Chitin is an abundant organic resource but shows poor solubility, leading to difficulty in utilization as materials. We have already reported that an ionic liquid (IL), 1-allyl-3-methylimidazolium bromide, dissolves chitin at concentrations up to ca. 5 wt %. However, the color of the resulting solution is blackened, mainly owing to the presence of bromide. On the other hand, some deep eutectic solvents (DESs) have been already reported to dissolve chitin. In this study, we found that DESs composed of imidazolium ILs and thiourea dissolved chitin without obvious coloring. DESs are systems formed from eutectic mixtures of hydrogen bond accepters and donors. We first prepared DESs by heating mixtures of imidazolium ILs with thiourea at 100 °C for 30 min with stirring. Predetermined amounts of chitin were then added to the DESs, and for the dissolution, the mixtures were left standing at room temperature for 24 h, followed by heating at 100 °C for 24 h with stirring. The dissolution processes were evaluated by CCD camera views, which revealed in most cases the dissolution of chitin at 2–5 wt % concentrations with the present DESs.

Chitin is a promising biomass resource and has high potential for industrial applications owing to its huge annual production in nature. However, it exhibits poor processability and solubility due to its very stable and crystalline character. Recently, ionic liquids (ILs) have attracted attention as solvents for structural polysaccharides - for example, 1-allyl-3-methylimidazolium bromide (AMIMBr) has been found to dissolve chitin. As few ILs are known to dissolve chitin, little research has been conducted on the dissolution mechanism involved. In this study, we have adopted a molecular dynamics (MD) approach to study the dissolution of chitin crystals in imidazolium-based ILs. The MD simulation in AMIMBr has demonstrated that the dissolution process involved peeling of chitin chains from the crystal surface, with Br- cleaving the chitin hydrogen bonds, and AMIM+ preventing a return to the crystalline phase after the peeling. By contrast, in imidazolium acetates, which has also been reported to dissolve chitin, although the molecular chains are peeled off, the peeled chains occasionally return to the crystalline phase. Furthermore, the MD trajectory analysis has revealed that the solubility of chitin is well correlated with the number of intermolecular hydrogen bonds by acetamido groups in the chitin crystal. It has been experimentally proven that mixing a small amount of 2-bromoethyl acetate, as a bromide generator, with 1-allyl-3-methylimidazolium chloride can enhance chitin solubility, which supports the dissolution mechanism indicated by the above theoretical results.

Herein, we report the fabrication of paperlike chitin sheets (chitin paper) by the extensive entanglement of selfassembled nanochitins formed using a bottom-up approach involving stepwise regeneration from a solution of chitin in a deep eutectic solvent composed of 1-allyl-3-methylimidazolium chloride and thiourea, with additives such as benzylamine. The successful process involves preliminary sedimentation, promoted addition of the additives to the solution, with subsequent dispersive regeneration with methanol. The appearance of the regenerated chitins, namely, sheet or powder, depends on the amount of benzylamine used. The nanoscale morphology of the chitin paper was confirmed by scanning electron microscopy, and it depends on the additive used. The mechanical properties of the chitin paper were evaluated by tensile testing and compared with those of cellulose filter paper.

In this study, we have performed the preparation of reswellable amorphous porous celluloses through regeneration from hydrogels. The cellulose hydrogels were first prepared from solutions with an ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl), in different concentrations. Lyophilization of the hydrogels efficiently produced the regenerated celluloses. The powder X-ray diffraction and scanning electron microscopic measurements of the products suggest an amorphous structure and porous morphology, respectively. Furthermore, the pore sizes of the regenerated celluloses, or in turn, the network sizes of cellulose chains in the hydrogels, were dependent on the concentrations of the initially prepared solutions with BMIMCl, which also affected the tensile mechanical properties. It was suggested that the dissolution states of the cellulose chains in the solutions were different, in accordance with the concentrations, which representatively dominated the pore and network sizes of the above materials. When the porous celluloses were immersed in water, reswelling was observed to regenerate the hydrogels.

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