Nanoscale insights into cellulose-imidazolium ionic liquid interactions via atomic force microscopy

The interaction force of cellulose is a key contributing factor to the mechanical properties and functionality of cellulose-based products, while quantitative understanding the nanoscale interaction of cellulose is challenging through experimentation. Herein, atomic force microscope (AFM) force spectroscopy was innovatively used to quantitatively measure the intermolecular forces of cellulose in air/water/ionic liquids (ILs), respectively. The results showed that the intermolecular forces were reduced by 60.3% and 34.0%, when the hydroxy groups of cellulose were replaced by methyl and hydroxyethyl groups, demonstrating the dominance of hydrogen bond and van der Waals forces in cellulose cohesion. Meanwhile, cation-anion of ILs cooperativity significantly decreased the interaction forces with 84.5% (39.13 nN → 6.08 nN), further demonstrating that the hydrogen bond formation ability, hydrophobic and van der Waals forces play significant roles in cellulose dissolution. Additionally, a dual-site interaction mechanism was proposed by combining the results of NMR and molecular dynamics simulations. Furthermore, the swelling to single-chain stripping evolution of fiber in ILs was visualized by in-situ AFM, and used as the spinning solution to successfully prepare regenerated cellulose fibers with good mechanical properties. Therefore, this work not only provided important guidance for new insights into the interaction mechanism of cellulose at the nanoscale, but also for screening and designing ILs for cellulose dissolution.