Ionic liquids (ILs) dissolve cellulose in a more efficient and environmen- tally acceptable way than conventional methods in aqueous solution. An understanding of how ILs act on cellulose is essential for improving pretreat- ment conditions in biorefineries and thus detailed knowledge of the inter- actions between the cations, anions and cellulose is necessary. In order to explore the effects of ILs on cellulose and to compare to cellulose-water so- lutions, we perform all-atom molecular dynamics simulations of an entire cellulose microfibril in 1-butyl-3-methylimidazolium chloride and in water, thus shedding light on the initiation of the dissolution process. Moreover, we use replica-exchange simulations of a single cellulose chain in order to study the end state of that process. The results indicate that the concerted action of anions and cations is key to the efficient dissolution of cellulose in ILs. Chloride anions predominantly interact with cellulose hydroxyl groups and disrupt the intrachain O3H� � �O5 hydrogen bonds. Complementary to these polar interactions on the cellulose microfibril surface, the organic cations stack on the hydrophobic faces and intercalate on the hydrophilic faces, thus facilitating the initiation of fiber disintegration. The decomposition of in- teraction energies shows that both Coulomb and van der Waals interactions between cellulose and the IL are more favorable than those between cellulose and water. Analyses of single-chain structural properties such as the degree of ring puckering or the torsion angles of hydroxymethyl groups or of glyco- sidic bonds all reveal distinctions between BmimCl and water solution. The results indicate that the single cellulose chain is more dynamic in BmimCl than in water. Estimation of the intramolecular entropy contribution to the solvation free energy suggests that the configurational entropy of cellulose chains may play a role in the more favorable solvation in ILs.

Dissolution of cellulose in ionic liquids: A molecular dynamics study

248th National Meeting (2014)

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