Atomistic modeling of the adsorption of benzophenone onto cellulosic surfaces

The interaction between cellulosic material and benzophenone was studied by molecular modeling. A model of the crystalline part of a native microfibril was built from previously published coordinates of the Ibeta allomorph. This model presents three faces, namely (200), (110), and (1(1) over bar 0), of about the same surface area. The energetical and geometrical characteristics of the benzophenone adsorption onto this microfibril were studied with a Monte Carlo protocol. It was shown that the interaction does occur on the three faces and was stabilized by both van der Waals and electrostatic forces. On the hydrophobic (200) face, a large number of interacting sites without specific geometry were sampled by the adsorbing molecule. The hydrophilic surfaces, (110) and (1(1) over bar 0), also have many interaction sites, but in contrast, the orientation of the adsorbed molecules is more strict. These two hydrophilic surfaces display equivalent behavior. Three surfaces (crystalline (1(1) over bar $0) and (200) and amorphous) subjected to periodic boundary conditions were also generated to study the process of the benzophenone monolayer formation. The calculated data showed that locally the amorphous surface displayed very favorable topology for benzophenone adsorption in which both van der Waals and electrostatic interactions were maximized. After fulfillment of these optimal sites, the amorphous surface behaves like the crystalline surfaces for which the adsorption sites are nonspecific. Finally, the interface between cellulose/benzophenone monolayer and water was studied by molecular dynamics. The density profiles showed that the benzophenone molecules penetrated the amorphous phase while they remained at the surface in the crystalline models.