Cellulose is the most abundant renewable biopolymer in nature. It has particularly high thermal, mechanical and chemical stabilities (Q.-F. Guan et al., 2020; Guan, Yang, Han, Ling, & Yu, 2020). Its abundant hydroxyl and hemiacetal functional groups make it an ideal substance for chemical reactions and modifications (Klemm, Heublein, Fink, & Bohn, 2005; Pourmoazzen et al., 2020). Nanocelluloses include cellulose nanocrystals, cellulose nanofibers and bacterial cellulose nanofibers (Y. Habibi, 2014; Kontturi et al., 2018). The main benefits attributed to nanocellulose are its impressive mechanical properties, reinforcing capabilities, abundance, low density, biodegradability and ability to form hydrogen bonds, which enable interparticle network formation (Dufresne, 2013; Eyley & Thielemans, 2014).
Despite all of these promising characteristics of cellulose, its high hydrophilicity is a limitation for its widespread application. Changing its surface energy through hydrophobic modification is an effective method for solving this problem (Eyley & Thielemans, 2014; Youssef Habibi, Lucia, & Rojas, 2010). Methods that involve the use of surfactants (Feizi & Fatehi, 2021), castor oil (Shang et al., 2018), silylation (Khanjanzadeh et al., 2018), acetylation (Lin, Huang, Chang, Feng, & Yu, 2011), amidation (Kebede, Imae, Sabrina, Wu, & Cheng, 2017; Ramaraju, Imae, & Destaye, 2015), esterification (Q. Chen, Shi, Chen, & Cai, 2020), polycaprolactone diol (Zhou et al., 2018) and quaternary ammonium salts (Salajková, Berglund, & Zhou, 2012) are the most commonly used methods for the hydrophobic modification of cellulose.
The amidation reaction, commonly known as amine grafting, is one of the most used transformation reactions in organic chemistry and has been successfully applied to modify nanocellulose surfaces (Le Gars, Delvart, Roger, Belgacem, & Bras, 2020; Pattabiraman & Bode, 2011). The most typical method of amine grafting includes forming an N-hydroxysuccinimidyl ester to activate the carboxylic acid moieties of TEMPO-oxidized nanocellulose surfaces, which is then utilized to react with a primary amine to produce the amide product (Araki, Wada, & Kuga, 2001; Filpponen & Argyropoulos, 2010; Le Gars et al., 2020). Octadecylamine (ODA) has been widely utilized for hydrophobic applications due to its long alkyl chain and reactive amino group (Dunlop, Sabo, Bissessur, & Acharya, 2021; Li, Wang, Wang, Qin, & Wu, 2019; Liu et al., 2022; Wang, Hui, & Su, 2022). Roy et al. (Roy et al., 2018) prepared a superhydrophobic material by using CNFs and ODA in glutaraldehyde (GA) coupling. Majdoub et al. (Majdoub et al., 2021) used carboxylated cellulose nanocrystals (CNCs) that were hydrophobically modified via a simple and coupling agent-free amidation reaction using ODA as the chemical reagent. In this activation process, 1-ethyl-3-(3-dimethylamino- propyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) are usually used as coupling agents. However, multistep requirements, such as solvent exchange and EDC and NHS coupling, are needed. In addition, the reagents are toxic, high cost and harmful to the environment.
Before hydrophobic modification, oxidizing cellulose with 2,2,6,6-tetramethylpiperidinyloxyl radical (TEMPO) is desirable, as it provides more active groups for further modification (Eyley & Thielemans, 2014). In this work, we investigate a brand-new, environmentally friendly method for hydrophobizing CNF surfaces in water. First, TEMPO-oxidized CNFs (TOCNFs) were crosslinked by citric acid to generate more carboxyl groups. Then, utilizing ODA as a chemical reagent, a straightforward and coupling agent-free amidation process was used to hydrophobically modify the carboxylated CNFs. Fourier transform infrared spectroscopy (FTIR) was used to investigate the surface modification. The degree of crystallinity of the cellulose derivatives was determined using X-ray diffraction (XRD). The size and morphology of the original and modified CNFs were examined using scanning electron microscopy (SEM). The relative hydrophobicity of the modified CNFs was analysed through water contact angle (WCA) analysis. Additionally, depending on the degree of amidation modification, oil-resistance paper and pH-responsive indicator paper were used to study the two modified CNFs (ODA-CA-CNF, ODA-CNF), respectively.