Enhancement of the nanofibrillation of birch cellulose pretreated with natural deep eutectic solvent

In this study, we demonstrate a new bio-derived and non-toxic deep eutectic solvent composed of betaine hydrochloride (Bh) and glycerol (Gl) as a pretreatment medium for birch cellulose (Betula pendula) to prepare cellulose nanofibers (CNFs) using microfluidization. The co-solvent could readily penetrate into cellulose to swell the fibrillar structure and weaken the interaction within the hydrogen bond network. Moreover, the cationization of glycerol and cellulose by betaine hydrochloride further enhances the swelling process. All of these effects promote the nanofibrillation of cellulose and reduce the energy demand in CNF production. A high CNF mass yield of up to 72.5 % was obtained through co-solvent pretreatment using a Bh-to-Gl mole ratio of 1:2 at 150 °C for 1 h. The mole amount of betaine hydrochloride was noted to affect the nanofibrillation process and stability of the CNF suspension. The obtained CNFs possessed a cationic charge of 0.05–0.06 mmol/g, a diameter of 17–20 nm, and a degree of crystallinity of 67.7–74.4 %. The CNFs displayed good thermal stability comparable to that of the pristine cellulose. Thus, this study provides a green and efficient swelling strategy for producing CNFs with a low cationic charge density.