Intermolecular
Interactions and 3D Structure in Cellulose–NaOH–Urea
Aqueous System
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Abstract
The dissolution of cellulose in NaOH/urea
aqueous solution at low
temperature is a key finding in cellulose science and technology.
In this paper, <sup>15</sup>N and <sup>23</sup>Na 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<sup>–</sup> anions and amino groups of urea through
hydrogen bonds and no direct interaction between urea and cellulose.
Moreover, Na<sup>+</sup> ions can interact with both cellulose and
urea in an aqueous system. These interactions lead to the formation
of cellulose–NaOH–urea–H<sub>2</sub>O inclusion
complexes (ICs). <sup>23</sup>Na relaxation results confirmed that
the formation of urea–OH<sup>–</sup> clusters can effectively
enhance the stability of Na<sup>+</sup> ions that attracted to cellulose
chains. Low temperature can enhance the hydrogen bonding interaction
between OH<sup>–</sup> ions and urea and improve the binding
ability of the NaOH/urea/H<sub>2</sub>O clusters that attached to
cellulose chains. Cryo-TEM observation confirmed the formation of
cellulose–NaOH–urea–H<sub>2</sub>O 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