Hydrophobicity or Superhydrophobicity—Which is Right Choice for Stabilizing Underwater Superoleophilicity?
Extremely water-repellent, bio-inspired superhydrophobic interfaces—that inherently display super affinity for oily phase under water, provided simple basis for selective filtration of bulk oil/oily phase from oil/water mixtures. However, the extreme water repellency in superhydrophobic interfaces appeared as “Achilles' heel” for the separation of practically more relevant and complex form of oil/water mixture—that is oil-in-water emulsion, as the suspended oil droplets in bulk aqueous phase are inaccessible to the selectively oil-absorbent superhydrophobic interface. Moreover, such underwater super oil affinity inherently embedded in the superhydrophobic coating is known to be completely compromised over continuous exposure to either aqueous phase for less than 2 days or at elevated temperature (>50°C), due to spontaneous displacement of metastable-trapped air from superhydrophobic interface. In this current report, a moderately hydrophobic (water contact angle ≤130°) multilayer coating, which inherently allows coexistence of discontinuous trapped air and aqueous phase, unusually displayed both underwater superoleophilicity and under-oil superhydrophobicity. The stability of underwater superoleophilicity in such hydrophobic multilayers, was chemically tailored through facile reaction between acrylate and selected alkylamines at ambient condition, and the duration for complete transition from superoleophilicity to superoleophobicity under water significantly improved to 100 days, in comparison to superhydrophobic multilayers (2 days). Moreover, this moderately hydrophobic interface, which displayed unusual and uninterrupted underwater superolephobicity even at highly (90°C) elevated temperature, was further explored in successful separation of both bulk-oil spills and oil-in-water emulsions based on both the energy efficient filtration and absorption principles, and such dual mode operations with single interface are practically impossible to achieve with any existing biomimicked approaches. Thus, the performance of moderately hydrophobic multilayers is superior over biomimicked superhydrophobicity in mainly two important and relevant aspects—a) unprecedented stability of underwater extreme oil-affinity and b) ability for separating both oil-in-water emulsion and bulk oil/water mixtures in severe settings—following energy efficient and environmentally friendly separation processes.