Cellulose nanofibril aerogels derived from Pickering emulsion templates with anisotropic droplet sizes

Abstract

Nanocellulose-based aerogels are considered as an ideal green absorbent because of their excellent absorptive performance as well as the renewability, biodegradability, and abundance of the raw material. Emulsion templating allows for the control of the overall porosity and morphology of the resultant aerogel, which may enhance the absorptive performance of the aerogel. This study explored the effect of anisotropic droplet sizes in an emulsion template on the microstructure and oil absorption performance of cellulose nanofibril (CNF) aerogels. By employing a two-step emulsification method—initial hand shaking followed by ultrasonication—two distinct droplet sizes, i.e., > 50 µm and < 20 µm, of n-hexane-CNF oil-in-water (O/W) Pickering emulsions were produced at CNF concentrations of 0.1 – 0.5 %. These O/W emulsions were subsequently freeze-dried to produce emulsion-templated CNF aerogels. The resultant CNF aerogels exhibited densities ranging from 2.3 to 6.0 mg/cm³ that were lower than the theoretical density of cellulose aerogels, possibly due to the presence of the large droplets (>50 µm) in the emulsion template. The density of the aerogels approached to the theoretical value at the CNF concentration of 0.5 %, which can be attributed to the higher prevalence of the small droplets (<20 µm) in the emulsion template at increased CNF concentrations. Scanning electron microscopy and the Brunauer-Emmett-Teller analysis indicated that the resulting CNF aerogels retained the pore size and distribution of the emulsion template, with a reduced specific surface area compared to the emulsion-templated CNF aerogels with isotropic droplet size. The CNF aerogels exhibited the highest absorption capacities of 226 g/g and 169 g/g for chloroform and n-hexadecane, respectively. These findings demonstrate that controlling droplet sizes in emulsion templating can significantly influence the microstructure of resulting CNF aerogels, highlighting their potential as highly effective and sustainable oil absorbents.