Dissolution of fibrous protein in ionic liquids

Abstract

Keratin, one of the most abundant natural biopolymers, can be obtained from different sources such as feather, wool, hair, nail and horn. Due to the generation of large amounts of these biopolymers as a waste from textile, livestock and slaughter industries, it is of significance to recover these by-products and convert them into useful biomaterials. Organic solvents have been investigated in the past to dissolve keratin based products, but with limited success. Recently, ionic liquids have been used as an alternative to conventional organic solvents in biomass dissolution due to their unique physical and chemical properties; in this study, task specific ionic liquids have been developed and investigated as solvents for keratin based biopolymers (turkey feather and wool). Imidazolium (1-allyl-3-methylimidazolium chloride ([AMIM]Cl), 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-allyl-3-methylimidazolium dicyanamide ([AMIM][DCA])), and choline based ionic liquids ([choline][thioglycolate] and [bis-(2-ethylhexyl)][thioglycolate]) were synthesized, characterized and tested for feather and wool solubilisation. Apart from these aprotic ionic liquids, protic ionic liquids (such as hydroxyl ammonium based ionic liquids (eg dimethylethanolammonium formate (([DMEA][formate])) were also synthesized and employed in the dissolution of keratin. Overall, the most effective, were the imidazolium based ionic liquids containing dicyanamide or chloride anions, showing a significant solubility (up to 45 wt. % for feather and up to 20 wt. % for wool) for keratins. The dissolved keratins were regenerated by precipitation from water and characterised by spectroscopic methods. The influence of reducing agents and deep eutectic mixtures on keratin solubility was also investigated in this study. The addition of reducing agents to the ionic liquids increased the amount of dissolved keratin by 50-100 mg g-1. Deep eutectic solvent mixtures were shown to be a potentially less expensive alternative solvent for dissolution of keratin biopolymers, however there was no significant increase in keratin solubility compared to ionic liquids. The next stage of the study involved the production of regenerated keratin bio-materials in the form of films, fibers and gel. This process could form the basis of a commercial process. The method employed in this study involved the dissolution of keratin, casting the dissolved keratin onto a Teflon plate, soaking the plates into water to diffuse out the ionic liquid and drying the material for further mechanical testing. The recycling of the protic ionic liquid was also investigated. After the dissolution and regeneration of the keratin biopolymer, the residual IL was recovered by distilling the mixture to remove the solvent. The 1H NMR spectra of the protic ionic liquids before and after distillation showed no structural change which indicates the potential for recyclability of these materials. The distilled ionic liquids showed high yield and purity (~99% recovered).