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

• Published in: Cellulose (London) Vol. 22; no. 4; pp. 2221 - 2229
• Main Authors: Ru, Geying, Luo, Huan, Liang, Xinmiao, Wang, Liying, Liu, Chaoyang, Feng, Jiwen
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.08.2015; Springer Nature B.V
• Subjects: Aqueous solutions; Bioorganic Chemistry; cellulose; Ceramics; Chemistry; Chemistry and Materials Science; Chitin; Composites; Crystallization; deuterium oxide; Dissolution; Glass; Liquid phases; Low temperature; Natural Materials; nitrogen; NMR; Nuclear magnetic resonance; nuclear magnetic resonance spectroscopy; Organic Chemistry; Original Paper; Physical Chemistry; Polymer Sciences; Sodium hydroxide; stable isotopes; Sustainable Development; Temperature; urea; Ureas; NMR; Chitin; Low temperature dissolution; Mechanism; NaOH/urea aqueous solution
• ISSN: 0969-0239; 1572-882X
• DOI: 10.1007/s10570-015-0667-2

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 (¹H, ¹⁵N and ²³Na) 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 °C, and the residual liquid phase consists of certain concentrations of NaOH (10 wt%), urea (13 wt%) and D₂O at −40 °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 °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.