Effective Extraction of Keratin from Human Hair under Mild Conditions Using DES Based on Choline Chloride and Ethanolamine

Keratin, a naturally occurring biomaterial, has gained substantial recognition for its application in various domains. Given its abundant presence in human hair, the pursuit of efficient methods for dissolving and extracting keratin is motivated by the dual objectives of maximizing the utility of human hair and promoting ecological sustainability. In this work, a series of deep eutectic solvents (DES) were designed for extraction of keratin from human hair under mild experimental conditions. The effects of DES structure, molar ratios of HBA to HBD, dissolution temperature, and time on the dissolving ability of DES for human hair were investigated in depth. The results showed that ChCl/MEA can effectively dissolve human hair under mild conditions (80 °C, 4 h) with a solubility of 84% and a mass percentage of 20%. FT-IR, XRD, SDS-PAGE, and TGA results showed that the structure of the regenerated keratin was slightly destroyed but still exhibited stable thermal property. Analysis of the reduced density gradient (RDG) and surface electrostatic potential (ESP) verified that the hydrogen bond driven by electrostatic attraction is formed in DESs. Independent gradient model (IGMH) analysis confirmed that the weak interaction between human hair and DES is mainly caused by van der Waals interactions. Additionally, ChCl/MEA displayed an excellent performance in extracting keratin from human hair even after recycling for three cycles.