Wetting and swelling behaviour of acetylated thin chitosan coatings in aqueous media.

Chitosan nanocoatings (thickness range of 120-540 nm) were produced on glass, zinc and silicon substrates with dip-coating and spin coating techniques to study their pH-dependent wetting and swelling behaviour. The coatings were -acetylated with the methanolic solution of acetic anhydride to increase the degree of acetylation from 36 % to 100 % (according to ATR-FTIR studies). The measured contact angles of Britton-Robinson (BR) buffer solutions (pH 6.0, 7.4 and 9.0) were lower on the acetylated surfaces ( 50°), than that of their native counterparts ( 70°) and does not depend on the pH. Contrary, contact angles on the native coating deteriorated 10°-15° with increasing the pH. In addition, for native coatings, the decrease of the contact angles over time also showed a pH dependence: at pH 9.0 the contact angle decreased by 7° in 10 min, while at pH 6.0 it decreased by 13° and at a much faster rate. The constraint swelling of the coatings in BR puffer solutions was studied by scanning angle reflectometry. The swelling degree of the native coatings increased significantly with decreasing pH (from 250 % to 500 %) due to the increased number of protonated amino groups, while the swelling degree of acetylated coatings was 160 % regardless of the pH. The barrier properties of the coatings were studied by electrochemical tests on zinc substrates. The analysis of polarization curves showed the more permeable character of the acetylated coatings despite the non-polar character of the bulk coating matrix. It can be concluded that in the case of native coatings, 49 % of the absorbed water is in bound form, which does not assist ion transport, while in the case of acetylated coatings, this value is only 33 %.