Self-Assembly and Complexation of Cellulose/Ionic Liquid at High Cellulose Concentration: Anion Dependence

We investigated the state of cellulose that was dissolved in ionic liquid (IL) consisting of 1-methyl-3-propylimdiazolium cation ([C₃mim]⁺) paired with acetate ([OAc]⁻), dimethylphospate ([DMP]⁻), or chloride (Cl^–) anions using wide-angle X-ray scattering (WAXS), particularly focusing on the high cellulose concentration region (10–80 mol %). The anion species considerably altered the WAXS pattern of the cellulose/IL mixtures. [C₃mim]­[OAc] deconstructed cellulose crystal structure up to the 40 mol % cellulose concentration. Similar to the previously reported cellulose/1-ethyl-3-methylimidazolium acetate system ([C₂mim]­[OAc]), the mixtures of cellulose/[C₃mim]­[OAc] at 25–60 mol % induced a low angle peak, which corresponds to the self-assembly of the system. For [C₃mim]­[DMP] and [C₃mim]Cl mixtures, sharp Bragg peaks were observed, which demonstrated the formation of the complex crystal of cellulose/IL. The presence of moisture eventually deconstructed these crystal structures; while for [C₃mim]­Cl, an intermediate complex crystal containing H₂O molecules emerged. This work contributes to understanding the cellulose state dissolved in IL at high concentrations and to the development of crystal engineering on this most abundant and recalcitrant biopolymer.