Contact Time and pH Dependent Interactions of Chitosan in Aqueous Solutions

Chitosan is a cationic biomacromolecule that is the structural component of insect and crustacean exoskeletons. Due to its biocompatibility and biodegradability, chitosan is one of the most widely used biopolymers for various applications such as fabrics, cosmetics, water treatment, and food processing. Here, we investigated the effects of contact time and pH on the interactions mediated by chitosan in aqueous solutions using a surface forces apparatus (SFA). The chitosan film was adhesive to mica surfaces at all pHs tested (3.0, 6.5, and 8.5) and showed maximum adhesion energy (W_ad ~4.8 mJ/m²) at pH 3.0 after a contact time of 1 hr. Cohesion energy between chitosan films also showed a maximum value (W_co ~6.4mJ/m²) at pH 3.0 after a contact time of 1 hr. However, at high pH, the solubility of chitosan decreased significantly, causing the chitosan-chitosan interaction to be repulsive at all contact times. The contact time and pH dependence of the adhesion/cohesion interaction indicates that the confinement induces molecular group reorientations and interdigitations. Our results and characterization of these interactions should be applicable to the use of chitosan in various biomedical applications.