The interaction between water vapor and chitosan II: Computational study

Different models were used for both the exposed structures of chitosan and for water in the vapor state in the molecular modeling of the first step of adsorption of water on chitosan, taking into account water clustering phenomena upon adsorption, the water structure, and the effect of temperature on water structure (i.e., the presence of dimers or monomers). The degree of acetylation (DA) influences markedly the first step of water vapor adsorption on chitosan. Modeling results on the first step of adsorption were analyzed taking into account the ratio water dimer/water monomer and the species predominant as a function of ambient conditions (i.e., relative humidity (RH) and temperature (T)). From the experimental point of view, the ratio of water dimer/water monomer has a different effect on chitosans depending on its DA and adsorption temperature. We found that the adsorption of water monomer on a model of hydrated chitosan is favoured in almost −84 kJ mol−1 of steric energy, whereas the adsorption on dehydrated chitosan by only −21 kJ mol−1. The water-chitosan surface distance found in this work was 0.2 nm, whilst the literature values are between 0.245 nm and 0.309 nm for the hydrated chitosan crystalline structure. It was evident that at least two tetramers are required to represent the surface of chitosan.