Elucidation of cellulose swelling behavior in ZnCl₂-based inorganic salt hydrate and its application in tailoring functional specialty paper

Novel cellulose solvents, including polybasic inorganic salt hydrates (ISH), have been exploited for cellulose swelling, dissolution and functionalization. The ZnCl₂-based hydrate is deemed to one of effective cellulose solvents. However, the role of water ratio for tunning cellulose swelling or dissolution remains unclear. In this work, a series of binary ISH with variable water ratios are designed for this purpose. The effects of water on the hydrogen bond interactions and cellulose swelling or dissolution behaviors were comprehensively investigated via Kamlet-Taft parameters (i.e., β and π*) analyses and molecular dynamic simulation. Results show that such ISH only contributes to the fiber swelling when the molar ratio of water is 2.5, ascribed to the destruction of surface hydroxyl group for cellulose molecule under an unsaturated zinc hydrate. Upon the cellulose swelling in ZnCl₂-based ISH, followed by the regeneration in ethanol/water, a flexible yet robust specialty paper can be roll-to-roll prepared, and the tailored paper has an ultrahigh strength (107.5 MPa), toughness (18.8 %), enhanced dielectric strength (48.8 kV/mm) and good hydrophobicity (73.6°).