Modelling of Congo red adsorption on the hydrophobic surface of cellulose using molecular dynamics

This study describes the interaction between crystalline cellulose and the direct dye Congo red (CR) using molecular dynamics simulations. A model of a microfibril corner of cellulose Iβ, exposing one hydrophobic (1 0 0) and the two hydrophilic (1 1 0) and (1 −1 0) surfaces was built. The energetic and geometric features of the dye adsorption were investigated at different temperatures following different initial positions of the CR. The relative positions and orientations of the CR with respect to the cellulose surface were unambiguously characterized using three translation (shift, slide, and rise) and three rotation (roll, tilt, and twist) parameters. Changes of twist and roll parameters showed that there was a tendency for the once adsorbed dye molecules, to become more planar and parallel to the cellulose surface. Several stable adsorption sites, translated laterally with respect to one another were obtained and adsorbed CR molecules were laid with their long axis either parallel or inclined by 50° with respect to the cellulose chain axis. Both vacuum and explicit water environments were considered. In this latter case, there was an increase in the energy of interaction between CR and cellulose and the adsorbed dye molecule behaved more rigidly than in vacuum case.