Room-Temperature Dissolution and Mechanistic Investigation of Cellulose in a Tetra-Butylammonium Acetate/Dimethyl Sulfoxide System

The dissolution of cellulose in tetrabutylammonium acetate (TBAA) and dimethyl sulfoxide (DMSO) mixed solvent was studied at room temperature (approximately 25 °C). The ratio of TBAA in the mixed solvent system (WTBAA) was found to have great influence on the solubility of cellulose and the corresponding dissolution time. The mixed solvent of WTBAA = 0.15 possessed the highest cellulose solubility and shortest dissolution time. Various cellulosic materials were well-dissolved in the solvent with a maximum solubility up to 8.17 wt %. A mechanistic study regarding the interaction between the solvent system and the model compound cellobiose was conducted using 1H NMR, 13C NMR, ATR-FTIR, conductivity, and viscosity measurements. The results implied that TBAA existed at two different states in the mixed solvent as the ratio of TBAA varied (i.e., ion-split stage (WTBAA ≤ 0.15) and ion-paired stage (WTBAA ≥ 0.15)). WTBAA = 0.15 was the turning point of these two stages, and the mixed solvent displayed the best dissolving ability at this ratio. This finding suggests that a balance between the ion concentration and ion mobility is crucial to the dissolving ability of a mixed solvent. The solvation effect of the cosolvent DMSO helped to dissociate TBAA into free ions and facilitate the ion mobility. The hydroxyl protons of cellobiose were demonstrated to form strong hydrogen bonds with CH3COO–, which was key to the dissolution of cellulose. Finally, the interaction between cellobiose and DMSO in the TBAA/DMSO/cellobiose system was investigated and was demonstrated to be another important factor for the dissolution of cellulose by stabilizing the dissolved cellulose chain from further formation of inter- and intramolecular hydrogen bonds.