Structure of cellulose/direct dye complex regenerated from supercritical water

The regeneration of cellulose from supercritical water in the presence of direct dyes was studied by small- and wide-angle synchrotron X-ray scattering and cryo-transmission electron microscopy to understand the effects of dyes on the structure formation of cellulose. In addition, the interactions between cellulose and the direct dyes were characterized using molecular dynamics simulations. Peaks corresponding to cellulose II crystals were observed in the wide-angle X-ray diffraction pattern of cellulose regenerated from supercritical water without dyes, whereas these peaks were not observed in the diffraction patterns of samples with direct dye (Direct Red 28 or Direct Blue 1). This result indicated that the direct dyes prevented the crystallization of regenerated cellulose. The results of the molecular dynamics simulations indicated that the planes of glucose rings interacted with the aromatic moieties of the dyes and that the sulfonate groups of the dye molecules interacted with the hydroxyl groups of cellulose. In addition, the CH groups of the glucose rings and aromatic moieties of the dyes (e.g., naphthalene and biphenyl moieties) interacted weekly. When cellulose regenerates from solution, cellulose sheet structures formed via hydrophobic interactions appear as the initial structure. The direct dyes were found to affect the formation of this cellulose sheet structure because cellulose molecularly dissolved in supercritical water. In the Kratky plots for small-angle X-ray scattering, a peak was clearly observed for the cellulose and cellulose/DR28 samples in the region of smaller q (<0.5), indicating that the nanoscale assembly structures dispersed in these systems. Bundled sheet-like and twisted ribbon-like structures were observed in the supernatants of the cellulose and cellulose/DR28 samples. These dispersed structures were considered to be intermediates in the structural formation of cellulose.