Quantitative NMR investigation on the low-temperature dissolution mechanism of chitin in NaOH/urea aqueous solution

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.