Dissolution of cellulose in imidazolium carboxylate with long alkyl chain

In this work, we investigated the solubility of cellulose in ionic liquids (ILs) with long alkyl chains, specifically 1-methyl-3-octylimidazolium propionate ([C8mim][C2COO]), 1-methyl-3-pentylimidazolium hexanoate ([C5mim][C5COO]), and 1-ethyl-3-methylimidazolium nonanoate ([C2mim][C8COO]). Contrary to previous research on cellulose solubility determined through visual observation, X-ray scattering measurements indicated that these ILs amorphized the crystalline structure of cellulose up to 40 mol%, similar to 1-ethyl-3-methylimidazolium acetate ([C2mim][C1COO]). However, solid-state NMR measurements strongly suggested that ILs with long alkyl chains in the anion (e.g., [C2mim][C8COO]) do not fully disrupt the inter- and intra-hydrogen bonding network of cellulose at this high concentration. This incomplete disruption of the cellulose crystal by ILs with long alkyl chains in the anion may occur for two reasons: first, the intra-sheet interactions of the cellulose crystal involve strong hydrogen bonding that exceeds the strength of inter-sheet hydrophobic interactions; second, the bulky alkyl chain in the anion inhibits the formation of anion bridging (i.e., one anion simultaneously interacts with multiple hydroxyl groups of cellulose), thereby preserving the intact strong hydrogen bonding within cellulose.