Mechanistic understanding of interactions between cellulose and ionic liquids: A molecular simulation study

The interactions between cellulose and ionic liquids (ILs) have been investigated by molecular simulations. A crystalline Iβ structure is adopted for cellulose and two ILs are considered, namely 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] and 1-n-butyl-3-methylimidazolium acetate [BMIM][Ac]. The cellulose and ILs are mimicked by the AMBER force field with the atomic charges estimated from quantum chemical calculations. The density, crystalline lattice constants, thermal expansion coefficients and Young's modulus predicted for the cellulose crystal agree well with experimental data. The Young's modulus along the chain direction is high due to the strong glycosidic bonds in oligosaccharide chains. Hydrogen-bonding is observed between -OH groups, including the intra-chain O2H2∙∙∙O6 and O3H3∙∙∙O5 and the inter-chain O6H6∙∙∙O3. Upon contact with the two ILs and water, the number of hydrogen-bonds at the cellulose surface decreases, particularly for the inter-chain O6H6∙∙∙O3 in [BMIM][Ac]. The simulation results reveal that solvation leads to the breaking of hydrogen-bonds at the cellulose surface. Among the three solvents, [BMIM][Ac] appears to have the strongest capability to break the hydrogen-bonds in cellulose. This simulation study provides molecular insight into the interactions of cellulose with ILs and suggests that hydrogen-bonding is critical to govern cellulose dissolution.