The solubility of cellulose in binary mixtures of ionic liquids and dimethyl sulfoxide: Influence of the anion

The cellulose solubility in IL-DMSO binary solutions at ambient temperature was studied. The anion effect was investigated using [EMIM][OAc], [EMIM][DEP], [EMIM][MP], and [EMIM][BF4] ionic liquids. In addition, the correlation between the cellulose solubility and the Kamlet-Taft hydrogen bond basicity (β) was examined. Cellulose solubility in terms of grams of cellulose per mole of ionic liquid (g/mol-IL) in neat ionic liquids and their β values were higher in the order of [EMIM][OAc] > [EMIM][DEP] > [EMIM][MP]. By blending DMSO with these ionic liquids, the cellulose solubility was notably enhanced, with a maximum at around 0.2 IL mole fraction. It was found that the maximum cellulose solubility and the IL concentration where the maximum cellulose solubility appeared are interestingly related with the β values of the neat ionic liquids, indicating that solubility information in IL-DMSO binary solutions is influenced by the characteristics of the neat ionic liquids. In the case of [EMIM][BF4], cellulose was not dissolved at all even though DMSO was included, owing to the insufficient β value for cellulose dissolution. In comparing the variation of behavior between cellulose solubility and the β value in IL-DMSO binary solutions, the cellulose solubility increased from 1.0 to around 0.2 IL mole fraction, whereas the β value remained constant. This indicates that the increase in cellulose solubility was most likely attributed to the increase in ion mobility, rather than the influence of the β value. However, the cellulose solubility decreased sharply below around 0.2 IL mole fraction due to the solvation of ionic liquids by DMSO and dissociation of the cations and anions.