Nanostructural Manipulation of Polyphenol Coatings for Superwetting Membrane Surfaces

Functional coatings have gained significant attention in multiple environmental and energy-related research fields. One of the coatings with superwetting surfaces has received significant interest, owing to the favorable properties like self-cleaning and antifouling as well as the roles it plays in processes of water harvesting and oil–water separation. Hydrophilic polyphenol molecules show good adhesion to different substrates and provide multiple interactive sites, which serve as building blocks for the preparation of superwetting coatings. In this study, to realize the controlled formation of a polyphenol-based coating and to demonstrate the nanostructural dependence of its superwetting performance, tannic acid (TA) complexed with cations was employed to construct coating networks with either nanorough or nanosmooth surface morphology through a layer-by-layer (LbL) self-assembly method. Both nanostructures could be precisely controlled by adjusting the TA concentration and number of LbL cycles to observe the evolution of the wetting state of the coating. More importantly, while the nanosmooth and nanorough coatings exhibited similar surface chemistry, pore sizes, and superwetting properties, the separation efficiency for oil-in-water emulsions using the membrane with the nanorough coating is 2–5 and 2–10 times that of the one with a nanosmooth coating and the pristine one without a coating, respectively. The experimental results confirmed that the nanorough coating structure contributed to the superwetting state of the membrane surface and, therefore, possessed a stronger ability to repel oil than the nanosmooth coating during the separation process. This work demonstrates a novel strategy for the molecular self-assembly of polyphenols and may provide guidance for designing superwetting coatings.