Atomistic insights into the humidity response of nanocellulose: a molecular dynamics study.

Context: TEMPO-oxidized cellulose nanofibers (TOCNFs) show significant potential for developing high-performance resistive humidity sensors due to their hydrophilicity and structural adaptability. However, the underlying atomic-scale mechanisms governing their humidity response remain poorly understood. Using molecular dynamics simulations, this study investigates how crystal facets, nanopore widths, and humidity levels influence the surface wettability, water permeability, and swelling of TOCNFs. Our findings reveal that the (1 0) crystal facet exhibits the highest hydrophilicity, while the (100) facet is the least hydrophilic. Narrower nanopores and more hydrophilic facets enhance capillary adsorption, significantly influencing water penetration depth. Additionally, nanopore swelling is highly dependent on the crystal facet, with the (1 0) facet showing the most pronounced expansion. These insights provide a foundation for designing high-performance TOCNF-based humidity sensors.

Methods: The humidity response of TOCNFs is simulated using the large-scale atomic molecular massively parallel simulator (LAMMPS) package with the OPLS-AA force field to describe interatomic interactions. The open-source visualization tool OVITO is employed to visualize the atomic configurations.