Elucidation of the Specific Ion Effects and Intermediate Structures of Cellulose Fibers Swollen in Inorganic Salt...

The solubilities of many substances are significantly affected by specific ions, as demonstrated by the Hofmeister series of proteins. Cellulose has a resistant fibrillar structure that hinders its swelling and dissolution. Certain inorganic salt solutions are effective swelling agents and solvents for cellulose. However, the precise effects of these ions on cellulose are not fully understood. In this study, we studied the intermediate structures of cellulose fibers during their swelling process in ZnCl₂ and LiBr solutions via in situ X-ray diffraction. Two swollen phases with characteristic morphologies were observed for both salt treatments. Only the surfaces of the fibers were swollen in ZnCl₂, whereas the ions penetrated the fibers and formed complexes with cellulose while the morphology of the fibers was maintained in LiBr. Our findings clarify the reasons that ZnCl₂ has been used as an excellent swelling agent, whereas LiBr has been used as a good solvent for cellulose.