On the weak dependence of water diffusivity on the degree of hydrophobicity of acetylated hydroxypropyl xylan

We used molecular dynamics (MD) simulation to study the diffusion of water at low concentrations in a series of chemically modified xylans, a major hemicellulose, including hydroxypropyl xylan (HPX) and acetoxypropyl xylan (APX) which is essentially acetylated HPX, with different degrees of acetylation (i.e., different degrees of hydrophobicity) at 400 K, a temperature well above the glass transition temperatures of the materials. We used one HPX and three APX models. The three APX models were constructed using the HPX model by substituting 1, 2 or 3 hydroxyl moieties on its repeating units, respectively. mulation results showed that the diffusivity of water at low concentrations in the chemically modified xylans decreased slightly (∼20%) from HPX to APX with 3 acetylated hydroxyl moieties, a trend that has been experimentally observed for xylans and other cellulosic type materials. Further data analysis shows that acetylation decreases the ability of the xylan to form hydrogen bonds with water and its degree of swelling. And these two factors exert opposite effects on the diffusivity of water. In particular, the first factor increases the mobility (lower activation energy) of the water molecules, while the second factor reduces the free volumes available for diffusion, thereby decreasing the water mobility. This finding implies that it is not likely to obtain orders of magnitude change to the water diffusivity simply by the acetylation of all hydroxyl moieties on xylan. The high degree of swelling observed for HPX compared to those of APXs is attributed to the fact that many hydrogen bonds in HPX are broken by water. It is interesting to note that water in most of the hydrogen bonds formed between water and xylans acted as hydrogen bond acceptors rather than donors.