Mechanism of dissolution of cellulose in quaternary ammonium phosphate/dimethyl sulfoxide

Finding of new environmentally friendly cellulose solvent system is critical for efficient usage of cellulose. In this paper, cellulose solvent based on the mixture of di-tetrabutylammonium hydrogen phosphate and dimethyl sulfoxide (TBA₂HPO₄/DMSO) was developed. We found that TBA₂HPO₄/DMSO system has excellent solubility of cellulose. The solubility of cellulose in the system depends on TBA₂HPO₄/DMSO mass ratio. The increase of DMSO content leads to the faster dissolution of cellulose in TBA₂HPO₄/DMSO. Microcrystalline cellulose can be dissolved in TBA₂HPO₄/DMSO (30/70, w/w) within 5 min at 60 °C. The highest solubility of cellulose, 14.5 wt%, was achieved in TBA₂HPO₄/DMSO (68/32, w/w). Dissolution mechanism investigation indicated that TBA₂HPO₄ itself cannot dissolve cellulose. The addition of DMSO resulted the dissociation of TBA₂HPO₄ to release TBA⁺ and HPO₄^2- in the system. The HPO₄^2- is an excellent hydrogen bond acceptor, which can form hydrogen bonds with hydroxy groups of cellulose to destroy the hydrogen-bonding network of cellulose. The solvated TBA⁺ cations by DMSO attracted around cellulose chains via the ionic bond with HPO₄^2- anions that bounded to cellulose chains to isolate cellulose chains to result homogeneous cellulose solution. The TBA₂HPO₄/DMSO is a promising system for the preparation of regenerated cellulose materials.