Layer-by-layer modification of cellulose aerogels to optimize capillary spreading rates and liquid holding capacity

Due to their excellent wetting and liquid-spreading properties, cellulose-based aerogels have shown great potential as absorbent materials in many applications. However, there is still a very limited understanding of how the aerogels should be tailored to optimize liquid spreading and liquid storage properties. The present work focuses on characterizing liquid spreading at short contact times and tailoring the surfaces within the aerogel to increase the spreading properties. Aerogels from periodate oxidized cellulose nano fibrils (CNFs) were freeze-linked to attain wet stability. Subsequently, they were modified with the layer-by-layer (LbL) assembly method using poly(diallyldimethylammonium chloride) (PDADMAC) and well-defined SiO₂ nanoparticles to change their surface properties. The morphology of the untreated and treated aerogels, as determined from SEM images, indicates a complete surface coverage of PDADMAC/SiO₂ bilayers on the inner surfaces of CNF aerogels, showing that the LbL-treatment can be used to tailor the aerogel, i.e. to increase the specific surface area of the aerogel, by changing the number of bilayers. It has also been shown that the horizontal liquid spreading rate increases significantly after surface modification. In addition, a theoretical analysis of the spreading results indicates that this is due to the increase in the specific surface area of the surface-modified aerogels. Moreover, the spreading rate can be gradually tailored by changing the number of bilayers and the dimensions of the nanoparticles. Furthermore, we provide a new method to calculate the specific surface area of aerogel materials by combining the experimentally determined liquid spreading rate and a version of the well-known Kozeny–Carman equation.