The Dissolution of Cellulose in Ionic Liquids - A Molecular Dynamics Study

The use of ionic liquids for the dissolution of cellulose promises an alternative method for the thermochemical pretreatment of biomass that may be more efficient and environmentally acceptable than conventional techniques in aqueous solution. Understanding how ionic liquids act on cellulose is essential for improving pretreatment conditions and thus detailed knowledge of the interactions between solute and solvent molecules is necessary. Here, results from the first all-atom molecular dynamics simulation of an entire cellulose microfibril in 1-butyl-3-methylimidazolium chloride (BmimCl) are presented and the interactions and orientations of solvent ions with respect to glucose units on the hydrophobic and hydrophilic surfaces of the fiber are analyzed in detail, shedding light on the initiation stages of cellulose dissolution. Moreover, replica-exchange simulations of a single cellulose chain fully solvated in BmimCl and in water are performed for a total of around 13 μs in order to study the dynamics and thermodynamics of the end state of the dissolution.

The results indicate that chloride anions predominantly interact with cellulose hydroxyl groups and disrupt the intrachain O3H’···O5 hydrogen bonds, which are essential for the integrity of cellulose fibers. The cations stack preferentially on the hydrophobic cellulose surface, governed by non-polar interactions with cellulose, which can stabilize detached cellulose chains by compensating the interaction between stacked layers. Moreover, a frequently occurring intercalation of cations on the hydrophilic surface is observed, which by separating cellulose layers can also potentially facilitate the initiation of fiber disintegration. The single-chain simulations indicate that differences in cellulose solvation mechanisms between the two solvents exist. Although global size-related properties of the cellulose chain are comparable in the two solvents, local conformational properties of cellulose differ significantly between the BmimCl and water solutions. In general, the results indicate that solute-solvent interaction energies are more favorable and that the cellulose chain is more flexible in BmimCl than in water.

Taken together, the simulations explain how ionic liquids can facilitate cellulose dissolution: the synergistic action of anions and cations helps to initiate fiber deconstruction through specific interactions on the fiber surface and to solvate single cellulose chains through favorable solvent interactions and conformational flexibility.