Influence and mechanism of NaOH concentration on the dissolution of cellulose and extraction of CNF in alkaline solvents at 15 °C

Alkaline aqueous solutions facilitate the rapid extraction of high-performance cellulose nanofiber (CNF) at 15 °C. However, the role and mechanism of NaOH in this process remain unclear. This study investigated the dissolution process of cellulose by controlling the NaOH concentration (1–7 wt%) at 15 °C and explored the effect and underlying mechanism of the NaOH concentration on cellulose dissolution in alkaline solvents. The results showed that the addition of NaOH strengthened the hydrogen bonding network of the solvent, whereas the addition of urea and thiourea promoted the dissolution of cellulose. The higher the concentration of NaOH is, the smaller the volume of hydrate molecules formed; smaller molecules more readily enter the cellulose molecules and form new hydrogen bonds with cellulose molecular chains, thereby destroying the original hydrogen bonds between cellulose molecules and their crystal structure. During dissolution, cellulose decomposes into microfibrils, and nanosized cellulose separates; moreover, the diameter and length of the CNF decrease with increasing NaOH concentration. When the concentration of NaOH reaches 7 %, the cellulose is completely broken down into CNF, and the yield is >60 %. As dissolution progresses, the crystallinity of cellulose first increases and then decreases (75.95 %–81.00 %–77.02 %), but its crystal size remains unchanged.