Janus nanocellulose membrane by nitrogen plasma: Hydrophilicity to hydrophobicity selective switch

Cellulose nanofibrils are one of the keystone materials for sustainable future, yet their poor water repellency hinders their push into industrial applications. Due to complexity and poor economical outcome and/or processing toxicity of the existing hydrophobization methods, nanocellulose loses against its antagonist plastic in medical and food industries. Herein, we demonstrate for the first time the "one-side selective water-repellency activation" in nanocellulose membranes by the means of mild N₂-plasma treatment, exhibiting lowest wettability after 20 s of treatment. Hydrophobicity and accompanying Janus character were justified by the topological, chemical and structural reorganizations in cellulose nanofibrils. The findings suggest that the mechanism behind the hydrophilic/hydrophobic change primarily relies on the interplay between OH removal and appearance of SiCH₃, originating from the polysiloxanes-based substrate, as well as complementary CNH₂ groups formation. First-principles calculations show that NH₂ groups moderately increase hydrophobicity, while various SiCH₃ substitutions wholly change the character of the surface to repel water. Using nitrogen is shown to be crucial, as N(H)Si(CH₃)₃ groups induce greater hydrophobicity than simple OSi(CH₃)₃. Finally, the obtained materials absorb water on the hydrophilic side, while remaining hydrophobic on the other, exhibit high tensile strength, and protection against UV light, demonstrating applicability over wide range of applications.