Cellulose Nanocrystal Aqueous Colloidal Suspensions: Evidence of Density Inversion at the Isotropic Liquid Crystal Phase Transition
Rod-like cellulose nanocrystal aqueous colloidal suspensions can be prepared from eucalyptus kraft pulp. In article number 2108227, Maria Helena Godinho and co-workers show the formation of a liquid crystalline (LC) phase atop an isotropic phase, inverted from the typical high-density LC phase. The bounciness of the particles is attributed to the presence of air. The behavior observed is an indication that the LC phase is an indirect way to reveal cellulose nanocrystal/water interactions.Abstract
"The colloidal suspensions of aqueous cellulose nanocrystals (CNCs) are known to form liquid crystalline (LC) systems above certain critical concentrations. From an isotropic phase, tactoid formation, growth, and sedimentation have been determined as the genesis of a high-density cholesteric phase, which, after drying, originates solid iridescent films. Herein, the coexistence of a liquid crystal upper phase and an isotropic bottom phase in CNC aqueous suspensions at the isotropic–nematic phase separation is reported. Furthermore, isotropic spindle-like domains are observed in the low-density LC phase and high-density LC phases are also prepared. The CNCs isolated from the low- and high-density LC phases are found to have similar average lengths, diameters, and surface charges. The existence of an LC low-density phase is explained by the presence of air dissolved in the water present within the CNCs. The air dissolves out when the water solidifies into ice and remains within the CNCs. The self-adjustment of the cellulose chain conformation enables the entrapment of air within the CNCs and CNC buoyancy in aqueous suspensions."