Hydrophobicity or superhydrophobicity—which is the right choice for stabilizing underwater superoleophilicity?

Abstract

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