Over the past several decades, nanocellulose has attracted rapidly growing interest due to its excellent performance such as high tensile strength, high elastic modulus, large specific surface area, and low density combined with biodegradability and renewability(Moon, Martini, Nairn, Simonsen, & Youngblood, 2011; Rajinipriya, Nagalakshmaiah, Robert, & Elkoun, 2018). According to the morphology and formation techniques, cellulose nanomaterials mainly include cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BNC) (Du et al., 2019). Among all the formation methods of CNCs, strong acid hydrolysis is indeed a simple, common, and time-saving method (Cheng, Qin, Chen, Liu, & Ren, 2017). However, problems such as harsh corrosion of equipment, severe environmental pollution, large water consumption, and relatively low production yield need to be well addressed (Xie, Du, Yang, & Si, 2018). Some sustainable and environmentally friendly methods based on the replacement of the recyclable chemicals have been put forward recently to address the above drawbacks, such as solid acid hydrolysis (Yefei Liu et al., 2014; Torlopov, Udoratina, Martakov, & Sitnikov, 2017), organic acid hydrolysis (Chen, Zhu, Baez, Kitin, & Elder, 2016; Du et al., 2016; Xu et al., 2017), ionic liquid (Miao, Yu, Jiang, & Zhang, 2016), or deep eutectic solvents (DESs) treatment (Lim et al., 2021; Yongzhuang Liu, Chen, et al., 2017; Selkälä, Sirviö, Lorite, & Liimatainen, 2016; Sirviö, Visanko, & Liimatainen, 2016).
Deep eutectic solvents, found as green solvent, have shown quite excellent performance in the nanocellulose formation. DESs are composed of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA), which can easily form a uniform liquid at a relatively low temperature due to the strong hydrogen bond interaction between HBD and HBA (Abbott, Boothby, Capper, Davies, & Rasheed, 2004). Among all kinds of DESs, acidic DESs possess the most advantage in preparing cellulose nanocrystals (CNCs). Many researchers chose choline chloride as a hydrogen bond acceptor, while oxalic acid, lactic acid, toluenesulfonic acid, and levulinic acid as hydrogen bond donors to prepare varieties of acidic DESs (Yongzhuang Liu, Guo, et al., 2017; Sirviö, et al., 2016). These DESs can effectively produce cellulose nanocrystals with diameters range from several to tens of nanometers. Previous works reported that the DESs composed of oxalic acid/choline chloride could destroy the hydrogen bond network of cellulose by forming competing hydrogens bond between DESs and cellulose (S. Liu, Zhang, Gou, Zhang, & Wang, 2021; Yongzhuang Liu, Guo, et al., 2017). However, this inference is a qualitative description based on experimental phenomena, the mechanism of the CNCs formation by DESs has not been studied thoroughly yet. During the preparation of nanocellulose by DES pretreated cellulose, two main reactions occurred, namely, esterification between carboxylic acid and hydroxyl group at the C6 position on cellulose, and acidolysis reaction of cellulose under acidic conditions with β-1,4 glycosidic bond breakage, which makes cellulose fibers break along the longitudinal direction. Recently, fundamental research analyzed the interaction between DESs and cellulose molecules at the molecular level by quantum chemistry (Fu et al., 2020) and molecular simulations (Smirnov et al., 2020). Results demonstrated that the hydrogen bond interactions of the cellulose hydroxyl groups with the chloride ions were the key factors of the destruction of cellulose in the process of solvation. In addition, it has been shown that the interactions within the studied DESs are strengthened when low amounts of water existing (Kalhor, Zheng, Ashraf, Cao, & Yu, 2020). Moreover, Ahmadi et al. (Ahmadi et al., 2018) found that the hydrous DESs systems favored the formation of more hydrogen bonds by molecular dynamics simulations.
The properties of DESs would be changed by a few crystal water molecules in HBD so that the ability of CNCs formation will also be affected. However, the effect of water molecules existing in DESs on CNCs formation has not been studied in depth. It is necessary to investigate the differences between hydrous DESs and anhydrous DESs in the nanocellulose formation and the effect of water molecules during the formation process. In this work, two kinds of organic acid (with and without crystal water) were chosen to mix with choline chloride. The DESs were applied to the jute noil pretreatment and CNCs formation. The non-covalent interaction and chemical reaction between DESs and cellulose was investigated by experiment and molecular simulations to reveal the effect of different DESs and water molecules on CNCs formation.