NMR insights into the role of urea in cellulose dissolution: a stoichiometric study of cellobiose in NaOH/urea aqueous solution

The mechanism underlying the enhanced dissolution of cellulose in NaOH/urea aqueous solutions has long been a subject of debate. In this study, high-resolution solution-state 1 H and 13 C DEPTq NMR spectroscopy was employed to probe the intermolecular interactions within this ternary system, using cellobiose as a molecular model of cellulose. Chemical shift titration experiments revealed that urea does not directly interact with cellobiose via hydrogen bonding in neutral aqueous environments, but it can form a complex with cellobiose and NaOH characterized by a specific stoichiometric ratio of 1:2:1 (cellobiose/NaOH/urea) in the alkaline regime. The urea weakens the NaOH-cellobiose interaction and effectively shields the reducing-end hemiacetal groups from the ionic environment. It is suggested that the urea-modified solution shell consisting of Na + , OH - , urea, and water is responsible for the dissolution of cellulose.