Simulation of a cellulose fiber in ionic liquid suggests a synergistic approach to dissolution

Ionic liquids dissolve cellulose in a more efficient and environmentally acceptable way than conventional methods in aqueous solution. An understanding of how ionic liquids act on cellulose is essential for improving pretreatment conditions and thus detailed knowledge of the interactions between the cations, anions and cellulose is necessary. Here, to explore ionic liquid effects, we perform all-atom molecular dynamics simulations of a cellulose microfibril in 1-butyl-3-methylimidazolium chloride and analyze site-site interactions and cation orientations at the solute-solvent interface. The results indicate that Cl- anions predominantly interact with cellulose surface hydroxyl groups but with differences between chains of neighboring cellulose layers, referred to as center and origin chains; Cl- binds to C3-hydroxyls on the origin chains but to C2- and C6-hydroxyls on the center chains, thus resulting in a distinct pattern along glucan chains of the hydrophilic fiber surfaces. In particular, Cl- binding disrupts intrachain O3H-O5 hydrogen bonds on the origin chains but not those on the center chains. In contrast, Bmim(+) cations stack preferentially on the hydrophobic cellulose surface, governed by non-polar interactions with cellulose. Complementary to the polar interactions between Cl- and cellulose, the stacking interaction between solvent cation rings and cellulose pyranose rings can compensate the interaction between stacked cellulose layers, thus stabilizing detached cellulose chains. Moreover, a frequently occurring intercalation of Bmim(+) on the hydrophilic surface is observed, which by separating cellulose layers can also potentially facilitate the initiation of fiber disintegration. The results provide a molecular description why ionic liquids are ideal cellulose solvents, the concerted action of anions and cations on the hydrophobic and hydrophilic surfaces being key to the efficient dissolution of the amphiphilic carbohydrate.