Influence of post-oxidation reactions on the physicochemical properties of TEMPO-oxidized cellulose nanofibers before and after amidation

During TEMPO-mediated oxidation of cellulose, primary alcohols on the surface ideally undergo oxidation to carboxylic acids. However, it is well known that some of these hydroxyl groups partially oxidize to aldehydes. To increase the degree of oxidation (DO) of nanocellulose, these unreacted aldehydes can be fully oxidized to carboxylic acids employing various strategies. We performed post-oxidation reactions using NaClO₂, on TEMPO-Oxidized Cellulose Nanofibers (TOCN) to investigate how this reaction influenced the physicochemical properties of nanocelulose before and after amidation with octadecylamine (ODA). We report on an increase in the content of carboxylate in TOCN (1.57 mmol/g of cellulose) up 40% after post-oxidation with NaClO₂ (2.20 mmol/g of cellulose). These results indicate that with TEMPO-mediated oxidation, 1.57 mmol of hydroxymethyl groups per gram of cellulose were completely oxidized to –COOH, and after treatment with NaClO₂, about 0.63 mmol of partially-oxidized alcohols (aldehyde groups) on TOCN were fully oxidized to COOH. Theoretical HLB values of 11.04 and 11.13 for TOCN and post-oxidized TOCN (TOCN-P) indicate that these materials behave as hydrophilic o/w surfactants or emulsifying agents with high affinity for polar phases. They also form highly stable dispersions in water due to an increased charge density as indicated by ζ-potential values of -56 mV for TOCN and -64 mV for TOCN-P. Low contact angles (CA) of 24º and 21º for TOCN and TOCN-P films confirm their hydrophilic nature. After amidation reactions with ODA, we observe a 73% and an 82% decrease in carboxylate content for TOCN and TOCN-P. These values can be related with the degree of coupling (DC) leading to amide formation on the nanocellulose surface, i.e., about 1.14 and 1.80 mmol of amide units were coupled per gram of cellulose on TOCN-ODA and TOCN-P-ODA. ζ-potential values decreased 34% and 52% after amidation, to -37 mV and -31 mV for TOCN-ODA and TOCN-P-ODA, indicating a low stability of these materials in water dispersions. Theoretical HLB values < 8.6 suggest the ability of these materials to act as wetting/spreading agents with poor dispersibility in water. In addition, high CA (> 125º) films illustrate the effect of octadecylamine alkyl chains on the hydrophobic properties of the nanocellulose’s surface. Thermal analysis indicates that amidation reactions can effectively shield in the nanocellulose’s surface from thermal degradation from by increasing the decomposition temperature from around 296 °C for “naked” TOCN to > 330 °C for the “protected” amidated nanocellulose.