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

Abstract

We investigated the state of cellulose that was dissolved in ionic liquid (IL) consisting of 1-methyl-3-propylimdiazolium cation ([C 3 mim] + ) 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. [C 3 mim][OAc] deconstructed cellulose crystal structure up to the 40 mol % cellulose concentration. Similar to the previously reported cellulose/1-ethyl-3-methylimidazolium acetate system ([C 2 mim][OAc]), the mixtures of cellulose/[C 3 mim]-[OAc] at 25−60 mol % induced a low angle peak, which corresponds to the self-assembly of the system. For [C 3 mim]-[DMP] and [C 3 mim]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 [C 3 mim]Cl, an intermediate complex crystal containing H 2 O 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.

Formation and Substructure of Cellulose/Ionic Liquid Complex Crystals: Results from Wide-Angle X-ray Scattering and Solid-State 13C NMR Spectroscopy

“…5−6 nm −1 corresponds to the appearance of a self-assembled structure. 17,22 On the other hand, various sharp Bragg peaks were observed for cellulose I/[C 3 mim]Cl We re-measured these samples, and it was confirmed that the X-ray patterns were almost the same as those previously reported. 17 From Table 1, we can speculate on the preferential chemical structures of cations and anions for the formation of a complex crystal.…”

“…16 In 2020, we discovered cellulose complex crystals with two ILs, 1-methyl-3-propylimidazolium paired with chloride at high cellulose concentrations. 17 This could be the first report of cellulose I/IL as a two-component (i.e., cellulose/IL) system.…”

“…Celluloses I and II were dried under vacuum via heating, the water content of which was expected to be less than 1 wt %. 17 Dried cellulose and dried IL were placed in a glovebox filled with dried Ar. They were mechanically mixed (in the concentration range of 10−50 mol %) using a spatula for ca.…”

“…Because the cellulose I/[Amim]Cl complex crystal is formed under precise moisture control and long-time annealing, it was revealed that the crystal contains H 2 O molecules . In 2020, we discovered cellulose complex crystals with two ILs, 1-methyl-3-propylimidazolium paired with chloride ([C 3 mim]Cl) and dimethyl phosphate ([C 3 mim][DMP]), under dry conditions and a simple mixing procedure at high cellulose concentrations . This could be the first report of cellulose I/IL as a two-component (i.e., cellulose/IL) system.…”

“…To elucidate the conditions of cellulose/IL complex crystal formations and substructures of the complex crystals, in this study, we first investigated the complex crystal formation of cellulose with 12 ILs analogous to the previous work. 17 The cations of ILs include 1,3-dimethylimidazolium ([C 1 mim] + ), 1ethyl-3-methylimidazolium ([C 2 mim] + ), 1-allyl-3-methylimidazolium ([Amim] + ), and 1-butyl-3-methylimidazolium ([C 4 mim] + ). These cations were paired with chloride, acetate ([OAc] − ), and [DMP] − .…”