Facile Two-Step Strategy for the Construction of a Mechanically Stable Three-Dimensional Superhydrophobic Structure for Continuous Oil-Water Separation

Biomimetic materials in oil/water separation: Focusing on switchable wettabilities and applications

Clean water resources are crucial for human society, as the leakage and discharge of oily wastewater not only harm the economy but also disrupt our living environment. Therefore, there is an urgent need for efficient oil-water separation technology. Surfaces with switchable superwetting behavior have garnered significant attention due to their importance in both fundamental research and practical applications. This review introduces the fundamental principles of wettability in the oil-water separation process, the basic theory of switchable wettability, and the mechanisms involved in oil-water separation. Subsequently, the review discusses the research progress, challenges, and issues associated with three conventional types of special wettability materials: superhydrophobic/superoleophilic materials, superhydrophilic/superoleophobic materials, and superhydrophilic/underwater superoleophobic materials. Most importantly, it provides a detailed exploration of recent advancements in switchable wettability smart materials, which combine elements of traditional special wettability materials. These include stimulus-responsive smart materials, pre-wetting-induced materials, and Janus materials. The discussion covers key response factors, detailed examples of representative works, design concepts, and fabrication strategies. Finally, the review offers a comprehensive summary of switchable superwetting smart materials, encompassing their advantages and disadvantages, persistent challenges, and future prospects.

Coupling Carbon-Based Composite Phase Change Materials with a Polyurethane Sponge for Sustained and Efficient Solar-Driven Cleanup of Viscous Crude Oil Spill

The efficient cleanup of crude oil spills is a worldwide problem due to their high viscosity and low fluidity. Under the assistance of solar radiation, adsorbents with in situ heating function are becoming the ideal candidates to solve this problem. In this study, a new strategy coupling a polyurethane (PU) sponge with phase change materials (PCMs) is proposed to realize the efficient utilization of solar energy and crude oil cleanup. Wormlike carbon nanotubes/mesoporous carbon (CNTs/MC) with a core-shell structure was used to encapsulate polyethylene glycol (PEG), which was then introduced into the PU sponge for photothermal conversion and thermal storage. After coating with a polydimethylsiloxane (PDMS) layer, the sponge was further endowed with hydrophobic characteristics. Additionally, PDMS can function as a binder between PEG@CNTs/MC and sponge skeleton. The resulting PEG@CNTs/MC/PU/PDMS (named as PEG@CMPP) exhibited excellent photothermal conversion and high absorption capacity for high-viscosity crude oil. Most importantly, thanks to the heat storage properties of PEG, the stored heat can be sustainably transferred to the surrounding crude oil to promote its continuous absorption even under insufficient light intensity conditions. The crude oil absorption capacity of PEG@CMPP-3 reached approximately 0.96 g/cm3 even after the light source was removed, which manifested the distinctive advantages compared to the conventional photothermal adsorbent. The proposed approach integrates the high efficiency of solar-assisted heating and energy-conserving advantage, thereby providing a feasible strategy for highly efficient remediation of viscous crude oil spills.

Preparation and Hydrophobicity of Bionic Structures Based on Composite Infiltration Model

The wettability, surface energy, structure, and morphology of a material’s surface will affect the interaction process between the material and the organism. Moreover, these factors are not independent of each other, but will affect each other, which together determine the biological surface of the material. Although two classic theories of surface wettability control have been established, including the Wenzel model and the Cassie–Baxter model, the mechanism of the microstructure parameters on the surface wettability has not been considered. This paper established a two-dimensional mathematical model of the composite wetting pattern based on microstructure parameters, revealed the mechanism of the microstructure parameters on the surface wettability, and then used ultra-precision cutting and molding composite preparation methods to quickly and efficiently prepare bionic structures, and the hydrophobic character of the microstructure was characterized by the contact angle meter, which provides theoretical support and preparation technology for the modification of the hydrophobic character of the material.

Universal Rapid Demulsification by Vacuum Suction Using Superamphiphilic and Underliquid Superamphiphobic Polyurethane/Diatomite Composites

A process for universal rapid demulsification by vacuum suction using an as-prepared superamphiphilic and underliquid superamphiphobic polyurethane (PU)/diatomite composite has been developed and is used to demulsify kerosene-in-water and water-in-kerosene emulsions with and without a surfactant. The results show that the demulsification rate of all the emulsions exceeds 98.5% in long-term operation, with a stable demulsification speed exceeding 0.303 L/m² min. When a superhydrophobic channel for separation is added, the oil/water separation efficiency exceeds 99.0%, and the final products are qualified oil and water. This attractive universal demulsification capability of PU/diatomite originates from its underliquid superamphiphobicity, which attracts a continuous phase to form a stable liquid film and thus repels dispersed phase droplets, which have a similar interaction with the surface but are much less abundant. The vacuum forces emulsion droplets into the microstructure of the PU/diatomite cake, where they are compressed, coalesce, and finally demulsified. This observed mechanism suggests a promising strategy to avoid the negative effects of oil fouling in demulsification and achieve large-scale universal continuous rapid demulsification.

Mussel-Inspired Durable TiO2/PDA-Based Superhydrophobic Paper with Excellent Self-Cleaning, High Chemical Stability, and Efficient Oil/Water Separation Properties

Oceanic oil spill and the discharge of industrial oily wastewaters can cause significant threats to the ecological environment and human health. Herein, we design a durable TiO₂/PDA-based superhydrophobic paper for efficient oil/water separation. Bioinspired from mussel adhesive proteins, the mechanical durability of the as-prepared superhydrophobic paper is enhanced by the deposition of polydopamine (PDA) onto cellulosic fibers via self-polymerization of dopamine. The TiO₂/PDA-based superhydrophobic paper shows a high water contact angle of 168.2° and an oil contact angle of ∼0°, exhibiting excellent superhydrophobicity and superoleophilicity. Furthermore, the as-prepared superhydrophobic paper possesses excellent chemical stability, thermal stability, and mechanical durability in terms of being immersed in corrosive solutions and solvents and boiling water and being subjected to the sandpaper abrasion test, respectively. More importantly, the separation efficiency of the TiO₂/PDA-based superhydrophobic paper for an oil/water mixture is 97.2%, and it maintains a separation efficiency above 94.3% even after 15 cyclic separation processes. Furthermore, the separation efficiency for water-in-oil emulsions is higher than 93.7% after 15 cyclic separation tests, showing its excellent recyclable stability for water-in-oil emulsions. Therefore, the rationally designed TiO₂/PDA-based superhydrophobic paper shows great potential in the practical applications of self-cleaning, antifouling, and oil/water separation.

Recent advances in superhydrophobic polyurethane: preparations and applications

Even though polyurethane (PU) has been widely applied, its superhydrophobicity is inadequate for certain applications. As such, the development of superhydrophobic polyurethane (SHPU) has recently attracted significant attention, with numerous motivating reports in recent years. However, a comprehensive review that summarizes these state-of-the-art developments remains lacking. Thus, this review aims to fill up this gap by reviewing the recent preparation methods for SHPU based on superhydrophobic theories and principles. Three main types of methods used in promoting the hydrophobicity of PU are emphasized in this review; (1) incorporation of silicide or fluoride to lower the surface energy, (2) creation of micro/nano-scale rough surfaces by electrospinning or grafting of nanoparticles, and (3) integrating the earlier two methods to develop a synergistic approach. Furthermore, this review also discussed the various applications of SHPU in oil spill treatment, protective coating, self-healing materials and sensors.

A durable superhydrophobic porous polymer coated sponge for efficient separation of immiscible oil/water mixtures and oil-in-water emulsions

Oil spills and organic solvents leakages have led to serious environmental problems, which calls for the emerging materials for the separation of oil/organic solvents from water effectively. Herein, a superhydrophobic/superoleophilic, SHMP-1@Sponge, with water contact angle (WCA) of 156° and oil contact angle of 0°, was fabricated by dip coating polymer SHMP-1 powder onto the skeleton of 3D melamine sponge. The SHMP-1@Sponge featured large specific surface area (556 m²/g) as well as high chemical and mechanical durability. SHMP-1@Sponge can absorb up to 40-105 times of its own weight of light and heavy oils/organic solvents in seconds, and it can be recycled for 25 times by squeezing. Moreover, the separation efficiency of immiscible oil/water mixtures and oil-in-water emulsions by SHMP-1@Sponge are > 99.5％. SHMP-1@Sponge shows tremendous absorption capacity for chloroform-in-water emulsions (1460 mg/g) compared with nitrobenzene-in-water (1290 mg/g) and diesel-in-water emulsions (980 mg/g), which is the strongest superhydrophobic absorbent for surfactant-stabilized oil-in-water emulsions reported to date. The durable SHMP-1@Sponge fabricated by loading superhydrophobic polymer with large surface area onto 3D sponge makes it a promising material for oil/water separation in realistic aquatic environments.

Facile Preparation of Robust Superhydrophobic/Superoleophilic TiO2-Decorated Polyvinyl Alcohol Sponge for Efficient Oil/Water Separation

Oily wastewater and oil spills pose a threat to the environment and human health, and porous sponge materials are highly desired for oil/water separation. Herein, we design a new superhydrophobic/superoleophilic TiO₂-decorated polyvinyl alcohol (PVA) sponge material for efficient oil/water separation. The TiO₂-PVA sponge is obtained by firmly anchoring TiO₂ nanoparticles onto the skeleton surface of pristine PVA sponge via the cross-linking reactions between TiO₂ nanoparticles and H₃BO₃ and KH550, followed by the chemical modification of 1H,1H,2H,2H-perfluorodecyltrichlorosilane. The as-prepared TiO₂-PVA sponge shows a high water contact angle of 157° (a sliding angle of 5.5°) and an oil contact angle of ∼0°, showing excellent superhydrophobicity and superoleophilicity. The TiO₂-PVA sponge exhibits excellent chemical stability, thermal stability, and mechanical durability in terms of immersing it in the corrosive solutions and solvents, boiling it in water, and the sandpaper abrasion test. Moreover, the as-prepared TiO₂-PVA sponge possesses excellent absorption capacity of oils or organic solvents ranging from 4.3 to 13.6 times its own weight. More importantly, the as-prepared TiO₂-PVA sponge can separate carbon tetrachloride from the oil-water mixture with a separation efficiency of 97.8% with the aid of gravity and maintains a separation efficiency of 96.5% even after 15 cyclic oil/water separation processes. Therefore, the rationally designed superhydrophobic/superoleophilic TiO₂-PVA sponge shows great potential in practical applications of dealing with oily wastewater and oil spills.

Design of Organic-Free Superhydrophobic TiO2 with Ultraviolet Stability or Ultraviolet-Induced Switchable Wettability

Superhydrophobic TiO₂ with great application potential is mainly obtained by surface modification with low surface energy organics, which is easily degraded under sunlight irradiation, which results in the loss of superhydrophobic properties. Herein, we developed a room-temperature pulsed chemical vapor deposition (pulsed CVD) method to develop amorphous TiO₂-deposited TiO₂ nanoparticles. The ultraviolet stability/ultraviolet-induced reversible wettability switch had been simultaneously realized by different and controllable deposition cycles of amorphous TiO₂. The superhydrophobic properties of the organic-free TiO₂ were determined by the micrometer-nanometer-sub-nanometer multiscale structure, the multiscale pore structure, and the large Young's contact angle resulting from carboxylic acid adsorption. Also, we found that the adsorption rate and adsorption stability of oxygen and water at the surface oxygen vacancies were the key to facilitate the reversible switching between superhydrophilic and superhydrophobic states, which was well demonstrated by experimental characterization and theoretical simulation. In addition, we also found that the resistance of dense amorphous TiO₂ films on the TiO₂ surface to the migration of photogenerated electrons and holes was the key to maintain the stable superhydrophobic properties of superhydrophobic TiO₂ under ultraviolet illumination. The powders were strongly ground and the coating surface was rubbed on the surface of the sandpaper, which still maintained superhydrophobic properties, providing favorable conditions for the application of superhydrophobic TiO₂. This work modulates the ultraviolet stability and dark/ultraviolet-induced switchable superhydrophobicity/superhydrophilicity of coated TiO₂ by simply adjusting the number of deposition times in a pulsed CVD process for the first time, thus contributing to the development of organic-free superhydrophobic TiO₂.

Photopolymerized superhydrophobic hybrid coating enabled by dual-purpose tetrapodal ZnO for liquid/liquid separation

Low-cost and scalable superhydrophobic coating methods provide viable approaches for energy-efficient separation of immiscible liquid/liquid mixtures. A scalable photopolymerization method is developed to functionalize porous substrates with a hybrid coating of tetrapodal ZnO (T-ZnO) and polymethacrylate, which exhibits simultaneous superhydrophobicity and superoleophilicity. Here, T-ZnO serves dual purposes by (i) initiating radical photopolymerization during the fabrication process through a hole-mediated pathway and (ii) providing a hierarchical surface roughness to amplify wettability characteristics and suspend liquid droplets in the metastable Cassie-Baxter regime. Photopolymerization provides a means to finely control the conversion and spatial distribution of the formed polymer, whilst allowing for facile large-area fabrication and potential coating on heat-sensitive substrates. Coated stainless-steel meshes and filter papers with desired superhydrophobic/superoleophilic properties exhibit excellent performance in separating stratified oil/water, oil/ionic-liquid, and water/ionic-liquid mixtures as well as water-in-oil emulsions. The hybrid coating demonstrates desired mechanical robustness and chemical resistance for their long-term application in large-scale energy-efficient separation of immiscible liquid/liquid mixtures.

Underwater suspended bifunctionalized polyethyleneimine-based sponge for selective removal of anionic pollutants from aqueous solution

Highly selective and efficient removal of ionic pollutants, including ionic organic compounds and heavy metal ions from water, is still a huge challenge due to the complex nature of polluted water. To meet this challenge, we presented the synthesis of bifunctionalized polyethyleneimine-based sponges through cryo-polymerization via BDDE as the crosslinker followed by bifunctional modification with glycidyl trimethylammonium chloride (GTAC) and phenyl glycidyl ether (PGE), which simultaneously afford quaternary ammonium cation (strongly basic and hydrophilic) and phenyl (hydrophobic) functionalities, respectively. As a result, a hybrid hydrophilic-hydrophobic sponge is generated that could stably be suspended underwater due to the co-operative effect of the water-absorbing hydrophilic domain and the hydrophobic domain generating buoyancy. The quaternized and phenyl-functionalized PEI-based sponge (S_QP-PEI) demonstrated highly selective and efficient removal of anionic pollutants from water, including diclofenac sodium (DIC), methyl orange (MO) and chromium (Cr(VI)) with co-existing interferences. The Langmuir isotherms revealed the maximum adsorption capacities of 342.7 mg/g, 491.9 mg/g, and 242.7 mg/g for DIC, MO, and Cr(VI), respectively. The studies of adsorption mechanism suggested that the bifunctional S_QP-PEI sponge indeed afford both strong anion-exchange interaction and π-π interaction toward organic pollutants DIC and MO, and the strong anion-exchange interaction can be the dominated adsorption mechanism for anionic DIC, MO and Cr(VI) species. The suspended S_QP-PEI also demonstrated excellent reusability, which shows the potential of S_QP-PEI for real applications.

Multifunctional superhydrophobic adsorbents by mixed-dimensional particles assembly for polymorphic and highly efficient oil-water separation

Supra-wetting materials, especially superhydrophobic absorption materials, as an emerging advanced oil-water separation material have attracted extensive concern in the treatment of oil spillage and industrial oily wastewater. However, it is still a challenge to fabricate robust and multifunctional superhydrophobic materials for the multitasking oil-water separation and fast clean-up of the viscous crude oil by an environment-friendly and scalable method. Herein, a solid-solid phase ball-milling strategy without chemical reagent-free modification was proposed to construct heterogeneous superhydrophobic composites by using waste soot as the solid-phase superhydrophobic modifier. A series of covalent bond restricted soot-graphene (S-GN) or soot-Fe₃O₄ (S-Fe₃O₄) composite materials with a peculiar micro-nano structure are prepared. Through "glue+superhydrophobic particles" method, the prepared soot-based composite particles are facilely loaded on the porous skeleton of the sponge to obtain multifunctional superhydrophobic adsorbents. The reported superhydrophobic adsorbents exhibited robust chemical and mechanical stability, convenient magnetic collection, the high oil absorption capacity of 60-142 g g^-1, durable recyclability (>250 cycles), efficient separation efficiency (>99.5%) and outstanding self-heated performance, which enable them to be competent for oil-water separation in multitasking and complex environment (floating oils, continuous oil collection, oil-in-water emulsion, and viscous oil-spills).

Underoil Superhydrophilic Metal Felt Fabricated by Modifying Ultrathin Fumed Silica Coatings for the Separation of Water-in-Oil Emulsions

Although various superhydrophobic/superoleophilic porous materials have been developed and successfully applied to separate water-in-oil emulsions through the size-sieving mechanism, the separation performance is restricted by their nanoscale pore size severely. In this study, the wettability of underoil water on fumed silica was experimentally observed, and the underlying mechanism was investigated by carrying out theoretical analysis and molecular dynamic (MD) simulations. Further, we present a novel, facile, and an inexpensive technique to fabricate an underoil superhydrophilic metal felt with microscale pores for the separation of water-in-oil emulsions using SiO₂ nanoparticles (NPs) as building blocks. The as-prepared underoil superhydrophilic coating is closed-packed and ultrathin (the thickness is approximately hundreds of nanometers), as well as capable of being coated on a metal felt with complex structures without blocking its pores. The as-prepared metal felt could adsorb water droplets directly from oil, which endowed it with the ability to separate both surfactant-free and surfactant-stabilized water-in-oil emulsions with high separation efficiency up to 99.7% even though its pore size is larger than that of the emulsified droplet. The filtration flux for the separation of span 80-stabilized emulsion is up to ∼4000 L·m^-2·h^-1. Its separation performance is better than most of the other traditional membranes and superwettable materials used for the separation of water-in-oil emulsions. Moreover, the as-prepared metal felt retained outstanding separation performance even after 30 cycles of use, which demonstrated its excellent reusability and durability. Additionally, the distinctive wettability of underoil superhydrophilicity endued coated metal felt with superior antifouling properties toward crude oil. Overall, this study not only provides a new perspective on separating water-in-oil emulsions but also gives a universal approach to develop unique wettability surfaces.

Water deteriorates lubricating oils: removal of water in lubricating oils using a robust superhydrophobic membrane

Water is recognized as a contaminant in lubricating oils. Recently, interfacial materials with special wettability have been broadly developed for oil-water separation. However, solving lubricating oil failure caused by water remains a challenge. Here, a robust superhydrophobic membrane is presented to effectively remove water in lubricating oils to recover their lubricating capability. Compared to pure lubricating oils without or with an additive, lubricating oils collected from their emulsions using the superhydrophobic membrane have an equivalent friction coefficient and wear volume, which are much lower than that of lubricating oils contaminated by water. Water in lubricating oils accelerates the oxidation of metallic substrates and wear corrosion. Moreover, the metallic ions dissolved in water-containing lubricating oils induce the catalytic dehydrogenation of lubricating oils, leading to the deposition of a good deal of carbon-based wear debris. Importantly, the prepared membrane shows steady performance in regard to extreme water repellency, high-efficiency purification of lubricating oils, and low wear volume even after harsh mechanical damage. Robust interfacial materials have potential advantages in practically solving lubricating oil failure caused by water.

Plant-Inspired Layer-by-Layer Self-Assembly of Super-Hydrophobic Coating for Oil Spill Cleanup

A versatile, facile, energy-saving, low-cost and plant-inspired self-assembly strategy was used to prepare super-hydrophobic coating in this study. Concretely, an appealing super-hydrophobicity surface was obtained by designing a molecular building block phytic acid (PA)-Fe (III) complex to anchor the substrate and hydrophobic thiol groups (HT). The facile and green modification method can be applied to variety of substrates. The as-prepared PA-Fe (III)-HT coated melamine composite sponge possesses both super-hydrophobic and superlipophilicity property. Moreover, it displays superior efficiency to separate the oil-water mixture and splendid oil spill cleanup.

Smart Amphiphilic Random Copolymer-Coated Sponge with pH-Switchable Wettability for On-Demand Oil/Water Separation

To prepare intelligent controllable oil/water separation materials with high mechanical stability and good recyclability, we fabricated a novel pH-controlled wettability melamine sponge by using a facile dip-coating method. The coated sponge exhibits reversibly switchable wettability between superhydrophilicity-superoleophobicity through acidic surrounding and superhydrophobicity-superoleophilicity under neutral or alkaline conditions. The as-prepared sponge possesses excellent absorption capacity (46.06-122.81 g/g) and oil/water separation efficiency (above 98%). The coated sponge also has good mechanical stability and recyclability which means it can be reused for absorption and oil/water separation. This smart porous material, which can flexibly transform wettability on demand, has great application prospects in oil/water separation.

Separation Mechanism and Construction of Surfaces with Special Wettability for Oil/Water Separation

Oil leakage and the discharge of oil/water mixtures by domestic and industrial consumers have caused not only severe environmental pollution and a threat to all species in the ecosystem but also a huge waste of precious resources. Therefore, the separation of oil/water mixtures, especially stable emulsion, has become an urgent global issue. Recently, materials containing a special wettability feature for oil and water have drawn immense attention because of their potential applications for oil/water separation application. In this paper, we systematically summarize the fundamental theories, separation mechanism, design strategies, and recent developments in materials with special wettability for separating stratified and emulsified oil/water mixtures. The related wetting theories that unveil the physical underlying mechanism of the oil/water separation mechanism are proposed, and the practical design criteria for oil/water separation materials are provided. Guided by the fundamental design criteria, various porous materials with special wettability characteristics, including those which are superhydrophilic/underwater superoleophobic, superhydrophobic/superoleophilic, and superhydrophilic/in-air superoleophobic, are systemically analyzed. These superwetting materials are widely employed to separate oil/water mixtures: from stratified oil/water to emulsified ones. In addition, the materials that implement the demulsification of emulsified oil/water mixtures via the ingenious design of the multiscale surface morphology and construction of special wettability are also discussed. In each section, we introduce the design ideas, base materials, preparation methods, and representative works in detail. Finally, the conclusions and challenges for the oil/water separation research field are discussed in depth.