents for the dissolution and regeneration of the keratin were developed. Ionic liquids (ILs) have been evidenced to be the green solvents to extract keratin from waste human hair. In this work, five novel Protic ILs were designed and synthesized in one step with inexpensive raw materials under mild conditions. The effect of different anions and cations, temperature, and water content on the dissolution ability of Protic ILs for human hair was investigated in depth. The best IL [MEA]HCOO with high solubilization capacity (9 h, 130 °C) for human hair was finally obtained by considering the time required for complete hair dissolution and the properties of regenerated keratin. The results of FTIR, XRD, and TGA showed that the α-helix structure of regenerated keratin was not destroyed. The recycling result indicates that the dissolution ability of [MEA]HCOO for human hair kept stable after 5 times recovery. Furthermore, the density functional theory (DFT) calculations and independent gradient model (IGM) analysis uncover the dissolution of human hair by ILs through synergistic interaction between the cations and anions of ILs.