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Our NMR experiments show that chitin can dissolve well in aqueous KOH through a freeze-thawing process, and the dissolution power of the alkali solvent systems is in the order of KOH > NaOH > LiOH aqueous solution, which is totally contrary to that of cellulose in the alkali aqueous solution (i.e., LiOH > NaOH ≫ KOH). In this work, we systematically study the dissolution process in KOH and KOH/urea aqueous solutions. Chitin has good solubility (solubility ~80 %) in 8.4–25 wt% KOH aqueous solution at −30 °C. The role of urea also has been investigated: unlike aqueous chitin-NaOH solutions, urea indeed enhances the solubility of chitin in KOH aqueous solutions, but the increased degree becomes unobtrusive with decreasing temperature and increasing dissolution time; the DA decline curves of chitin-KOH and chitin-KOH/urea aqueous solutions are nearly overlapping, indicating that the effect of the urea on the degree of acetylation of chitin in KOH aqueous solutions is small, similar to the NaOH/urea solvent.

The low-temperature dissolution mechanism of chitin in NaOH/urea aqueous solution has not been well established yet, especially at a temperature that lowers the crystallization temperature of both NaOH and urea. Using multiple nuclei (H-1, N-15 and Na-23) nuclear magnetic resonance (NMR) methods, we find that the ternary NaOH/urea/D2O system, different from the fully frozen binary NaOH-D2O system, exhibits solid-liquid phase coexistence below -40 degrees C, and the residual liquid phase consists of certain concentrations of NaOH (10 wt%), urea (13 wt%) and D2O at -40 degrees C. Our NMR results further demonstrate that chitin dissolves in the residual liquid phase (containing 5-10 wt% NaOH and 8-12 wt% urea) at low temperatures (-40 and -70 degrees C). An important role of urea is enhancing the low-temperature stability of NaOH aqueous solutions and then increasing the dissolution temperature range of chitin, thus providing an essential low-temperature liquid environment for chitin dissolution.

Hydrophobic cellulose nanocrystals (CNs) have been prepared by grafting isocyanate-terminated castor oil, a kind of natural vegetable oil, onto their surface. The existence of castor oil component in the modified cellulose nanocrystals was verified by Fourier transform infrared spectroscopy, solid-state C-13 NMR spectra and X-ray photoelectron spectroscopy. At the same time, X-ray diffraction and transmission electron micrographs further proved that the crystalline structure and large aspect ratio of cellulose nanocrystals were essentially preserved after chemical grafting. Furthermore, the surface of modified cellulose nanocrystals appeared to be hydrophobic as indicated by contact angle measurements. The value of the polar component of surface energy decreased from 21.5 mJ/m(2) to almost zero via grafting castor oil. These novel hydrophobic castor oil-grafted cellulose nanocrystals appear as valuable alternatives to formulate bionanocomposites with non-polar polymers for optimized performances.