Ultra-porous nanocellulose foams with tailored wetting properties

Cellulose nanofibrils are biodegradable, renewable, intrinsically amphiphilic and possess outstanding mechanical properties, thanks to their high weight-to-strength ratio, obtained through cellulose pulp disintegration. Nanocellulose foams are attracting an increasing attention for their potential in a wide variety of applications, where low density and high surface areas are required, including filtration processes, gas adsorption and selective liquid absorption for remediation of polluted areas. One commonly used technique to fabricate nanocellulose foams is based on ice templating, through the well-known freeze-drying process, in which an aqueous suspension of cellulose nanofibrils is frozen by liquid nitrogen and vacuum-dried, to obtain dry foams. However, vacuum drying is a high energy-consuming step, and currently represents a bottleneck for the process scale-up, needed for industrially relevant applications. As such, the current study approaches the issue of how to improve the ice template strategy, avoiding the vacuum-drying step, while achieving extremely high porosity. To address the issue, we present a novel straightforward freeze-thawing-drying procedure, taking advantage of urea as additive to the aqueous cellulose nanofibril suspension and of solvent exchange. Such method allows the production of mechanically stable, lightweight cellulose-based structures, avoiding foam collapse, upon facile thawing-drying steps. Functionalization of porous foams imparts a hydrophobic-oleophilic wetting property, which can be used for selective oil absorption, as demonstrated by impact tests with multiphase water-in-oil compound drops