Mechanistic analysis of nanocellulose formation tuned by deep eutectic solvents

Clean energy and green solvents have attracted wide attention due to their non-toxic, biodegradable, and recyclable properties. Deep eutectic solvent (DES), as a green solvent, has advantages in the formation of nanocellulose. To reveal the formation mechanism during cellulose nanocrystal (CNC) preparation, different carboxylic acid DESs are compared in the optimal experimental conditions. Experimental observations show that oxalic acid (OA) DESs can fabricate CNCs with higher yield, higher crystalline index than that of citric acid series. Moreover, crystal water molecules in DESs promote the reaction activity of DESs in the CNC formation. To understand the interaction among the DES/cellulose complex, molecular dynamics simulations and quantum chemical calculations were applied to investigate the arrangement of CNCs in the atomic scale. The radial distribution function and intermolecular interactions indicate that the non-covalent intermolecular interactions between DESs and cellulose are strong, which could be further enhanced by the crystal waters in DESs. Reaction pathways during the formation of CNCs were revealed by computational simulations, which show that OA is more prone to react with cellulose in esterification and acidolysis reactions. Both computational and experimental results demonstrate that the OA DESs are more beneficial in the production of CNCs. The synergistic effects of chemical reactions and non-covalent interactions favor the formation of CNCs by DESs.