Swelling and dissolution behaviors of cellulose in phosphate-based ionic liquids-H2O binary systems: In-situ observation and molecular dynamics simulation

Abstract

Swelling and dissolution of cellulose in ionic liquids (ILs) as the pretreatment processes are key steps for the high-value conversion and utilization of cellulose. However, the molar ratio of H2O to ILs has a significant effect on the swelling and dissolution of cellulose, and its molecular-scale mechanism is still unclear, which limits screening out the suitable ILs and recovery of ILs from coagulation bath in the spinning process. Herein, three phosphate-based ILs were chosen to couple with water as solvents for swelling and dissolution of cellulose by combining in-situ observation and molecular dynamics (MD) simulation. The results show that the redissolution time of swollen cellulose is greatly reduced, especially [Emim]DEP-H2O has the best swelling and dissolution performance for cellulose due to its high β value and low viscosity. Meanwhile, when the molar ratio of IL to H2O was more than 1: 1, the IL-H2O system still has the ability to dissolve cellulose. While with the molar ratio further decreased, cellulose was no longer dissolved and began to swell, and the fiber diameter obtained the maximum swelling rate with 40.1% at a molar ratio of 1: 3. Moreover, MD simulation was used to calculate the interaction between IL/water/cellulose, which further illustrated that the combination of IL with water molecular has a significant influence on the behavior of cellulose in IL-H2O systems at the molecular scale. Thus, the potential swelling and dissolution mechanism of cellulose in ILs-H2O binary systems was proposed based on experimental and simulation results. Furthermore, the characterization results of swollen cellulose indicate that the crystal structure and degree of polymerization (DP) were almost unchanged, but the specific surface area and pore size increased significantly, and the thermal stability and heat absorption were decreased, all the results prove that the swelling process increased the accessibility of cellulose and benefited to the subsequent dissolution. Consequently, preferable swelling and dissolving parameters of experiments and simulations will provide a new idea for the selection of suitable ILs and the recovery of ILs from a coagulation bath in the dry-wet spinning process.