Surface chemistry-dominated underwater superoleophobic mesh with mussel-inspired zwitterionic coatings for oil/water separation and self-cleaning

Mussel-inspired surface modification has been received great attention due to the universal adhesive properties of catechols for fabrication of multifunctional coatings, especially for gluing hydrophilic polymers to fabricate underwater-superoleophobic materials utilizing in oil/water separation. Despite the extensive research carried out on this topic, the similarity and discrepancy between catecholamine and catecholic amino acid on surface modification and post functionalization have not been fully addressed yet. In this work, underwater superoleophobic surfaces have been successfully developed by a two-step dip-coating method with mussel-inspired coatings and subsequent zwitterionic sulfobetaine methacrylate (SBMA) grafting onto stainless steel meshes and used in oil/water separation. Here, dopamine and 3,4-dihydroxy-L-phenylalanine were both served as mussel-adhesives. More specifically, small molecule zwitterion rather than polyzwitterion was functionalized onto mussel-inspired coatings to minimize the effect of surface topography on surface wettability. The modified surfaces were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle measurements to observe the surface morphology, estimate the surface roughness, and evaluate the wettability, respectively. It showed that SBMA modified meshes with polydopamine (PDA) or poly(3,4-dihydroxy- L-phenylalanine) (PDOPA) layer possessed quite different surface roughness, while both presented excellent oil repellency in water with underwater oil contact angles of 153 degrees-160 degrees, indicating a less dependence on surface roughness. Although by using the small molecule as the hydrophilic functionalized groups, the asprepared meshes exhibited good self-cleaning and oil/water separation performance (separation efficiency > 98% for hexane and > 97% for soybean oil) and outstanding recyclability with 98% separation efficiency after 30 cycles. This method provides insight into different properties of polycatechols and simplifies the fabrication process through the use of small molecule zwitterion rather than zwitterionic polymer. Besides, the modified meshes also exhibited excellent stability for long-term use. The resulting underwater superoleophobicity and robust self-cleaning ability promise an ideal candidate for oil/water separation and oil contamination restriction.