Hydration-Induced Orientation and Pitch Modulation in Chiral Nematic Cellulose Nanocrystal Films Revealed by Grazing-Incidence X-ray Scattering

Rod-like cellulose nanocrystals (CNCs) self-assemble into chiral nematic liquid-crystal phases above a critical concentration. Upon drying, these structures can be preserved as iridescent solid films exhibiting structural color. Because of the hydrophilic nature of CNCs, such films are sensitive to ambient humidity. Although humidity-induced pitch modulation in CNC films is well established, the extent to which adsorbed water enables axial reorientation of CNC crystals in solid films has not been quantitatively determined. Here, plane-resolved grazing-incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are combined to directly monitor crystal rotation and correlate these structural changes with optical responses. Water is found to modulate CNC assembly through two synergistic mechanisms: (i) increasing interparticle spacing via hydration-induced expansion, and (ii) enabling axial reorientation of crystals partially from the (11̅0) to the (110) plane through enhanced rotational freedom. These transformations increase the helical pitch, producing a redshift from blue to red and near-infrared. The dual role of water as a spacer an rotational mediator explains CNC self-assembly dynamics and establishes a designframework for humidity-responsive photonic materials based on sustainable colloidal architectures.