Intermolecular Interactions and 3D Structure in Cellulose–NaOH–Urea Aqueous System

The dissolution of cellulose in NaOH/urea aqueous solution at low temperature is a key finding in cellulose science and technology. In this paper, ¹⁵N and ²³Na NMR experiments were carried out to clarify the intermolecular interactions in cellulose/NaOH/urea aqueous solution. It was found that there are direct interactions between OH^– anions and amino groups of urea through hydrogen bonds and no direct interaction between urea and cellulose. Moreover, Na⁺ ions can interact with both cellulose and urea in an aqueous system. These interactions lead to the formation of cellulose–NaOH–urea–H₂O inclusion complexes (ICs). ²³Na relaxation results confirmed that the formation of urea–OH^– clusters can effectively enhance the stability of Na⁺ ions that attracted to cellulose chains. Low temperature can enhance the hydrogen bonding interaction between OH^– ions and urea and improve the binding ability of the NaOH/urea/H₂O clusters that attached to cellulose chains. Cryo-TEM observation confirmed the formation of cellulose–NaOH–urea–H₂O ICs, which is in extended conformation with mean diameter of about 3.6 nm and mean length of about 300 nm. Possible 3D structure of the ICs was proposed by the M06-2X/6-31+G­(d) theoretical calculation, revealing the O3H···O5 intramolecular hydrogen bonds could remain in the ICs. This work clarified the interactions in cellulose/NaOH/urea aqueous solution and the 3D structure of the cellulose chain in dilute cellulose/NaOH/urea aqueous solution