Hydrogen-Bond-Induced Inclusion Complex in Aqueous Cellulose/LiOH/Urea Solution at Low Temperature

• Published in: Chemphyschem Vol. 8; no. 10; pp. 1572 - 1579
• Main Authors: Cai, Jie, Zhang, Lina, Chang, Chunyu, Cheng, Gongzhen, Chen, Xuming, Chu, Benjamin
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16.07.2007; WILEY‐VCH Verlag; Wiley
• Subjects: Applied sciences; Calorimetry, Differential Scanning; cellulose; Cellulose - chemistry; Cellulose - ultrastructure; Cellulose and derivatives; Chemical Phenomena; Chemistry, Physical; Exact sciences and technology; Hydrogen Bonding; hydrogen bonds; inclusion compounds; Lithium Compounds - chemistry; Magnetic Resonance Spectroscopy; Microscopy, Electron, Transmission; Models, Chemical; Natural polymers; Physicochemistry of polymers; Solubility; Solutions; solvent effects; Spectroscopy, Fourier Transform Infrared; Temperature; Thermodynamics; transmission electron microscopy; Urea - chemistry; Water; X-Ray Diffraction; solvent effects; Cellulose; Temperature effect; Experimental study; Dissolution; hydrogen bonds; Mechanism; Intermolecular interaction; Urea; inclusion compounds; Aqueous solution; transmission electron microscopy; Lithium Hydroxides; Hydrogen bond
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200700229

It was puzzling that cellulose could be dissolved rapidly in 4.6 wt % LiOH / 15 wt % urea aqueous solution precooled to −12 °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, 13C 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. Wrapped up: Aqueous LiOH/urea precooled to −12 °C dissolves cellulose, whereas the same solvent without prior cooling does not. At low temperature, hydrogen‐bonded networks form, creating a channel inclusion complex (IC) hosted by urea hydrate, which leads to good dispersion of cellulose in the solution. The picture shows a TEM image and a schematic depiction of the IC.