A molecular scale analysis of TEMPO-oxidation of native cellulose molecules

Abstract

The native cellulose, through TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation, can be converted into individual fibers. It has been observed that oxidized fibers disperse completely and individually in water. It is believed that electrostatic repulsive forces might be responsible for such observations. In order to study the TEMPO-oxidation of cellulose molecules, we used Density Functional Theory (DFT) calculations and Flory-Huggins theory combined with molecular dynamics (MD). The surface electrostatic potential in native cellulose and TEMPO-oxidized cellulose were calculated using DFT calculations. We found that TEMPO-oxidized cellulose accommodates a threefold screw conformation where the negatively charged (-COO-) functional groups are pointed away from the surface in all spatial directions. This spatial orientation causes that TEMPO-oxidized cellulose molecules repulse each other due to strong negatively charged surface. At the same time, the spatial orientation increases the hydrophilicity in TEMPO-oxidized cellulose molecules. These observations explain the improved dispersion in water and separability of TEMPO-oxidized cellulose molecules. We obtained large and positive Flory-Huggins interaction parameters for TEMPO-oxidized cellulose molecules indicating their higher dispersion once in water.

Influence of reaction variables on the surface chemistry of cellulose nanofibers derived from palm oil empty fruit bunches

“…24 Recent theoretical studies have shown that COO − groups in TOCN dispersions can cause a triple screw conformational change, increasing the repulsion between the cellulose particles and their hydrophilicity. 71 In our case, TOCN could potentially undergo conformational changes affecting the surface COO − density and the z potential value. Another factor to consider is the presence of silica (SiO 2 ) in the TOCN suspensions from the EFB matrix (Fig.…”

A review of nanocellulose modification and compatibility barrier for various applications

“…Nevertheless, a previous study found that adding a significant number of sodium carboxylate groups to the surface of nanocellulose increases the possibility of aldehyde group formation. 160 Few studies of TEMPO-mediated oxidation techniques have been undertaken to address the compatibility issues between nanocellulose and polymer matrix. One of them employs the reaction condition of TEMPO/NaClO/NaBr at pH 10 in the presence of water.…”

“…158,159 The generation of interfibrillar repulsive forces of dense surface charges causes individualization between fibrils, which aids in their liberation and retention in the stable colloidal form. 160,161 During the reaction, the C6 hydroxy groups (C6-OH) of nanocellulose (-CH 2 OH) preferentially oxidize to sodium carboxylate groups (-COONa). 132,161 The oxidation process of C6-OH groups initiates the dissociation of sodium atoms from -COONa, leaving the negatively charged carboxylate (-COO-) groups on TEMPO-oxidized nanofibrils, which later act as scaffolds, aiding in the assembly of the metal ions via an ion exchange process to increase nanocellulose functionalities.…”

“…Nevertheless, a previous study found that adding a significant number of sodium carboxylate groups to the surface of nanocellulose increases the possibility of aldehyde group formation. 160…”

A molecularly enhanced proof of concept for targeting cocrystals at molecular scale in continuous pharmaceuticals cocrystallization

“…Solving the above problem requires reliable insight into the interplay of 1) the critical raw material attributes, 2) the critical process parameters, and 3) the drug product’s critical quality attributes. This in turn necessitates models that utilize a bottom-up approach ( 32 , 33 ) [where the material properties are calculated from scratch using, e.g., density functional theory [DFT] and molecular dynamics [MD] ( 34 )] to establish a process design space without prior experimental information. After producing this design space, a process optimization strategy can be synthesized for any specific operational parameters.…”