Multifunctional Superwettable Material with Smart pH Responsiveness

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24668−24682

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Multifunctional Superwettable Material with Smart pH Responsiveness for Efficient and Controllable Oil/Water Separation and Emulsified Wastewater Purification Mengnan Qu,* Lili Ma, Jiaxin Wang, Yi Zhang, Yu Zhao, Yichen Zhou, Xiangrong Liu, and Jinmei He* College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China

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S Supporting Information *

ABSTRACT: Developing multifunctional superwettable materials is highly demanded in the oil/water separation field but remains challenging due to the critical limitations of complex fabrication strategy and high cost. Herein, based on the costeffective kaolin nanoparticles, we present a convenient and mild strategy for fabricating a smart superwettable material with multiple excellent performances, such as pH-responsive water wettability, self-cleaning property, favorable buoyancy, and air purification performance. By virtue of the dual rough surface structure and special chemical composition, the resultant material surface exhibits a superior pH-dependent wettability, which can be reversibly switched between superamphiphobicity and superhydrophilicity−superoleophobicity for many times in accordance with the pH value of the corresponding aqueous solution. As a result, the obtained superwettable material with reversible and controllable water wettability can be applied in efficient and continuous separation of multiple types of oil/water mixtures, especially the highly emulsified oil/water emulsions, via in situ or ex situ wettability change. To our knowledge, the smart material with the wetting property of superamphiphobicity that can be used for continuous emulsified wastewater purification has been rarely discussed in the emerging research works. In addition, the as-prepared material presents universal applicability to diversiform substrates and exhibits robust durability and stability against highconcentration salt solutions and rigorous mechanical abrasion. All of these above-mentioned advantages indicate that the asprepared superwettable material will hold great potential in various practical applications, including oily wastewater remediation, smart aquatic device fabrication, liquid droplet manipulation, guiding liquid movement, and optimizing multiple operations in industrial fields. KEYWORDS: superwettable, pH responsiveness, multifunctional, oil/water separation, wastewater purification limit their recyclability, and induce secondary pollution.8 Additionally, the difficulty to clean the oil-polluted materials further increases material and operating costs.9 In view of this problem, researchers of the field have paid more attention to developing materials with superhydrophilic− superoleophobic property.10,11 When an oil/water mixture contacted the superwetting material, the water phase of the mixture could quickly spread and penetrate through the material, whereas oil was totally blocked on the material surface. Therefore, the superwettable material can be well protected from the contamination of oil and remain clean during the whole separation process by virtue of its superior oil repellency, which shows attractive potential in practical oil/ water separation and has important significance for fundamental research. In addition, the superwetting materials with

1. INTRODUCTION Nowadays, with the vigorous development of industrialization and economy, oil/water separation and wastewater purification have been crucial and urgent issues because of the frequent marine oil-spill accidents, oily wastewater discharge, and industrial chemical leakage.1,2 In the light of interfacial science, superwettable functional materials with different wettabilities for water and oils, such as superhydrophobicity/superoleophilicity and superhydrophilicity/superoleophobicity, have provided facile and effective solutions to these environmental issues.3−5 To date, the existing superwettable materials for oil/water separation mostly possess a lotus leaf-inspired superhydrophobic−superoleophilic surface, which shows high affinity to oil but super repellency to water; thus, the oil pollutants in water can be efficiently removed.6,7 However, it should be noted that these special surfaces are easily contaminated by oils (especially oils with high viscosity) during the separation process owing to their intrinsic oleophilicity, which will significantly reduce their performance, © 2019 American Chemical Society

Received: February 28, 2019 Accepted: June 18, 2019 Published: June 18, 2019 24668

DOI: 10.1021/acsami.9b03721 ACS Appl. Mater. Interfaces 2019, 11, 24668−24682

Research Article

ACS Applied Materials & Interfaces

Among all of these external stimuli systems, the pHresponsive smart materials have recently received extensive attention in triggering wettability transition because of their rapid response capability, convenient operation, and facile surface wettability recovery without additional modification.33,34 The typical pH-responsive functional groups, such as carboxyl, amine, sulfonate, and pyridine groups,35,36 which can undergo reversible protonation and deprotonation according to the pH values of an aqueous solution, contribute to the controllable and reversible surface wettability transition. For instance, Sun et al.37 reported an intelligent sponge with switchable water wettability through introducing the pHresponsive groups of −OH and −COOH on the sponge surface. Luo et al.38 utilized the electrospinning method to prepare a pH-responsive fibrous membrane coated with copolymer poly(methyl methacrylate)-block-poly(4-vinylpyridine). Zhang et al.39 fabricated a multifunctional superwettable material with intelligent-control surface wettability via spraying a cross-linked mixture containing a pH-responsive copolymer onto a cotton fabric. Benefiting from the elaborate design of the rough hierarchical structure and successful introduction of the pH-responsive polymer segments, the as-prepared materials exhibit excellent pH-induced switchable superwettability, which holds great potential for developing a smart and convenient separation method for efficient and controllable oil/water separation. Nevertheless, the fabrication of pH-responsive surfaces remains challenging despite the wide range of research reported. The mechanism of surface wettability transformation, the precise control of surface wettability change, and the stability and reproducibility of the switching behavior with increasing cycles of external stimuli still need to be further investigated.40 Besides, as far as we know, most of these materials exhibit an ex situ pH responsiveness, which is required to be pretreated by an aqueous solution of proper pH value to obtain the desired surface wettability. Few reports have referred to superwettable materials that simultaneously exhibited in situ and ex situ pH responsiveness for handling oily wastewater under diverse conditions and situations.34 Herein, based on the cost-effective kaolin nanoparticles, we have successfully developed a smart superwettable nanocomposite material with excellent pH responsibility via a facile and mild strategy. By adjusting the pH conditions of surrounding aqueous solutions from acid to alkaline, the surface wettability of the resultant material can transit from superamphiphobicity to superhydrophilicity−superoleophobicity within a short time. To our knowledge, the special conversion between these two different extreme wettabilities has been rarely discussed in the current reported literature,41 which is supposed to result from the cooperation of the pHsensitivity surface composition and micro-/nano-dual roughness on the resultant surface. The superior pH responsiveness and reversible wettability transition property of the superwettable material provide greater potentials for controllable and selective oil/water separation and emulsified wastewater purification with high separation efficiency, favorable antifouling property, and reusability. Moreover, the resultant material can combine both advantages of these two extreme wettabilities and shows superior oil repellency, smart water wettability, good antifouling property, and favorable buoyancy and durability, all of which endow the as-prepared material with wide application scope and potentially reduced operation costs toward oil contamination treatment.29 Thus, we believe

excellent superoleophobicity also possess superior potential applications in antioil coatings,12 oil droplet manipulation,13 guiding oil movement,14 bioadhesion,15 and floating on oil.16 Although very encouraging progress has been achieved through unremitting effort and investigation, very few reports have discussed superhydrophilic−superoleophobic materials. On the one hand, the complicated morphology and low surface energy are considered two indispensable factors for fabricating the superoleophobic surface, as first confirmed by Tuteja and his co-workers.17 It also means that to obtain a super oilrepellent surface, it may have a higher requirement for sufficient surface roughness and ultralow-surface-energy materials to resist the oily organic liquids of extremely low surface tension.18 On the other hand, a special surface that simultaneously possesses superhydrophilicity and superoleophobicity is theoretically energy unfavorable because since the surface tension of water is remarkably greater than that of oils, an oil-repellent surface is usually also water-repellent.19,20 Thus, to achieve such unusual wetting performances, a superoleophobic surface may be designed to produce specific interactions with water, which further increases the fabrication difficulty and has become a great limitation to its extensive application. Till now, multifarious bionic superwettable surfaces have been developed and fabricated, which show broad application potential in various fields, especially in oil/water separation. However, critical limitations still remain because most of these materials can only separate the immiscible oil/water mixture, which is not competent for treating the stabilized oily emulsion with complex phase states and higher viscosity.21 Moreover, these superwetting materials with fixed wettability patterns commonly lack flexibility in separating oily wastewater from diverse sources of pollution. All of these above-mentioned drawbacks significantly restrict their large-scale practical application. In this respect, endowing these superwettable surfaces with multiple functions and smart properties has become the main trend of this research field.22 More recently, a variety of smart surfaces with controllable superwettability and diversified properties have been developed by accurately regulating their surface morphologies or surface chemical composition according to external stimuli such as light,23 pH value,24 electric field,25 temperature,26 solvent quality,27 and magnetic field.28 In response to appropriate external triggers, the surface wettability of the smart superwettable material to water and oil can switch between two opposite states, which are desirable for the intelligent-controlled and selective oil/ water separation. Generally, the basic design principle of intelligent separation materials with switchable surface wettability is introducing the stimuli-responsive polymeric materials into porous materials.29 For example, Xin et al.30 fabricated a light-/heat-responsive polyvinylidene fluoride nanofiber membrane doped with TiO2 for controllable oil/ water separation. Feng et al.31 developed a superwetting mesh coated with poly(acrylic acid) that could switch its surface wettability according to the concentration of mercury ion. Li et al.32 prepared a magnetically responsive superhydrophobic surface consisting of poly(dimethylsiloxane) prepolymers and carbonyl iron particles. Although considerable progress has been made in the development of smart separation materials, the real widespread application of these superwettable materials is still unrealized since most of the stimulus-response wetting transitions require special facilities and complicated operations. 24669


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Figure 1. Schematic illustration of the pH-responsive wettability of the superwettable material surface. 2.3. Oil-in-Water Emulsion Separation. To separate the oily emulsions, a glass tube (4 cm in diameter) filled with the superamphiphobic kaolin particle layer was used as the separation device, and the beaker placed below the separation device served as a collection vessel. Then, 30 mL of the prepared emulsion (pH = 13) was poured into the separation device, and the separation experiment was conducted under external pressure (0.02 bar) provided by a double-bond ball. Separation time of the middle 10 mL of emulsion was recorded, and the corresponding separation flux was calculated by

that this research will provide new ideas to design multifunctional materials for various practical and technological applications, especially the controllable separation of multiple types of oil/water mixtures.

2. EXPERIMENTAL SECTION The chemicals, instruments, and corresponding measurements applied in the experimental procedure have been described in detail in the Experimental Section of the Supporting Information. 2.1. Fabrication of the Superwettable Materials. First, 0.2 mL of bis[3-(trimethoxysilyl)propyl]ethylene diamine was added in 9.8 mL of ethanol; subsequently, the obtained solution was subjected to magnetic stirring at 50 °C for 30 min. Therefore, a bis[3(trimethoxysilyl)propyl]ethylene diamine/ethanol solution (2% v/v) was prepared. Second, kaolin (4.50 g) was dispersed in the mixed solution containing PFOA (0.65 g) and ethanol (8.5 mL) by ultrasonic oscillation for 30 min. Then, the as-prepared bis[3-(trimethoxysilyl)propyl]ethylene diamine/ethanol solution (2.8 mL) was added in the mixture and magnetically stirred at 55 °C for 3 h. Afterward, the resulting suspension was cooled in air to room temperature and then evenly drop-coated on the cleaned substrates, such as glass, fabric, cotton, foam, and so on. After the coated samples were cured at ambient condition, they were then dried in an 80 °C oven for 2 h. Finally, the smart coated materials with excellent superamphiphobicity were obtained (for more information, see Figure S1, Supporting Information). 2.2. Preparation of the Oil-in-Water Emulsions. To obtain the oily emulsions without addition of a surfactant, 5 mL each of hexadecane, sunflower oil, industry white oil, olive oil, n-hexane, and toluene were added into 60 mL of water and ultrasonically dispersed for 1 h. Then, the resultant mixture was continuously stirred for 5 h so that an oily emulsion that could remain stable for 24 h under ambient conditions was prepared. For the preparation of surfactant-stabilized oily emulsions, Span80 (0.5 g) was selected as the emulsifier and dissolved in 60 mL of water; later on, 1 mL each of hexadecane, sunflower oil, industry white oil, olive oil, n-hexane, and toluene were added and ultrasonically dispersed for 0.5 h. Afterward, each mixture was continuously stirred for 5 h. The resulting emulsions were highly stable, and no obvious demulsification could be observed for more than 1 week.

flux =

V St

(1)

where V is the volume of the feed emulsion (10 mL), S is the active area of the particle layer (4π cm2), and t is the separation time. The oil rejection rate of the superwettable materials for the oily emulsion was calculated by

ij C yz R = jjj1 − c zzz × 100% j C0 z{ k

(2)

where R is the oil rejection rate of the separation process, and Co and Cc are, respectively, oil concentrations of the feed emulsion and the collected filtrate after separation.

3. RESULTS AND DISCUSSION 3.1. Fabrication and Wetting Behaviors of the pHResponsive Superwettable Materials. Generally, surface roughness and low surface energy are two crucial factors for the fabrication of superwettable surface.42 Herein, a smart superwettable material with superior pH responsiveness was fabricated by the combination of kaolin, PFOA, and bis[3(trimethoxysilyl)propyl]ethylene diamine, as schematically illustrated in Figure 1. In the presence of bis[3(trimethoxysilyl)propyl]ethylene diamine as the silane coupler, the kaolin particles were able to compactly accumulate together and contributed plentiful micro-/nanoscale aggregate structures on the as-prepared material surface. Besides, the imino groups of the bis[3-(trimethoxysilyl)propyl]ethylene diamine exhibited high activity, which could be subjected to amidation reaction with the carboxyl-containing PFOA readily; 24670


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Figure 2. (a) Time-dependence WCAs of the superwettable materials treated by different pH solutions. (b) Water and hexadecane droplets on the acidic (pH 1) solution-treated superwettable material surface, and the WCA was about 158°. (c) Water and hexadecane droplets on the alkali (pH 13) solution-treated superwettable material surface, and the water droplets could spread on the material surface within 2 s, showing a WCA of about 0°. (d) Reversible water wettability of the as-prepared superwettable material with a cyclic treatment of acidic (pH 1) and alkali (pH 13) solutions. (e) Contact angle measurements for water and various oils on the superwettable material surfaces treated by acidic (pH 1) and alkali (pH 13) solutions, respectively.

thus, the long fluorocarbon chains with comparatively low surface tension were successfully grafted onto the surfaces of kaolin particles, further resulting in a superamphiphobic surface that could repel both oil and water when combined with the constructed hierarchical surface roughness (Figure 1). More importantly, it was found that the as-prepared materials exhibited good pH responsiveness, which could transit between superamphihobicity and superhydrophilicity−superoleophobicity after being treated by different pH solutions. The special wettability switchable property was mainly related to the amide groups of the obtained surface. Specifically, after the treatment of neutral and low pH solutions, the amide groups attached to the perfluorooctyl chains could maintain stability and induced a substantial decrease of surface energy, resulting in the superamphiphobicity of the material surface. However, these amide groups containing perfluorooctyl polymers were easily hydrolyzed and formed a great deal of perfluorooctyl carboxyl groups on the as-prepared material surface after being treated by high pH solutions. Meanwhile, the obtained perfluorooctyl carboxyl groups were readily deprotonated to the perfluorooctyl carboxylate anion under the influence of alkali solutions, which showed superior water affinity and oil repellency, endowing the material surface with simultaneous superhydrophilicity and superoleophobicity. The attractive responsiveness provides the as-prepared materials with great potential for controllable and selective separating oil/water mixtures from diverse sources. To study the specific transition of water wetting behaviors on the as-prepared material surface treated with different pH solutions for 30 min, water contact angles (WCAs) of the corresponding surfaces were measured, as shown in Figure 2a. When the as-prepared material was treated with pH ≤ 9 solutions, its WCA was higher than 150° and did not show any obvious change during 480 s, featuring a stable super-

hydrophobicity. However, when the pH value of the treated solution was increased to 11, the original WCA of the asprepared material gradually decreased to 120°, and on further increasing the pH value higher than 12, the original WCA rapidly decreased to 0° within 2 s, indicative of an excellent superhydrophilicity. The remarkable wettability change was mainly affected by the amide groups on the as-prepared material surface, which could be hydrolyzed in a strong alkali environment and formed massive perfluorooctyl carboxylate on the surface, inducing wettability transformation of the treated surface from superhydrophobicity to superhydrophilicity. Therefore, the smart material exhibited better responsiveness and excellent water affinity undergoing treatment of solutions with pH > 12. Figure 2b shows the wetting property of the asprepared material treated by a pH 1 acidic solution, where water and hexadecane droplets were all in near-perfect spheres, presenting a WCA about of 158°. In comparison, when the asprepared material was treated by an alkali solution with pH 13, the water droplets applied on the treated surface could quickly spread because of the hydrogen-bonding effect between the water molecules and the carboxylate, exhibiting a WCA of about 0°, whereas the hexadecane droplets remain stable on the surface (Figure 2c). Importantly, when the alkali-treated material was further treated by a pH 1 acidic solution, the material surface wettability recovered back to superamphiphobicity. The switchable water wettability between the two extreme states could be repeated for 20 cycles with a slight fluctuation in the responsiveness, as presented in Figure 2d, indicating that the as-prepared materials possessed stable switchable ability and sensitive response to pH change. For further investigation of the surface wettability of the asprepared materials that were separately treated by acidic (pH 1) and alkali (pH 13) solutions, water and various oils were applied onto the surfaces of the corresponding materials 24671


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Figure 3. (a−h) Low- and high-magnification SEM images of the (a, e) original kaolin material, (b, f) superwettable kaolin material, (c, g) superwettable kaolin material treated by acid (pH 1) solution, and (d, h) superwettable kaolin material treated by alkali (pH 13) solution. (i) FTIR spectra of the (i1) original kaolin material, (i2) superwettable kaolin material treated by acid (pH 1) solution, (i3) superwettable kaolin material without any treatment, and (i4) superwettable kaolin material treated by alkali (pH 13) solution. (j) XPS spectra of the original kaolin material, superwettable kaolin material, and superwettable kaolin material treated by alkali (pH 13) solution. (k, l) N 1s profiles of the (k) superwettable kaolin material and (l) superwettable kaolin material treated by alkali (pH 13) solution.

Figure 3a,e shows the SEM images of the pristine kaolinmaterial-coated glass; it can be found that the nanoscaled kaolin particles agglomerated to plentiful microscopically fluffy structures and loosely adhered to the glass substrate, demonstrating superhydrophilicity. However, after the modification of the PFOA and bis[3-(trimethoxysilyl)propyl]ethylene diamine, the resultant material surface became comparatively rough from an overall perspective (Figure 3b). Besides, plenty of small protrusions could be observed uniformly scattered on the microscaled rough structures at high magnification (Figure 3f), forming micro- and nano-dualscale roughness on the corresponding surface, which was capable of trapping a substantial amount of air in the protrusions and enhanced the surface liquid wettability to extremes. Furthermore, the surface morphologies of the

(Figure 2e). It can be found the WCAs of the two treated surfaces were quite different, whereas their oil contact angles (OCAs) were all greater than 150°, demonstrating a good water wettability change and stable superoleophobicity. 3.2. Surface Morphology and Chemical Composition. The intrinsic wetting behavior of a superwettable material is considered to be mainly governed by the hierarchically micronanostructured rough morphology and low-surface-energy component of the material surface.43,44 Herein, to investigate the influence of microsurface morphologies and chemical compositions on surface wettability, the scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and Xray photoelectron spectroscopy (XPS) of the corresponding materials were carried out. 24672


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Figure 4. Schematic illustration of the liquid-wetting modes of the superwettable material surface. (a) For nonbasic water, the resultant material surface shows superhydrophobicity (WCA ∼ 158°); thus, water droplets can be sustained on the surface because Δp > 0. (b) For alkaline water, the as-prepared material presents superhydrophilicity with a WCA of about 0°, so water can permeate the coated material surface owing to Δp < 0. (c) After water permeation, the pore structures of the coated surface are filled with alkali water, which is beneficial to repel oil; thus, the coated material presents superoleophobicity, and oil is blocked on the surface as Δp > 0. (d) In air, the as-prepared material exhibits stable superoleophobicity (OCA > 154°); thus, oil can be supported on the coated surface because Δp > 0. The inserted photographs in (a, b, and d) show the nonbasic water droplet (a), basic droplet (b), and oil droplet (d) on the corresponding material surface, respectively.

(Figure 3i2), the intensities of the N−H peak (3500 cm−1) and the −OH peak (3520 cm−1) obviously weakened, indicating the more complete amidation of PFOA. Therefore, the obtained material exhibited excellent superamphiphobicity, which was consistent with the contact angle results in Figure 2e. As the modified kaolin material was treated by the alkali solution (Figure 3i4), the adsorption peaks of N−H (3500 cm−1) and −OH (3520 cm−1) groups became much more intensified; meanwhile, the characteristic peak of −CO in PFOA appeared at 1698 cm−1, all of which demonstrated the hydrolysis of amide groups. Moreover, the resultant carboxyl groups of PFOA were easily deprotonated to carboxylate in an alkaline environment, which could be verified by the absorption peaks of −CO in carboxylate at 1660 and 1412 cm−1. Therefore, the carboxylate having great water affinity in combination with the fluorine polymers of pretty low surface energy jointly resulted in the superhydrophilicity and superoleophobicity of the treated material surface. In addition, to further clarify the chemical compositions of the superwettable materials, we recorded and analyzed the XPS spectra in Figure 3j. It was clear that the pristine kaolin material mainly consisted of Al, Si, C, and O elements. While for the modified superamphiphobic kaolin material, the typical peaks of N 1s, F 1s, and F KLL appeared at 399.5, 686.4, and 842.5 eV, respectively, which were mainly assigned to perfluorooctyl amide groups of the modified material surface. This result indicated that the modified kaolin surface was covered with abundant fluoride segments, which was beneficial for improving the amphiphobicity of the superwettable material. Compared with the superamphiphobic kaolin material, two additional peaks at 1072.3 and 456.2 eV arose on the surface of the alkali-treated superwettable kaolin material that separately corresponded to Na 1s and Na KLL. Moreover, it should be noted that for the resultant superamphiphobic kaolin material, the N 1s peak presented at 399.8 eV (C−N) was assigned to perfluorooctyl amide groups (Figure 3k).

modified kaolin materials after treatment of acidic (pH 1) and alkali (pH 13) solutions were also characterized, which are separately presented in Figure 3c,g,d,h. On the whole, the surface morphologies of the treated superwettable materials did not have obvious changes. Plentiful nanoscaled protrusions could be observed distributed on the surface microstructures (Figure 3g,h), contributing to the high roughness. All of these results demonstrated that the dual rough structures of the prepared superwettable materials were fairly stable and could not be subject to strong acid or alkali, which were beneficial for achieving the superwetting properties of the modified materials. To understand better the superwetting behaviors of the asprepared materials at different states, the chemical compositions of the materials in various conditions were verified by FTIR spectral analysis, as illustrated in Figure 3i. For the pristine kaolin material (Figure 3i1), the wide absorption band located at 3460 cm−1 was assigned to −OH groups on kaolin surfaces, demonstrating intrinsic hydrophilicity of the surfaces. After the modification of PFOA and bis[3-(trimethoxysilyl)propyl]ethylene diamine (Figure 3i3), several new absorption peaks appeared on the modified kaolin material surfaces. The new peak emerging at 1682 cm−1 was mainly ascribed to the −CO groups of amide I bands, and the peak discovered around 1077 cm−1 corresponded to the stretching vibration of C−F in PFOA. The additional vibration peaks at 2947, 2826, and 3500 cm−1 were assigned to −CH3, −CH2−, and N−H groups of bis[3-(trimethoxysilyl)propyl]ethylene diamine. All of these results confirmed that the obtained amide groups attached to the low surfaceenergy perfluorooctyl chains were successfully introduced onto the functionalized kaolin surfaces, contributing to a satisfactory superamphiphobicity of the superwettable material. Besides, it should be noted that the additional weak peak appearing at 3520 cm−1 was attributed to the −OH groups in residual PFOA. However, after the modified kaolin material was treated by an acidic solution 24673


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Figure 5. (a) Schematic abrasion test of the coated glass sample with a load of 200 g on the 600-grit sandpaper. (b) Variations of CAs and SAs of water and hexadecane measured in accordance with the abrasion length of the coated glass surface on the sandpaper. The inserted photograph shows the water (blue) and hexadecane (yellow) on the coated glass surface after a 220 cm abrasion length. (c) Potentiodynamic polarization curves for the pristine Cu sample, single-side-coated Cu sample, and double-side-coated Cu sample. (d) Nyquist plots of the pristine Cu sample, single-side-coated Cu sample, and double-side-coated Cu sample. (e) Bode impedance plots and (f) phase plots of the pristine Cu sample, singleside-coated Cu sample, and double-side-coated Cu sample.

where γ is the surface tension, R is the radius of the meniscus, l and A are separately the perimeter and area of the pore, respectively, and θa is the advancing contact angle of oil or water on the material surface. According to the equation, it can be seen that when θa > 90°, the Δp must be greater than 0, that is, the as-prepared material can withstand a certain pressure of liquid (negative capillary effect) unless external pressure is applied.47 Whereas when θa < 90°, the Δp is apparently less than 0, which means that the as-prepared material is not able to support any pressure (capillary effect); therefore, the liquid will spontaneously permeate through the material surface. In this work, the θa of nonbasic water on the as-prepared material surface is above 150°; as a result, the water droplet maintains a stable sphere on the coated glass surface and will not permeate spontaneously because of Δp > 0 (Figure 4a). However, for the alkali water, the θa is nearly 0° (Figure 4b), which reveals that the water can spread on the coated glass surface owing to Δp < 0. After water permeation, the pores of the material surface will be fully filled with the water phase, which is beneficial to repel oils, as shown in Figure 4c. Under this circumstance, the θa of oil on the as-prepared material surface is obviously greater than 90°, and the Δp is calculated to be larger than 0; thus, oil is

However, after the treatment of the alkali solution, the N 1s peak containing two components appeared at 399.8 and 400.5 eV (Figure 3l), which belonged to the C−N group and the N− H group of bis[3-(trimethoxysilyl)propyl]ethylene diamine, respectively, demonstrating that the perfluorooctyl amide groups were hydrolyzed. All of these results were well consistent with the results of FTIR. Based on the analysis mentioned above, it can be concluded that the excellent superamphiphobicity and pH-induced reversible water wettability of the superwettable material are benefited from the synergistic effect of the pH-sensitive surface compositions and the micro-/nanorough structures of the material surface. To thoroughly understand these unique wetting properties, we have modeled the water and oil wetting processes of the resultant superwettable material surface. The corresponding wetting mechanisms are well demonstrated by the concept of the intrusion pressure (Δp), which can be expressed as45,46 Δp =

lγ(cos θa) 2γ =− R A

(3) 24674


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Figure 6. Buoyancy and loading capacity tests of the coated glass (a, d), coated filter paper (b, e), and coated fabric (c, f) separately on oil and water. (g, h) The side-view photographs of the coated glass carrying a load on water and oil, respectively; the corresponding microscopic images (left) show the shape of the coated glass at higher magnification and exhibit limited dimple depths at maximum supportable weight. (i) Corresponding schematic illustration of the side view of the coated substrate carrying a load on water or oil. (j) Load-bearing capacity of different coated substrates on water and oil.

sandpaper back and forth until the superamphiphobicity was lost. It was noted that the glass sample with its coated side facing the sandpaper was pulled straight ahead for 20 cm one time at a speed of 2 cm·s−1, after which water and hexadecane droplets were applied onto the abraded surface to characterize its wetting behavior. Furthermore, the variations of water and oil contact angles (W/OSAs) and their sliding angles (W/ OSA) with abrasion distance of the coated glass surface are shown in Figure 5b. It can be seen that after a total wearing distance of 220 cm, the WCA of the abraded surface changed slightly from 161.0 ± 0.7 to 155.0 ± 1.5° and OCA changed from 152.0 ± 1.3 to 148.0 ± 1.5°; meanwhile, the WSA and OSA remained relatively steady below 10°. Therefore, the worn surface still maintained favorable water and oil repellency despite such a long-distance abrasion, demonstrating superior mechanical durability, which was advantageous to help the

resisted on the material surface. In addition, it should be highlighted that in air, the θa of oil is about 154° (Figure 4d), inducing a higher Δp to maintain the oil droplet’s stability on the material surface. 3.3. Mechanical Durability and Corrosion Resistance. Durability is an important criterion for particle-based superwetting materials to be applied in practice for a long service time.48 However, the micro-/nanorough structures constructed on the superamphiphobic surfaces are usually too fragile to resist the rigorous mechanical damages, which has become a bottleneck and impedes the large-scale applications of the superwettable material. In this work, we utilized the mechanical abrasion test reported in our previous work to evaluate the mechanical durability and stability of the resultant superamphiphobic material.49 As illustrated in Figure 5a, a coated glass slide with a load of 500 g weight was placed on the 600-grit sandpaper; then, they were pulled as a whole on the 24675


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impedance modulus and phase angle plots of the corresponding Cu samples are presented in Figure 5e,f. It shows that the impedance magnitude (Figure 5e) of the double-side-coated Cu sample was higher than that those of the pristine and single-side-coated Cu samples in the whole frequency range, demonstrating a superior corrosion resistance.55,56 Meanwhile, the phase angle (Figure 5f) of the double-side-coated Cu sample was high and wide in the mid-frequency range, which was well consistent with the above corrosion results. 3.4. Buoyancy and Loading Capacity Tests. As is known, the water strider with unique superhydrophobic legs is able to jump, float, and slide on the water surface; meanwhile, it shows outstanding loading capacity that can support about 15 times its body weight on water just by a single leg.57,58 Such a special property implies that developing miniaturized aquatic devices that can move freely on both water and oil has great potential to significantly improve buoyancy and reduce water or oil drag. To investigate the buoyancy and load-carrying capabilities on water and oil induced by the superwettable materials, glass slides, filter paper, and fabric were utilized as different types of substrates. Notably, the samples were all top-coated, and the weights were gently placed on the middle of the samples to further determine the load capacity of the coated materials. As shown in Figure 6a−c, the coated glass slide, filter paper, and fabric loaded with weights of 1.38, 5.49, and 9.12 g, respectively, could steadily float on sunflower oil despite their densities being significantly higher than that of oil. Correspondingly, the coated glass slide, filter paper, and fabric could also stay on the water surface (Figure 6d−f) and carry loads of 2.73, 9.63, and 12.37 g, respectively, without sinking. Furthermore, taking the coated glass samples as the example, it can be observed that the coated glass slide with a weight of 5.65 g could carry a load of 1.38 g on oil and made an oil dimple of 2.2 mm (Figure 6g). Similarly, on water, the coated glass slide could carry a load of 2.73 g and produced a dimple of 2.6 mm (Figure 6h). The above results indicated that, under load, both of the oil and water surfaces were deformed and formed an obvious dimple (Figure 6i), which could induce significantly enhanced buoyancy force.59 Therefore, the coated samples were capable of supporting a certain amount of weight without sinking. Besides, the load-carrying capabilities of the different coated substrates on water and oil were presented and are compared in Figure 6j. One can find that the coated samples had a better load-bearing performance on water than on oil. It was mainly attributed to the greater water contact angle (WCA ∼ 158°), higher density (0.997 g·cm−3), and larger surface tension (72.8 mN·m−1) of water (for more information, see Figure S2, Supporting Information), which resulted in a greater supporting force and led to better loading capacity.60,61 A comparison of the load-bearing properties of different kinds of coated samples showed that the coated filter paper exhibited more outstanding load-carrying capability, which could support a weight of 28.26 N·m−1 on oil and 49.67 N·m−1 on water. We ascribe this to its relatively light mass, which is beneficial to improving its loading capacity. 3.5. Controllable Separation of the Oil/Water Mixture. In comparison with the typically superamphiphobic materials, the as-prepared superamphiphobic material with smart pH responsiveness offers an additional advantage of realizing oil/water separation, which has been seldom reported in emerging works.41 Herein, with the resultant superwettable materials as the separation membrane, we designed a special

superwetting kaolin material gain more opportunity in practical applications. Apart from robust durability, corrosion resistance is also an important factor that determines the ability to apply superwetting materials in surface corrosion prevention. For this reason,50,51 it is imperative to investigate the corrosionresistance capacities of the resultant superamphiphobic materials. Herein, the electrochemical measurements of the pristine Cu sample and Cu samples coated with the resultant superamphiphobic materials were conducted in 3.5 wt % NaCl aqueous solution. Figure 5c shows the potentiodynamic polarization plots of the uncoated and coated Cu samples, from which we can further obtain the corresponding electrochemical parameters of the different samples, as shown in Table S1. The corrosion potential (Ecorr) and corrosion current density (Icorr) obtained from Tafel linear extrapolation were separately represented by the abscissa and ordinate values of the intersection of the anodic slope (βa) and the cathodic slope (βc). As can be observed in Figure 5c and Table S1, the pristine Cu samples presented much lower Ecorr (−0.289 V) and higher Icorr (−8.015 × 104 A·cm−2), which could be attributed to the initiation of corrosion. Compared with the polarization plot of the pristine Cu sample, the coated Cu samples exhibited an apparent shift to the anodic region, revealing the improved corrosion resistance. Notably, the double-side-coated Cu sample showed maximum Ecorr (−0.015 V) and minimum Icorr (2.619 × 106 A·cm−2) values, which signified a lower corrosion probability and a slow corrosion rate. Besides, for the coated Cu samples, it can be found that the values of the anodic slope were greater than those of the cathodic slope, implying that the hydrogen evolution reaction could not occur on the coated Cu surfaces. These results evidently demonstrate the excellent corrosion resistance of the superwettable materials, which can provide a passive hydroxide film on the Cu samples and effectively protect the copper surface from being attacked by the corrosion ions owing to their superior water repellency.52 Besides, to further evaluate the corrosion-resistance performance of the obtained superamphiphobic material, the electrochemical impedance spectra of the uncoated and coated Cu samples immersed in a 3.5 wt % NaCl aqueous solution were recorded, as shown in Figure 5d. In Nyquist spectra, a larger semicircle diameter typically indicates a lower corrosion rate, which is related to the charge transfer resistance.53 It can be found in Figure 5d that the Nyquist spectrum of the doubleside-coated Cu sample exhibited a large semicircle over the whole frequency, indicating an excellent anticorrosion performance. Moreover, the semicircle diameter of the coated Cu samples in the high-frequency range significantly increased in comparison with that of the uncoated Cu sample, which implied that the as-prepared superwettable coating materials could supply more charge transfer resistance to the sample surface. However, in the mid-frequency range, the single-sidecoated Cu sample showed nearly equal corrosion resistance to that of the pristine Cu sample, revealing a decreased anticorrosion performance, which could be ascribed to the localized corrosion of the superwettable material on the coated side of the sample. As a whole, all of the above results fully demonstrated that the resultant double-side-coated Cu samples possess an excellent corrosion resistance. This may be attributed to the excellent water repellency of the superamphiphobic materials, which could prevent the corrosive ions from attacking the metal surface. 54 Furthermore, the 24676


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ACS Applied Materials & Interfaces device for continuous oil/water separations via ex situ and in situ wettability changes; the specific separation process is shown in Figure 7. Notably, the manual valve of the separation device was kept open during the whole separation process, which would not have any impact on separation. Moreover, to accelerate the separation process, an external pressure of about 0.02 bar was applied onto the device. For ex situ pH-responsive oil/water separation (Figure 7a), the superwettable kaolin particles that filled in the glass tube (4 cm in diameter) were pretreated by alkali, thus showing a favorable superhydrophilicity−superoleophobicity. When a mixture of n-hexane (30 mL, yellow) and water (30 mL, pH = 7, blue) was poured into the separation device, it quickly formed distinct oil/water layers as presented in Figure 7ai. Then, owing to the superhydrophilicity of the treated particle layer, the bottom water solution gradually filtrated through the particle layer and dropped into the below beaker (Figure 7aii). However, the upper n-hexane was blocked on the particle layer and collected in the device because of the favorable superoleophobicity of the pretreated particles (Figure 7aiii). Thus, the separation of oil/water was performed continuously and successfully. For in situ pH-responsive oil/water separations (Figure 7b), the superwettable kaolin particles were directly filled in the glass tube without any ex situ treatment and exhibited an excellent superamphiphobicity. When an oil/water mixture containing n-hexane (15 mL, yellow) and water (15 mL, pH = 7, blue) was poured into the separation device, both of the oil and water were retained on the particle layer despite the effect of the external pressure (Figure 7bi). Then, on pouring the alkali water solution (15 mL, pH = 13, red) into the device (Figure 7ii), the pH condition of the bottom water solution changed to alkaline (pH = 12.7, red) (Figure 7biii). Expectedly, after several minutes of in situ alkalizations, the wettability of the particle layer was converted to superhydrophilic−superoleophobic, resulting in the penetration of the bottom water solution through the particle layer (Figure 7biv,v). Finally, the upper n-hexane layer was retained and held in the separation device (Figure 7bvi). Therefore, each component of the mixture was separated successfully, which indicated the superior oil/water separation performance and favorable pH responsiveness of the smart superwettable materials. 3.6. Evaluation of Oily Emulsion Separation Performance. In view of the smart switchable surface wettability and selective oil/water wetting behavior of the resultant superwettable materials, we also applied complex oil/water emulsions to further evaluate their in situ responsive separation capability, which would be of great significance for practical application. Specifically, two forms of typical oil-in-water emulsions with and without surfactant were used as the emulsions to be separated, and all of these emulsions exhibited high alkalinity (pH 13) and good stability. The separation experiments were conducted under an external pressure of about 0.02 bar to accelerate the separation rates, and the manual valve of the device kept opening during the whole separation process. Taking the surfactant-free and surfactant-stabilized emulsions of hexadecane/water as the examples, the corresponding separation processes are shown in Figure 8a,b. It should be noted that before the separation, the as-prepared superamphiphobic kaolin particles were compactly filled in the glass tubes without any pretreatment. In this case, when 30 mL of the surfactant-free hexadecane/water emulsion (pH 13) was poured into the filtration device, the continuous water phase in

Figure 7. Photographs of the controllable oil/water separation. (a) Ex situ pH-responsive oil/water separation. An oil/water mixture containing n-hexane (yellow) and water (pH = 7, blue) was poured into the separation device filled with the alkali pretreated modified kaolin particles (ai). After a few minutes, the water solution penetrated through the particle layer and dropped into the below beaker (aii), whereas the upper oil was retained in the device (aiii) owing to the superior water affinity and oil repellency of the particle layer. (b) In situ pH-responsive oil/water separation. An oil/water mixture containing n-hexane (yellow) and water (pH = 7, blue) was poured into the separation device filled with the untreated modified kaolin particles (bi), and both of the oil and water were held on the particle layer surface because of the superamphiphobicity of the particles. After pouring the aqueous solution (dyed red with methyl blue) with pH 13 into the tube (bii), the pH of the bottom water solution (in red color) was adjusted to 12.7 (bii), and the water layer was filtrated through the particle layer and was collected in the below beaker (biii, iv), whereas the upper n-hexane layer was repelled from passing through the particle layer (bv, vi) due to the superhydrophilicity and superoleophobicity of the particles. 24677


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Figure 8. (a, b) Photographs of the separation process of the surfactant-free hexadecane/water emulsion (ai−aiii) and the Span80-stabilized hexadecane/water emulsion (biv−bvi). Both of the emulsions were alkaline (pH 13). (c, d) Photographs and optical microscopic images of the hexadecane/water emulsion (c) and Span80/hexadecane/water emulsion (d), respectively, before and after separation.

original milky-white feed emulsion. The successful separation result can be further corroborated by the relevant optical microscope images, as shown in Figure 8d. Lots of spherical oil droplets with a size of hundreds of nanometers to several micrometers could be found uniformly distributed in the feed emulsion. After filtration, no oil droplets could be observed in the optical image of the filtrate, indicating that the oil that dispersed in the original emulsion was successfully removed and intercepted by the particle layer. To confirm the separation results precisely, dynamic light scattering (DLS) measurements of the corresponding oil-inwater emulsions (pH 13) before and after filtration were obtained (Figure 9a,b). For the surfactant-free hexadecane/ water emulsion (Figure 9a), the feed has a wide droplet size range from 120 nm to 7 μm, whereas the filtrate presents a droplet size of 0.5−10 nm. For the Span80/hexadecane/water emulsion (Figure 9b), the droplet size of the feed distributes within 5 nm to 4 μm, whereas the filtrate shows a smaller droplet size of 2.5−14 nm. All of these results revealed that these two kinds of oil-in-water emulsions were separated in high quality by the as-prepared materials. Additionally, the separation efficiency of the resultant superwetting material for

the emulsion was exposed to the surface of the modified kaolin particle layer (Figure 8ai) and induced a switch in surface wettability of the particle layer from superhydrophobicity to superhydrophilicity. Thus, after a few minutes, the clear filtrate could be observed to permeate through the particle layer and gradually gathered in the beaker (Figure 8aii). Ultimately, the original milky emulsion was completely separated and became transparent (Figure 8aiii), which could be further confirmed by the corresponding optical microscopic images in Figure 8c. It could be found that before the separation (left), there were numerous tiny oil droplets of varying sizes distributed in the feed emulsion, whereas no visible and obvious oil droplets were observed in the collected filtrate (right), demonstrating that all oil droplets of hexadecane were separated from the emulsion. Generally, the emulsions stabilized by surfactants are difficult to be demulsified and separated owing to their high stability.21 Herein, the separation experiment of the asprepared superwettable materials for the Span80-stabilized hexadecane-in-water emulsion is performed as shown in Figure 8b. It can be seen that the filtrate became clean and transparent after the separation, which was obviously different from the 24678


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Figure 9. (a, b) DLS measurements of droplet size before and after (inset) filtration of the (a) surfactant-free and (b) Span80-stabilized hexadecane-in-water emulsion. (c) Oil rejection rate of the superwettable materials toward different oil/water emulsions. All of the emulsions were alkaline (pH 13). (d) Cycling separation performance of the superwettable materials for the Span80/hexadecane/water emulsion.

could be almost recovered after a simple wash of deionized water, and there was no significant influence on the corresponding oil rejection rate during the cycling tests. Moreover, it was found that with the cycle number increasing, the permeation flux slightly decreased, whereas the oil rejection rate improved. The possible reason was that with the increase in cycle number, the oil droplets retained on the surface of the particle layer could not be completely removed by pure water and further held on the layer surface, resulting in smaller pore size of the skin layer. These nanoscaled pores not only increased the permeation resistance but also prevented the tiny oil droplets in emulsion from filtrating through the particle layer, thus leading to a decreased permeation flux and an increased oil rejection rate of the filter layer. With respect to the separation results of the above two types of emulsions, we assume that the excellent separation performance mainly benefits from the superior switchable surface wettability and size-sieving capacity of the functionalized superwettable particle layer. On the one hand, the tightly packed superwettable particles have generated plentiful tiny pores of micro-/nanometer size on the skin layer. In this case, when the emulsion droplets came in contact with the particle layer, the rough pore structure of the skin layer could easily lead to the deformation of the emulsion, which significantly decreased the stability of the emulsion droplets. On the other hand, the continuous alkali water phase of the deformed emulsion droplets had more surface area to interact with the particle layer surface and endowed the surface with superhydrophilicity. Therefore, the water droplets in the emulsions could readily penetrate through the particle layer and formed an antioil water film on the layer surface, whereas oil droplets and surfactant in the emulsions were intercepted and blocked by the porous oil-repellent skin layer. Based on the combined effects of favorable size-sieving capacity, smart in situ pH responsiveness, and super water affinity and underwater oil repellency of the filtration layer surface, the oils dispersed in

various oil-in-water emulsions was carried out in Figure 9c. The oil rejection rate of the superwettable kaolin materials for surfactant-free emulsions, such as sunflower oil/water, industry white oil/water, olive oil/water, n-hexane/water, and hexadecane/water, was higher than 99.2%, comparable to that of the most reported oil/water separation materials.38,39 In addition, for the corresponding surfactant-stabilized emulsions, such as Span80/sunflower oil/water, Span80/industry white oil/water, Span80/olive oil/water, Span80/n-hexane/water, and Span80/hexadecane/water, the oil rejection rate could also be more than 98.6%, regardless of their low surface tension and high viscosity, indicative of a superior separation performance. The reusability of the superwettable materials is an essential factor in determining their practical application performance. In this respect, an in situ pH-responsive cycling separation test of the resultant superwetting materials for Span80-stabilized hexadecane/water emulsion (pH 13) was performed, as shown in Figure 9d. It is noteworthy that the as-prepared materials were directly filled in the glass tube without any pretreatment, and after every emulsion separation cycle, the particle layer in the tube was rinsed with deionized water several times without being taken out and then applied for the next separation experiment directly. As a result, the permeation flux and oil rejection rate of each separation cycle were relatively stable without obvious decrease during the whole cycling tests (Figure 9d). In detail, the particle layer maintained a flux of 17.5 L·m−2·L−1 and an oil rejection rate of 99.35% even after 6 cycles of separation, indicating the favorable reusability of the superwettable material. For a specific separation cycle, the permeation flux decreased with the increment of separation time owing to the oil filtration cake gradually forming on the particle layer surface. Fortunately, the oil contaminants and surfactants intercepted by the particle layer could be almost all taken away by water because of the excellent antioil fouling property of the layer surface. Therefore, the permeation flux 24679


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ACS Applied Materials & Interfaces Notes

the emulsion were successfully separated, highlighting a great potential in wastewater treatment. Besides the excellent emulsified wastewater purification property, the superwettable materials also exhibited favorable air filtration performance (for more information, see Figure S3, Supporting Information), which could effectively remove the tiny particulate contaminants from the polluted air environment, achieving the purpose of air purification and improving the air quality. Therefore, the as-prepared superwettable materials will be a good candidate for removing contaminants in both water and air environments.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the continuing financial support from National Natural Science Foundation of China (Grant No. 21473132), the Youth Innovation Team of Shannxi Universities, the Outstanding Youth Science Fund of Xi’an University of Science and Technology (Grant No. 2019YQ2-09), and Huyang Scholar Program of Xi’an University of Science and Technology.

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4. CONCLUSIONS In summary, a multifunctional superwettable material with multiple brilliant performances, such as pH-responsive water wettability, self-cleaning property, favorable buoyancy, and air purification, has been successfully prepared via a convenient and mild strategy. With the synergistic effect of the dual rough structures and unique chemical compositions, the as-prepared material surface exhibits controllable water wettability, which can be reversibly switched between superamphiphobicity and superhydrophilicity−superoleophobicity with the pH of an aqueous solution. The special pH-dependent water wettability endows the as-prepared material with an additional property of controllable oil/water separation and emulsified oily sewage purification, compared with the typical superamphiphobic materials. Moreover, the as-prepared material exhibits favorable durability and stability against concentrated salt solutions and rigorous mechanical abrasion, which are beneficial to its long-term applicability. Because of the superior oil and water repellency, the as-prepared material possesses satisfactory buoyancy and significantly enhanced load-carrying capacity in both oil and water environments, which shows both theoretical significance and practical importance in the development of smart aquatic devices that can freely float on water or oil. Considering the above-mentioned advantages, the as-prepared smart superwettable material will hold great potential in multiple fields, in particular for oil wastewater remediation, oil-spill cleanup, and optimizing multiple operations in industrial fields.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03721.

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REFERENCES

(1) Stokstad, E. Looking Beyond the Spill, Obama Highlights LongTerm Restoration. Science 2010, 328, 1618−1619. (2) Ju, G.; Liu, J.; Li, D.; Cheng, M.; Shi, F. Chemical and Equipment-Free Strategy to Fabricate Water/Oil Separating Materials for Emergent Oil Spill Accidents. Langmuir 2017, 33, 2664−2670. (3) Cheng, M.; Ju, G.; Jiang, C.; Zhang, Y.; Shi, F. Magnetically Directed Clean-Up of Underwater Oil Spills through a Functionally Integrated Device. J. Mater. Chem. A 2013, 1, 13411−13416. (4) Zhang, S.; Jia, G.; Gao, S.; Jin, H.; Zhu, Y.; Zhang, F.; Jin, J. Cupric Phosphate Nanosheets-Wrapped Inorganic Membranes with Superhydrophilic and Outstanding Anticrude Oil-Fouling Property for Oil/Water Separation. ACS Nano 2018, 12, 795−803. (5) Zhang, Q.; Jiang, J.; Gao, F.; Zhang, G.; Zhan, X.; Chen, F. Engineering High-Effective Antifouling Polyether Sulfone Membrane with P(PEG-PDMS-KH570)@SiO2 Nanocomposite via In-Situ SolGel Process. Chem. Eng. J. 2017, 321, 412−423. (6) Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. A Multifunctional Gelatin-Based Aerogel with Superior Pollutants Adsorption, Oil/ Water Separation and Photocatalytic Properties. Chem. Eng. J. 2019, 358, 1539−1551. (7) Su, X.; Li, H.; Lai, X.; Zhang, L.; Liao, X.; Wang, J.; Chen, Z.; He, J.; Zeng, X. Dual-Functional Superhydrophobic Textiles with Asymmetric Roll-Down/Pinned States for Water Droplet Transportation and Oil-Water Separation. ACS Appl. Mater. Interfaces 2018, 10, 4213−4221. (8) Wei, C.; Dai, F.; Lin, L.; An, Z.; He, Y.; Chen, X.; Chen, L.; Zhao, Y. Simplified and Robust Adhesive-Free Superhydrophobic SiO2-Decorated PVDF Membranes for Efficient Oil/Water Separation. J. Membr. Sci. 2018, 555, 220−228. (9) Hou, Y.; Wang, Z.; Guo, J.; Shen, H.; Zhang, H.; Zhao, N.; Zhao, Y.; Chen, L.; Liang, S.; Jin, Y.; Xu, J. Facile Fabrication of Robust Superhydrophobic Porous Materials and Their Application in Oil/ Water Separation. J. Mater. Chem. A 2015, 3, 23252−23260. (10) Deng, Y.; Zhang, G.; Bai, R.; Shen, S.; Zhou, X.; Wyman, L. Fabrication of Superhydrophilic and Underwater Superoleophobic Membranes via an In Situ Crosslinking Blend Strategy for Highly Efficient Oil/Water Emulsion Separation. J. Membr. Sci. 2019, 569, 60−70. (11) Zhang, X.; Zhao, Y.; Mu, S.; Jiang, C.; Song, M.; Fang, Q.; Xue, M.; Qiu, S.; Chen, B. UiO-66-Coated Mesh Membrane with Underwater Superoleophobicity for High-Efficiency Oil-Water Separation. ACS Appl. Mater. Interfaces 2018, 10, 17301−17308. (12) Zhang, G.; Jiang, J.; Zhang, Q.; Gao, F.; Zhan, X.; Chen, F. Ultralow Oil-Fouling Heterogeneous Poly(ether sulfone) Ultrafiltration Membrane via Blending with Novel Amphiphilic Fluorinated Gradient Copolymers. Langmuir 2016, 32, 1380−1388. (13) Lu, X.; Kong, Z.; Xiao, G.; Teng, C.; Li, Y.; Ren, G.; Wang, S.; Zhu, Y.; Jiang, L. Polypyrrole Whelk-Like Arrays toward Robust Controlling Manipulation of Organic Droplets Underwater. Small 2017, 13, No. 1701938. (14) Yang, X.; Breedveld, V.; Choi, W. T.; Liu, X.; Song, J.; Hess, D. W. Underwater Curvature-Driven Transport between Oil Droplets on Patterned Substrates. ACS Appl. Mater. Interfaces 2018, 10, 15258− 15269.

Chemicals, instruments, and corresponding measurements applied in the experimental procedure; universal applicability to various substrates and self-cleaning capabilities of the resultant superwettable materials; supporting force analysis of the coated samples floating on water and oil; particulate matter removal performance of the superwettable materials; electrochemical measurements for the pristine and coated Cu substrates (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Q.). *E-mail: [email protected] (J.H.). ORCID

Mengnan Qu: 0000-0002-0684-4162 24680


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Oil/Water Separation. ACS Appl. Mater. Interfaces 2016, 8, 31281− 31288. (35) Ju, G.; Cheng, M.; Shi, F. A pH-Responsive Smart Surface for the Continuous Separation of Oil/Water/Oil Ternary Mixtures. NPG Asia Mater. 2014, 6, No. e111. (36) Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. Renewable, BiomassDerived, Honeycomblike Aerogel as a Robust Oil Absorbent with Two-Way Reusability. ACS Sustainable Chem. Eng. 2017, 5, 10307− 10316. (37) Cheng, Z.; Lai, H.; Du, Y.; Fu, K.; Hou, R.; Chong, L.; Zhang, N.; Sun, K. pH-Induced Reversible Wetting Transition between the Underwater Superoleophilicity and Superoleophobicity. ACS Appl. Mater. Interfaces 2014, 6, 636−641. (38) Li, J.; Zhou, Y.; Luo, Z. Smart Fiber Membrane for pH-Induced Oil/Water Separation. ACS Appl. Mater. Interfaces 2015, 7, 19643− 19650. (39) Fu, Y.; Jin, B.; Zhang, Q.; Zhan, X.; Chen, F. pH-Induced Switchable Superwettability of Efficient Antibacterial Fabrics for Durable Selective Oil/Water Separation. ACS Appl. Mater. Interfaces 2017, 9, 30161−30170. (40) Wang, B.; Liang, W.; Guo, Z.; Liu, W. Biomimetic Superlyophobic and Super-lyophilic Materials Applied for Oil/Water Separation: A New Strategy Beyond Nature. Chem. Soc. Rev. 2015, 44, 336−361. (41) Xu, Z.; Zhao, Y.; Wang, H.; Wang, X.; Lin, T. A Superamphiphobic Coating with an Ammonia-Triggered Transition to Superhydrophilic and Superoleophobic for Oil-Water Separation. Angew. Chem., Int. Ed. 2015, 54, 4527−4530. (42) Li, X.; Reinhoudt, D.; Crego-Calama, M. What Do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36, 1350−1368. (43) Zhou, Y.; Li, J.; Luo, Z. Toward Efficient Water/Oil Separation Material: Effect of Copolymer Composition on pH-Responsive Wettability and Separation Performance. AIChE J. 2016, 62, 1758− 1771. (44) Zhang, L.; Zhang, Z.; Wang, P. Smart Surfaces with Switchable Superoleophilicity and Superoleophobicity in Aqueous Media: Toward Controllable Oil/Water Separation. NPG Asia Mater. 2012, 4, No. e8. (45) Liu, B.; Lange, F. F. Pressure Induced Transition between Superhydrophobic States: Configuration Diagrams and Effect of Surface Feature Size. J. Colloid Interface Sci. 2006, 298, 899−909. (46) Cheng, Z.; Wang, J.; Lai, H.; Du, Y.; Hou, R.; Li, C.; Zhang, N.; Sun, K. pH-Controllable On-Demand Oil/Water Separation on the Switchable Superhydrophobic/Superhydrophilic and Underwater Low-Adhesive Superoleophobic Copper Mesh Film. Langmuir 2015, 31, 1393−1399. (47) Tian, D.; Zhang, X.; Wang, X.; Zhai, J.; Lei Jiang, L. Micro/ nanoscale Hierarchical Structured ZnO Mesh Film for Separation of Water and Oil. Phys. Chem. Chem. Phys. 2011, 13, 14606−14610. (48) Dong, J.; Wang, Q.; Zhang, Y.; Zhu, Z.; Xu, X.; Zhang, J.; Wang, A. Colorful Superamphiphobic Coatings with Low Sliding Angles and High Durability Based on Natural Nanorods. ACS Appl. Mater. Interfaces 2017, 9, 1941−1952. (49) Qu, M.; Ma, X.; Hou, L.; Yuan, M.; He, J.; Xue, M.; Liu, X.; He, J. Fabrication of Durable Superamphiphobic Materials on Various Substrates with Wear-Resistance and Self-Cleaning Performance from Kaolin. Appl. Surf. Sci. 2018, 456, 737−750. (50) Xiang, T.; Han, Y.; Guo, Z.; Wang, R.; Zheng, S.; Li, S.; Li, C.; Dai, X. Fabrication of Inherent Anticorrosion Superhydrophobic Surfaces on Metals. ACS Sustainable Chem. Eng. 2018, 6, 5598−5606. (51) Argade, G. R.; Kandasamy, K.; Panigrahi, S. K.; Mishra, R. S. Corrosion Behavior of a Friction Stir Processed Rare-Earth Added Magnesium Alloy. Corros. Sci. 2012, 58, 321−326. (52) Ishizaki, T.; Masuda, Y.; Sakamoto, M. Corrosion Resistance and Durability of Superhydrophobic Surface Formed on Magnesium Alloy Coated with Nanostructured Cerium Oxide Film and

(15) Lin, Y.; Hu, Z.; Zhang, M.; Xu, T.; Feng, S.; Jiang, L.; Zheng, Y. Magnetically Induced Low Adhesive Direction of Nano/Micropillar Arrays for Microdroplet Transport. Adv. Funct. Mater. 2018, 28, No. 1800163. (16) Qin, L.; Zhao, J.; Lei, S.; Pan, Q. A Smart “Strider” Can Float on Both Water and Oils. ACS Appl. Mater. Interfaces 2014, 6, 21355− 21362. (17) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing Superoleophobic Surfaces. Science 2007, 318, 1618−1622. (18) Qu, M.; Ma, L.; Zhou, Y.; Zhao, Y.; Wang, J.; Zhang, Y.; Zhu, X.; Liu, X.; He, J. Durable and Recyclable SuperhydrophilicSuperoleophobic Materials for Efficient Oil/Water Separation and Water-Soluble Dyes Removal. ACS Appl. Nano Mater. 2018, 1, 5197− 5209. (19) Wang, S.; Liu, K.; Yao, X.; Jiang, L. Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications. Chem. Rev. 2015, 115, 8230−8293. (20) Zeng, J.; Wang, B.; Zhang, Y.; H. Zhu, H.; Guo, Z. Strong Amphiphobic Porous Films with Oily-Self-Cleaning Property beyond Nature. Chem. Lett. 2014, 43, 1566−1568. (21) Ge, J.; Zong, D.; Jin, Q.; Yu, J.; Ding, B. Biomimetic and Superwettable Nanofibrous Skins for Highly Efficient Separation of Oil-in-Water Emulsions. Adv. Funct. Mater. 2018, 28, No. 1705051. (22) Zhang, G.; Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. Amphiphilic Poly(ether sulfone) Membranes for Oil/Water Separation: Effect of Sequence Structure of the Modifier. AIChE J. 2017, 63, 739−750. (23) Zhu, H.; Yang, S.; Chen, D.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. A Robust Absorbent Material Based on Light-Responsive Superhydrophobic Melamine Sponge for Oil Recovery. Adv. Mater. Interfaces 2016, 3, No. 1500683. (24) Zeng, X.; Yang, K.; Huang, C.; Yang, K.; Xu, S.; Wang, L.; Pi, P.; Wen, X. Novel pH-Responsive Smart Fabric: From Switchable Wettability to Controllable On-Demand Oil/Water Separation. ACS Sustainable Chem. Eng. 2019, 7, 368−376. (25) Zheng, X.; Guo, Z.; Tian, D.; Zhang, X.; Jiang, L. Electric Field Induced Switchable Wettability to Water on the Polyaniline Membrane and Oil/Water Separation. Adv. Mater. Interfaces 2016, 3, No. 1600461. (26) Rao, Q.; Li, A.; Zhang, J.; Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. Multi-Functional Fluorinated Ionic Liquid Infused Slippery Surfaces with Dual-Responsive Wettability Switching and SelfRepairing. J. Mater. Chem. A 2019, 7, 2172. (27) Qing, W.; Shi, X.; Zhang, W.; Wang, J.; Wu, Y.; Wang, P.; Tang, C. Solvent-Thermal Induced Roughening: A Novel and Versatile Method to Prepare Superhydrophobic Membranes. J. Membr. Sci. 2018, 564, 465−472. (28) Lin, Y.; Hu, Z.; Zhang, M.; Xu, T.; Feng, S.; Jiang, L.; Zheng, Y. Magnetically Induced Low Adhesive Direction of Nano/Micropillar Arrays for Microdroplet Transport. Adv. Funct. Mater. 2018, 28, No. 1800163. (29) Chang, J.; Zhang, L.; Wang, P. Intelligent Environmental Nanomaterials. Environ. Sci.: Nano 2018, 5, 811−836. (30) Wang, Y.; Lai, C.; Wang, X.; Liu, Y.; Hu, H.; Guo, Y.; Ma, K.; Fei, B.; Xin, J. H. Beads-on-String Structured Nanofibers for Smart and Reversible Oil/Water Separation with Outstanding Antifouling Property. ACS Appl. Mater. Interfaces 2016, 8, 25612−25620. (31) Xu, L.; Liu, N.; Cao, Y.; Lu, F.; Chen, Y.; Zhang, X.; Feng, F.; Wei, Y. Mercury Ion Responsive Wettability and Oil/Water Separation. ACS Appl. Mater. Interfaces 2014, 6, 13324−13329. (32) Yang, C.; Wu, L.; Li, G. Magnetically Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation. ACS Appl. Mater. Interfaces 2018, 10, 20150−20158. (33) Li, J.; Zhou, Y.; Luo, Z. Polymeric Materials with Switchable Superwettability for Controllable Oil/Water Separation: A Comprehensive Review. Prog. Polym. Sci. 2018, 87, 1−33. (34) Dang, Z.; Liu, L.; Li, Y.; Xiang, Y.; Guo, G. In Situ and Ex Situ pH-Responsive Coatings with Switchable Wettability for Controllable 24681


Research Article

ACS Applied Materials & Interfaces Fluoroalkylsilane Molecules in Corrosive NaCl Aqueous Solution. Langmuir 2011, 27, 4780−4788. (53) Yuan, R.; Wu, S.; Yu, P.; Wang, B.; Mu, L.; Zhang, X.; Zhu, Y.; Wang, B.; Wang, H.; Zhu, J. Superamphiphobic and Electroactive Nanocomposite toward Self-Cleaning, Antiwear, and Anticorrosion Coatings. ACS Appl. Mater. Interfaces 2016, 8, 12481−12493. (54) Yazdi, E. G.; Ghahfarokhi, Z. S.; Bagherzadeh, M. Protection of Carbon Steel Corrosion in 3.5% NaCl Medium by Aryldiazonium Grafted Graphene Coatings. New J. Chem. 2017, 41, 12470−12480. (55) Mousavinejad, T.; Bagherzadeh, M. R.; Akbarinezhad, E.; Ahmadi, M.; Guinel, M. J. F. A Novel Water-Based Epoxy Coating using Self-Doped Polyaniline-Clay Synthesized under Supercritical CO2 Condition for the Protection of Carbon Steel Against Corrosion. Org. Coat 2015, 79, 90−97. (56) Conde, A.; De Damborenea, J. Evaluation of Exfoliation Susceptibility by means of the Electrochemical Impedance Spectroscopy. Corros. Sci. 2000, 42, 1363−1377. (57) Gao, X.; Jiang, L. Biophysics: Water-Repellent Legs of Water Striders. Nature 2004, 432, 36. (58) Liu, X.; Gao, J.; Xue, Z.; Chen, L.; Lin, L.; Jiang, L.; Wang, S. Bioinspired Oil Strider Floating at the Oil/Water Interface Supported by Huge Superoleophobic Force. ACS Nano 2012, 6, 5614−5620. (59) Hwang, G. B.; Patir, A.; Page, K.; Lu, Y.; Allan, E.; Parkin, I. P. Buoyancy Increase and Drag-Reduction through a Simple Superhydrophobic Coating. Nanoscale 2017, 9, 7588−7594. (60) Zhao, J.; Zhang, X.; Chen, N.; Pan, Q. Why Superhydrophobicity Is Crucial for a Water-Jumping Microrobot? Experimental and Theoretical Investigations. ACS Appl. Mater. Interfaces 2012, 4, 3706−3711. (61) Zhang, X.; Zhao, J.; Zhu, Q.; Chen, N.; Zhang, M.; Pan, Q. Bioinspired Aquatic Microrobot Capable of Walking on Water Surface Like a Water Strider. ACS Appl. Mater. Interfaces 2011, 3, 2630−2636.

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Multifunctional Superwettable Material with Smart pH Responsiveness

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