Binary mixtures of ionic liquids-DMSO as solvents for the dissolution and derivatization of cellulose: Effects of alkyl and alkoxy side chains

Article abstract of DOI:10.1016/j.carbpol.2019.02.024

The efficiency of mixtures of ionic liquids (ILs) and molecular solvents in cellulose dissolution and derivatization depends on the structures of both components. We investigated the ILs 1-(1-butyl)-3-methylimidazolium acetate (C₄MeImAc) and 1-(2-methoxyethyl)-3-methylimidazolium acetate (C₃OMeImAc) and their solutions in dimethyl sulfoxide, DMSO, to assess the effect of presence of an ether linkage in the IL side-chain. Surprisingly, C₄MeImAc-DMSO was more efficient than C₃OMeImAc-DMSO for the dissolution and acylation of cellulose. We investigated both solvents using rheology, NMR spectroscopy, and solvatochromism. Mixtures of C₃OMeImAc-DMSO are more viscous, less basic, and form weaker hydrogen bonds with cellobiose than C₄MeImAc-DMSO. We attribute the lower efficiency of C₃OMeImAc to “deactivation” of the ether oxygen and C2–H of the imidazolium ring due to intramolecular hydrogen bonding. Using the corresponding ILs with C2–CH₃ instead of C2–H, namely, 1-butyl-2,3-dimethylimidazolium acetate (C₄Me₂ImAc) and 1-(2-methoxyethyl)-2,3-dimethylimidazolium acetate (C₃OMe₂ImAc) increased the concentration of dissolved cellulose; without noticeable effect on biopolymer reactivity.