Although ILs have been demonstrated to be highly effective solvents for the dissolution of cellulose and lignocellulosic biomass, to date, the mechanism of this dissolution process remains not well understood. There is no definitive rationale for selecting ionic liquids that are capable of dissolving these biopolymers. Most current work is based on the hypothesis that cellulose insolubility is due to the strong intermolecular hydrogen bonds between cellulose chains. The dissolution of cellulose by a solvent is dependent on the destruction of these hydrogen bonds.

The outstanding solubility of ionic liquids to cellulose is due to the hydrogen basicity of anions, which can disrupt the hydrogen-bonding network among cellulose and lead to the dissolution. So far, there have been a few theoretical and experimental studies, including molecular dynamic studies and NMR analyses.

In 2007, Remsing et al. reported that 35/37Cl NMR relaxation measurements could be employed to study Cl-H hydrogen bonds in [Bmim] Cl [27]. It was found that the solvation of cellulose by the ionic liquid 1-n-butyl-3-methylimidazolium chloride ([Bmim]Cl) involves hydrogen bonding between the carbohydrate hydroxyl proton and the chloride ion in a 1:1 stoichiometry. Their further study demonstrated that the anions in these ILs are involved in specific interactions with the solutes, and govern the solvation process by analysis of Cl and C relaxation data for sugar solutions in both imidazolium chlorides and [Emim] [OAc] [28]. Variable-temperature NMR spectroscopy was also applied in the investigation on the dissolution mechanism of cellulose in 1-ethyl-3-methylimi — dazolium acetate ([Emim] [OAc]) in DMSO-d6. The results confirmed that the hydrogen bonding of hydroxyls with the acetate anion and imidazolium cation of EmimAc is the major force for cellulose dissolution in the ILs. The relatively small acetate anion favors the formation of a hydrogen bond with the hydrogen atoms of the hydroxyls, while the aromatic protons in the bulky cation imidazolium, especially H2, prefer to associate with the oxygen atoms of hydroxyls with less steric hindrance [29].

Since glucose is one of the main repeating units of polysaccharides, a better understanding of the interaction mechanism of glucose with ILs will provide in-depth understanding of the interaction of ILs and polysaccharides. In 2006, Youngs et al. investigated the molecular dynamics simulations of the solvation environment of isolated glucose monomers in a chloride-based IL (1, 3-dime — thylimidazolium chloride); the results revealed that the sugar prefers to bind to four chloride anions. Coordination shells involve only three anions, two of which are bridging chlorides. The low value of chloride: glucose ratio explains the unexpected high solvation degree of glucose in ILs [30]. Few glucose-glucose hydrogen bonds, but chloride anions hydrogen bonding to different glucose molecules simultaneously were found, partially explain the high solubility of glucose/cellulose in ILs [31].

As a natural polymer, cellulose is significantly amphiphilic and hydrophobic interactions are important for explaining the solubility pattern of cellulose. Lindman et al. presented strong evidence that cellulose is amphiphilic and that the low aqueous solubility must have a marked contribution from hydrophobic interactions [32]. Thus, we should reconsider the molecule interaction between lignocellulosic biomass and ILs. Liu et al. developed an all-atom force field for 1-ethyl-3-methylimidazolium acetate [Emim][OAc] and the behavior of cellulose in this IL was examined using molecular dynamics simulations of a series of (1-4) linked b-D-glucose oligomers (degree of polymerization n = 5, 6, 10, and 20). They found that there is strong interaction energy between the polysaccharide chain and the IL, and the conformation (b-(1,4)-glycosidic linkage) of the cellulose was altered. The anion acetate formed strong hydrogen bonds with hydroxyl groups of the cellulose, and some of the cations were found to be in close contact with the polysaccharides through hydrophobic interactions. These results supported the fact that the cations play a significant role in the dissolution of cellulose in anion acetate ILs [33]. Guo et al. calculated geometries, energies, IR characteristics, and electronic properties of the cellulose-anion (acetate, alkyl phosphate, tetrafluoro — borate and hexafluorophosphate) complexes using density functional theory calcu­lations (DFT). They found that the strength of interactions of anions with cellulose follows the order: acetate anion > alkyl phosphate anion > tetrafluoroborate anion > hexafluorophosphate anion, and the favorable sites of cellulose for the chloride anion attack are around the O2 and O3 hydroxyls [34].

Singh et al. reported that autofluorescent mapping of plant cell walls was used to visualize cellulose and lignin in pristine switchgrass (Panicum virgatum) stems to determine the mechanisms of biomass dissolution during ionic liquid pretreat­ment. Swelling of the plant cell wall, attributed to the disruption of inter — and intra­molecular hydrogen bonding between cellulose fibrils and lignin, followed by complete dissolution of biomass, was observed without using imaging techniques that require staining, embedding, and processing of biomass [35]. This could be applied to the elucidation of structural information of wood and wood components.