Hydrogen-bond-induced inclusion complex in aqueous cellulose/LiOH/urea solution at low temperature

It was puzzling that cellulose could be dissolved rapidly in 4.6wt%LiOH/15wt% urea aqueous solution precooled to -12 degrees C, whereas it could not be dissolved in the same solvent without prior cooling. To clarify this important phenomenon, the structure and physical properties of LiOH and urea in water as well as of cellulose in the aqueous LiOH/urea solution at different temperatures were investigated by means of laser light scattering, C-13 NMR spectroscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and transmission electron microscopy (TEM). The results reveal that a hydrogen-bonded network structure between LiOH, urea, and water can occur, and that it becomes more stable with decreasing temperature. The LiOH hydrates cleave the chain packing of cellulose through the formation of new hydrogen bonds at low temperatures, which result in a relatively stable complex associated with LiOH, water clusters, and cellulose. A channel inclusion complex (IC) hosted by urea could encage the cellulose macromolecule in LiOH/urea solution with prior cooling and therefore provide a rationale for forming a good dispersion of cellulose. TEM observations, for the first time, showed the channel IC in dry form. The low-temperature step played an important role in shifting hydrogen bonds between cellulose and small molecules, leading to the dissolution of macromolecules in the aqueous solution.