Inclusion complex formation of cellulose in NaOH-thiourea aqueous system at low temperature

It is puzzling that cellulose could be dissolved rapidly in 9.5 wt.-% NaOH/4.5 wt.-% thiourea aqueous solution pre-cooled to -5 degrees C, but is could not be dissolved in the same solvent mixture without prior cooling. To understand this phenomenon, changes in the structure and properties of NaOH and thiourea in water as well as solution behaviours of celluose in the solvent at low temperatures were investigated by means of C-13 NMR, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, transmission electron microscopy (TEM), and viscometry. The results indicated that an association of NaOH and thiourea in water through the hydrogen bonding strengthened with a decrease in temperature. The low temperature created a new hydrogen-bonded network structure associated with NaOH, thiourea, water cluster, and cellulose, leading to the cleaving of the close chain packing of cellulose. The results proved that a good dispersion of cellulose occurred at a low temperature as a result of the formation of an inclusion complex (IC) hosted by thiourea and NaOH, in which cellulose chain associated with NaOH hydrates as guest was encaged. IC could aggregate to form a sphere having a mean size of 60-160 nm. TEM observations, for the first time, revealed the shape and size of IC and its aggregate. The stability of the cellulose solution could be related to the thiourea IC.