Ionic interactions between sulfuric acid and chitosan membranes

Treatment of chitosan with crosslinking agents is a common strategy for improving the performance of chitosan membranes in industrial applications. The preparation of sulfuric acid–crosslinked chitosan membranes was studied by infrared spectroscopy, elemental analysis, and X-ray diffraction, to better understand the mechanism of sulfuric acid treatment. The results indicated that first H2SO4 protonated the chitosan amine groups, then the SO42- anions slowly interacted with chitosan NH3+ groups to form ionic bridges between the polymer chains. Kinetic analysis showed that protonation was the rate-limiting factor in the adsorption of sulfuric acid by the membrane. The strong interactions between chitosan NH3+ groups and SO42- anions were related to changes in membrane crystallinity. Protonation of 10-μm-thick chitosan membranes by 0.01 M sulfuric acid was accomplished within 5 min and complete ionic crosslinking required 30 min. The results of this study suggest better guidelines for treating chitosan membranes with divalent and multivalent acids.