Self-cleaning poly(L-dopa)-based coatings with exceptional underwater oil repellency for crude oil/water separation

As a new derivative material of polydopamine, poly(L-dopa) (PDopa) possesses abundant catechol, carboxyl and amino groups, endowing a new platform for construction of multifunctional coatings. In the present work, negatively charged PDopa was synthesized on a large scale by self-polymerization of L-3,4-dihydroxyphenylalanine (L-Dopa) in aqueous solutions. The PDopa-based coatings were fabricated via layer-by-layer assembly of positively charged polyelectrolyte and negatively charged PDopa. The as-obtained coatings demonstrate excellent underwater oil repellency with oil contact angles of 165 ± 2° and ultra-low contact angle hysteresis arising from electrostatically-induced and hydrogen bonding-induced hydration layers formed between functional groups of PDopa and water. Unlike traditional underwater superoleophobic coatings realized by introducing hydrophilic nano/microstructures, the smooth PDopa-based coatings can clean up oil fouling simply by water action. The coatings applied onto steel meshes facilitate high-performance separation of both phase-separated and emulsified oil/water mixtures with high flux and separation efficiency solely driven by gravity. By employing the separation apparatus with a sloping plate structure, crude oil/water mixture collected from oilfield with 75–80% of water can be effectively separated under mixture temperature of 35–50 °C, with oil content in water of ~20 to ~70 ppm, and water content in oil of 1.9–4.3%.