Understanding the interaction mechanisms of chitosan films plays a central role in a wide range of its applications, such as bioadhesive, drug delivery, wound healing, tissue engineering, and wastewater treatment for heavy metal ions. Here, we investigated the molecular interactions between chitosan films in acidic solutions with different salt ions using a surface forces apparatus (SFA). The results showed that chitosan can be adsorbed to mica surfaces by electrostatic interaction under acidic conditions. The force measurements demonstrated that the interactions depend on the salt types, concentrations, and contact time. With the addition of 1 mM LaCl3 and NaCl into the acetic acid (HAc) buffer solution, the cohesion between chitosan films enhanced by about 45% and 20%, respectively, after a contact time of 60 min. The enhanced cohesion induced by the combination of partly intermolecular complexation formation in a bridge model and conformation adjustment of chitosan under contact time in 1 mM LaCl3 solution. However, the cohesion reduced rapidly and even disappeared when the salt concentration increased to 10 mM and 100 mM. We proposed that the cross-linked structures of chitosan mainly contribute to the significant reduction of chitosan cohesion in LaCl3 solution. In comparison, the decrease in cohesion capacity in NaCl solution mainly results from the enhanced hydration effect. Our findings may provide insights into the interaction mechanisms of chitosan films under nanoconfinement in acidic conditions and suggestions for the development of chitosan-based materials.
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