Strong adhesion and cohesion of chitosan in aqueous solutions

Abstract

Chitosan, a load-bearing biomacromolecule found in the exoskeletons of crustaceans and insects, is a promising biopolymer for the replacement of synthetic plastic compounds. Here, surface interactions mediated by chitosan in aqueous solutions, including the effects of pH and contact time, were investigated using a surface forces apparatus (SFA). Chitosan films showed an adhesion to mica for all tested pH ranges (3.0-8.5), achieving a maximum value at pH 3.0 after a contact time of 1 h (Wad ~ 6.4 mJ/m(2)). We also found weak or no cohesion between two opposing chitosan layers on mica in aqueous buffer until the critical contact time for maximum adhesion (chitosan-mica) was reached. Strong cohesion (Wco ~ 8.5 mJ/m(2)) between the films was measured with increasing contact times up to 1 h at pH 3.0, which is equivalent to ~60% of the strongest, previously reported, mussel underwater adhesion. Such time-dependent adhesion properties are most likely related to molecular or molecular group reorientations and interdigitations. At high pH (8.5), the solubility of chitosan changes drastically, causing the chitosan-chitosan (cohesion) interaction to be repulsive at all separation distances and contact times. The strong contact time and pH-dependent chitosan-chitosan cohesion and adhesion properties provide new insight into the development of chitosan-based load-bearing materials.

Figures

Figure 1

Chemical structures of chitin and chitosan. Chitin and Chitosan are random copolymers of d-glucosamine (m) and N-acetyl-D-glucosamine (n) unit. (Chitin n>m;Chitosan m>n). Red circles indicate the hydrogen bonding sites.

Figure 2

Force profiles between two physisorbed chitosan surfaces in (A) pH 3.0 and 150 mM acetic acid solution, (B) pH 6.5 and 150 mM sodium acetate buffer solution, and (C) pH 8.5 and 150 mM phosphate buffer solution with the contact times of 5 sec, 2 min, 10 min, and 1 hr. All force curves are the representative results at each experiment conditions.

Figure 3

AFM tapping mode images of chitosan films on freshly cleaved mica deposited at pH3.0 and incubated for 1hr (A) at pH 3.0, (B) at pH 6.5, and (C) at pH 8.5.

Figure 4

Force profiles between physisorbed chitosan surfaces against mica in (A) pH 3.0 and 150 mM acetic acid solution, (B) pH 6.5 and 150 mM sodium acetate buffer solution, and (C) pH 8.5 and 150 mM phosphate buffer solution with the contact times of 5 sec, 2 min, 10 min, and 1 hr. All force curves are the representative results at each experiment conditions.

Figure 5

Normalized cohesion/adhesion force (Energy) vs contact time curve for symmetric (chitosan vs chitosan) and asymmetric (chitosan vs mica) cases under the applied load of ~4 mN/m at pHs of 3.0 (black), 6.5 (red) and 8.5 (blue). Arrows indicate critical contact time, tcrit, where surfaces become adhesive.

Figure 6

Approach curves for symmetric and asymmetric cases at pHs of 3.0, 6.5, and 8.5. The calculated Debye lengths were ~2.5, ~0.8, and ~0.8 nm, respectively. The larger decay lengths compared to the Debye lengths indicate that steric contribution is significant on approach for all cases.

Figure 7

(A) Molecular structure of chitosan (left and center) extended two fold helix and (right) relaxed two-fold helix as determined by X-ray crystallography (B) Schematic representation of the putative cohesion (top right, chitosan vs chitosan) and adhesion (bottom right, chitosan vs mica) mechanisms of chitosan film in wet conditions. Atoms color coded as: cyan:carbon; red:oxygen; blue:nitrogeon; white:hydrogen; yellow:silicon. Putative hydrogen bonds are shown as dotted lines.