Fabrication of cellulose nanofibril aerogels via emulsion templating and supercritical carbon dioxide drying

Cellulose nanofibril (CNF) aerogels were fabricated using CNF-stabilized oil-in-water (O/W) Pickering emulsions as templates, followed by sequential solvent exchange with ethanol and supercritical carbon dioxide (SC-CO₂) drying. Stable CNF-based O/W emulsions were prepared using a 0.1% CNF aqueous suspension as the continuous water phase, and either n -hexane or n -hexadecane as the dispersed oil phase, yielding average droplet sizes of 14.1 ± 4.4 μm and 10.1 ± 2.2 μm, respectively. The emulsions had an oil volume fraction of 40%, classifying them as medium internal phase emulsions. The O/W emulsion as a template compels cellulose nanofibrils to assemble around oil droplets, creating a hollow microcapsule-like structure. The emulsion-templated aerogels were then characterized for their morphology and pore structure. Thermogravimetric analysis confirmed complete removal of the oil phase in the aerogels. Compared to neat CNF aerogels and freeze-dried cryogels, the emulsion-templated aerogels contained microcapsule-like features observed in SEM images whose sizes and distributions resembled those of emulsion droplets. BET analysis showed that the emulsion-templated aerogels exhibited a maximum specific surface area (SSA) of 183 m²/g. This SSA was slightly lower than that of the neat CNF aerogel (216 m²/g), suggesting limited mesopore formation, which may be due to partial miscibility between the oil phase and ethanol during solvent exchange. Nonetheless, the emulsion-templating approach effectively produced CNF aerogels with hollow microcapsule-like features with nanofibrillar shells, creating hierarchical porosity with macro- and mesopores. To the best of our knowledge, this is the first report to integrate emulsion templating with SC-CO₂ drying to fabricate CNF aerogels, offering a promising strategy for the fabrication of bio-based materials with tailored pore structures.