Preparation of amphoteric cellulose nanofibers via betaine-oxalic acid pretreatment and high-pressure homogenization

In this study, the cellulose pulp was pretreated with a deep eutectic solvent (DES) composed of betaine and anhydrous oxalic acid with a molar ratio of 1:2, and then homogenized to produce amphoteric cellulose nanofibers (CNF). The DES is gentle for the pretreatment of cellulose pulp, but still can effectively solve blockage problem in the homogenization process. Importantly, some carboxyl groups from oxalic acid and quaternary ammonium groups from betaine were introduced onto the surface of cellulose by esterification in this pretreatment process. The CNF shows uniform size distribution, good thixotropy, and high thermal stability. In addition, the crystal structure of CNF maintained cellulose Iβ unchanged and the crystallinity index of CNF is slightly higher than that of bleached softwood kraft pulp (BSKP). Steady rheological analysis and modulus analysis show that all amphoteric CNF suspensions are pseudoplastic fluid with shear-thinning behavior, and have weak fibril network structure but still exhibit solid like behavior under low shear conditions at a concentration of 1.2%. Therefore, an efficient green pretreatment method was developed for preparation of amphoteric CNF which shows a promising application in the field of various fluid materials.