Cai Jie, Zhang Lina, Chang Chunyu, Cheng Gongzhen, Chen Xuming, Chu Benjamin
Department of Chemistry, Wuhan University, Wuhan 430072, China.
Chemphyschem. 2007 Jul 16;8(10):1572-9. 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 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, 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.
令人困惑的是，纤维素能迅速溶解于预冷至-12℃的4.6 wt% LiOH/15 wt%尿素水溶液中，而在未经预冷的相同溶剂中却不能溶解。为阐明这一重要现象，通过激光光散射、13C核磁共振光谱、差示扫描量热法、傅里叶变换红外光谱、广角X射线衍射和透射电子显微镜（TEM）研究了不同温度下LiOH和尿素在水中的结构和物理性质以及纤维素在LiOH/尿素水溶液中的结构和物理性质。结果表明，LiOH、尿素和水之间可形成氢键网络结构，且该结构随温度降低而变得更稳定。LiOH水合物在低温下通过形成新的氢键破坏纤维素的链堆积，从而形成与LiOH、水簇和纤维素相关的相对稳定的络合物。预冷时，尿素形成的通道包合物（IC）可将纤维素大分子包裹在LiOH/尿素溶液中，因此为纤维素形成良好分散体提供了理论依据。TEM观察首次显示了干燥形式的通道IC。低温步骤在纤维素与小分子之间氢键的转移中起重要作用，导致大分子在水溶液中溶解。
