Insight into the Cosolvent Effect of Cellulose Dissolution
in Imidazolium-Based Ionic Liquid Systems
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Abstract
Recently,
it has been reported that addition of a cosolvent significantly
influences solubility of cellulose in ionic liquids (ILs), but little
is known about the influence mechanism of the cosolvent on the molecular
level. In this work, four kinds of typical molecular solvents (dimethyl
sulfoxide (DMSO), <i>N</i>,<i>N</i>-dimethylformamide
(DMF), CH<sub>3</sub>OH, and H<sub>2</sub>O) were used to investigate
the effect of cosolvents on cellulose dissolution in [C<sub>4</sub>mim][CH<sub>3</sub>COO] by molecular dynamics simulations and quantum
chemistry calculations. It was found that dissolution of cellulose
in IL/cosolvent systems is mainly determined by the hydrogen bond
interactions between [CH<sub>3</sub>COO]<sup>−</sup> anions
and the hydroxyl protons of cellulose. The effect of cosolvents on
the solubility of cellulose is indirectly achieved by influencing
such hydrogen bond interactions. The strong preferential solvation
of [CH<sub>3</sub>COO]<sup>−</sup> by the protic solvents (CH<sub>3</sub>OH and H<sub>2</sub>O) can compete with the cellulose–[CH<sub>3</sub>COO]<sup>−</sup> interaction in the dissolution process,
resulting in decreased cellulose solubility. On the other hand, the
aprotic solvents (DMSO and DMF) can partially break down the ionic
association of [C<sub>4</sub>mim][CH<sub>3</sub>COO] by solvation
of the cation and anion, but no preferential solvation was observed.
The dissociated [CH<sub>3</sub>COO]<sup>−</sup> would readily
interact with cellulose to improve the dissolution of cellulose. Furthermore,
the effect of the aprotic solvent-to-IL molar ratio on the dissolution
of cellulose in [C<sub>4</sub>mim][CH<sub>3</sub>COO]/DMSO systems
was investigated, and a possible mechanism is proposed. These simulation
results provide insight into how a cosolvent affects the dissolution
of cellulose in ILs and may motivate further experimental studies
in related fields