Facile fabrication of chitosan colloidal films with pH-tunable surface hydrophobicity and mechanical properties

Chitosan is an attractive biopolymer with great potential to be applied in biomaterial and food packaging fields. However, this natural polymer is only soluble in acidic condition at which the chitosan is highly hydrophilic in nature, thus limiting its widespread application. Here, we report a facile fabrication of surface hydrophobic chitosan films through pH adjustment, homogenization, and casting of the film-forming suspensions. With an increase in pH above its pKa (6.5), chitosan molecules experienced gradual aggregation due to the decrease of electrostatic repulsion and consequently the rise in hydrogen bonds and hydrophobic effect among and inside chitosan chains. Accordingly, the film water contact angle progressively increased from 81.8° to 93.8° as the pH increased from 6.0 to 7.0, indicating the formation of hydrophobic films at pH values close to neutral. In contrast, the pristine chitosan films at pH 4.0 were completely hydrophilic. FTIR observations confirmed the formation of stronger inter and intramolecular hydrogen bonds in the neutralized chitosan films than in the film at pH 4.0. The analyses of mechanical properties and microstructure further indicated that the mechanical properties and surface microstructure of the films were modulated by the pH adjustment of the chitosan suspensions. Findings from this study are relevant for the development of functional chitosan films with pH-tunable surface wettability.