Stable Superwetting Meshes for On-Demand Separation of Immiscible

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Stable Superwetting Meshes for On-Demand Separation of Immiscible Oil/Water Mixtures and Emulsions Mingming Liu, Yuanyuan Hou, Jing Li, and Zhiguang Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00658 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Stable Superwetting Meshes for On-Demand Separation of Immiscible Oil/Water Mixtures and Emulsions Mingming Liu,†,§ Yuanyuan Hou,†,§ Jing Li,*,† and Zhiguang Guo*,†,‡ †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, Lanzhou 730000, People’s Republic of China ‡

Ministry of Education Key Laboratory for the Green Preparation and Application of Functional

Materials, Hubei University, Wuhan 430062, People’s Republic of China §

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

ABSTRACT: Oil-water separation is of great importance for the treatment of oily wastewater, including immiscible light/heavy oil-water mixtures, oil-in-water or water-in-oil emulsions. Recently, interfacial materials (especially filtration membranes) with special wettability have been broadly developed to solve the environmental problems by virtue of their advantages in energy saving, high flux and good selectivity. However, the given wetting property (superhydrophilicity or superhydrophobicity) and pore size and poor stability of filtration membranes limit their widespread applications, which is far from meeting a wide variety of oil

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polluted water. Here polypyrrole-coated meshes with underwater superoleophobicity and underoil superhydrophobicity as well as controllable pore size were prepared by adopting cyclic voltammetry. It is found that the surface micro-/nanohierarchical structures play a critical role in the formation of underwater superoleophobicity and underoil superhydrophobicity. HCl is advantageous to the construction of highly rough surface rather than H2SO4 and H3PO4. The obtained filtration membranes can be used for the on-demand separation of oil-water mixtures, showing outstanding stability in harsh conditions, such as high temperature (80 ºC), low temperature (0 ºC), salt (0.5 M NaCl) and acid (1 M HCl), except for alkali (1 M NaOH).

INTRODUCTION Oily wastewater is one of water pollution problems with global attention. As reported by the International Tanker Owners Pollution Federation (ITOPF), one hundred and eighty medium spills (7-700 tonnes) and 43 large spills (> 700 tonnes) of various oils were recorded from 2000 to 2015, in which approximately 229 thousand tonnes lost to the environment. The oil spills can cause a wide range of impacts on social and economic activities, fisheries and mariculture, as well as marine environment. In addition, the development of chemical and automobile industries, densely covered gas stations, and the endless discharged domestic wastewater aggravate the environmental problem of oily wastewater. Thus, how to realize high-efficiency separation of oil and water has been a worldwide challenge. In the last ten years, interfacial materials with special wettability have attracted extensive interests, especially pressure driven filtration membranes.1-5 Combining with surface modification technologies, a variety of nanomaterials and nanotechnologies have been widely exploited to construct superwetting surfaces, generally superhydrophobic-superoleophilic or superhydrophilic-underwater superoleophobic materials.6-15


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For

example,

typical

superhydrophobic

polytetrafluoroethylene-coated

meshes6

and

nanoparticle-coated textiles with modifiers8 have been used for the oil-water separation, which is more suitable for immiscible heavy oil/water mixtures. As an optimal choice for light oil-water separation, interfacial materials with underwater superoleophobicity have been prepared, such as hydrogel-,7 zeolite-,9 and graphene oxide-coated10 meshes. However, due to their large pore sizes, these superwetting filtration membranes cannot apply to various emulsions. Recently, superhydrophobic and underwater superoleophobic filtration membranes with small pore size have been designed to separate water-in-oil and oil-in-water emulsions, respectively.16-25 To achieve the selective penetration of oil or water, smart materials as building blocks have been employed to construct filtration membranes with switchable wettability between superhydrophobicity and underwater superoleophobicity under external stimuli.26-36 For example, Tian et al. prepared aligned ZnO nanorod-coated meshes for photo-induced oil-water separation. Water remained on the obtained meshes that intelligently allowed water to pass through under UV illumination.27 Hg2+-responsive poly(acrylic acid)29 and CO2-response poly(N,Ndiethylaminoethyl methacrylate)31 were used to fabricate smart membranes for controllable separation of immiscible oil/water mixtures. To date, few smart membranes have been expanded for on-demand emulsion separation in response to external stimuli. A thermo-responsive polymer membrane was produced by coating poly(N-isopropylacrylamide) on the surface of polyurethane microfibers, which can separate oil-in-water emulsions at room temperature and water-in-oil emulsions at 45 ºC.36 There is a critical need for a facile synthetic method capable of making filtration membranes with controllable pore size and applicability to various oil-water mixtures without any continuous external stimulus,37-40 including immiscible light/heavy oil-water mixtures, oil-in-water or water-in-oil emulsions. In addition, the aforementioned smart


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membranes can hardly satisfy practical requirements under harsh environmental conditions for a long time use (such as high and low temperature, high-concentration salt, acid and alkali). In this work, we reported a simple electrochemical method to prepare polypyrrole (PPy)coated stainless steel meshes (SSMs). By Just adjusting the cyclic voltammetry (CV) number in 1 M HCl, the pore size of the obtained PPy-coated SSMs can be availably regulated from microscale to nanoscale. Without any continuous external stimulus, the filtration membranes with different pore sizes show underwater superoleophobic and underoil superhydrophobic. Therefore, under only gravity, various oil-water mixtures can be economically and effectively separated by using the PPy-coated meshes with appropriate pore sizes regardless of oil density (immiscible oil-water mixtures) or the type of disperse phase (emulsions), achieving the most comprehensive on-demand separation. Moreover, even though the PPy-coated meshes are immersed in various harsh solutions for one month, such as acid, salt, high and low temperature, the amphiphilic filtration membranes can still maintain excellent separation performance.

EXPERIMENTAL SECTION Materials. SSMs (2300 mesh size) were commercially available. Diesel was purchased from adjacent gas station. Other chemical reagents were analytical grade and used without further purification. Deionized water was used throughout the work. Preparation of PPy-Coated SSMs. PPy-coated meshes were prepared by electrochemical polymerization in a three-electrode system at room temperature. A saturated calomel electrode and two pieces of SSMs (3 cm in length and 2.5 cm in width) were chose as reference, and working and counter electrodes, respectively. The aqueous electrolytes were 60 mL of 0.1 M pyrrole in 1 M HCl, H2SO4 and H3PO4, respectively. CV was used by commanding potential


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from -0.2 to 0.9 V at a scan rate of 50 mV/s. Subsequently, the PPy-coated SSMs were taken out and thoroughly washed by deionized water to remove the residues, followed by drying in an oven. PPy-coated SSMs prepared in HCl, H2SO4 and H3PO4 are termed as SSMX-Cl, SSMX-S and SSMX-P, respectively, where X represents CV numbers. In addition, a slice of conductive glass as a working electrode was used to replace SSM and to be coated with PPy. Preparation of Oil-in-Water and Water-in-Oil Emulsions. Oil-in-water emulsions were prepared by mixing water and oil (hexane, n-hexadecane, and silicone oil) at a volume ratio of 100:1 with addition of 0.1 g/L Tween 80 under sharp stirring for 6 hours. Water-in-oil emulsions were prepared by mixing water and oil (dichloroethane, chloroform) at a volume ratio of 1:100 with addition of 2 g/L Span 80 under sharp stirring for 6 hours. In addition, diesel-in-water and water-in-diesel emulsions were prepared by mixing water and diesel at a volume ratio of 100:1 and 1:100, respectively, followed by sonication for 30 min and then stirring for 6 hours. All milky surfactant-stabilized emulsions were highly stable for 24 hours. Oil-Water Separation Experiments. The obtained PPy-coated meshes were fixed between two glass tubes and placed vertically. The immiscible oil-water mixtures (50%, v/v) were poured onto the surface of SSM5-Cl that was prewetted by water or oil. The flux was calculated by measuring the time after collecting 30 ml of water or oil. In addition, SSM25-Cl was used for the emulsion separation. Flux was determined by calculating the volume of filtrates per unit time according to the equation: flux = V/St, where V is the volume of filtrates, S is the area of membranes, and t is the testing time (2 min). All separation process was carried out under gravity and the height of immiscible oil-water mixtures and feed emulsions were kept at 10 cm. Characterization. All optical images were taken by a digital camera (Sony, DSC-HX200). The morphology of PPy-coated meshes was investigated by a field-emission scanning electron


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microscope with Au-sputtered specimens (FESEM, JEOL JSM-6701F). The accelerating voltage was 5 kV and the current was 10 µA. The element distribution maps of PPy-coated membranes were got by energy dispersive spectroscopy (EDS, Kevex). The chemical compositions were investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) and Fourier transform infrared spectra (FTIR, Thermo Scientific Nicolet iS10). Optical microscope images of the feed emulsions and the corresponding filtrates were obtained by using OLYMPUS BX51 microscope. Oil contact angles (OCAs) and water contact angles (WCAs) were measured on a JC20001 contact angle system (Zhongchen digital equipment Co., Ltd. Shanghai, China). Organic contents in the collected water were analyzed by measuring chemical oxygen demand (COD, HACH DRB 200). The purity of the collected oils was examined by using a Karl Fischer titrator (Metrohm 831 KF, Switzerland). The sizes of the feed emulsions were calculated by dynamic light scattering (DLS) analysis with a Zetasizer Nano ZS (Malvern 3600, UK). All measurements were repeated for 3-5 times and the results were reproducible with relative errors less than ±5%.

RESULTS AND DISCUSSION An electrochemical method was adopted to fabricate PPy-coated SSMs with different pore sizes by controlling the CV numbers. Figure 1 shows CV curves of 0.1 M pyrrole in 1 M HCl aqueous solution. The CV potential was set between -0.2 and 0.9 V. Pyrrole is easily polymerized by electrochemical oxidation. A reduction peak appears at about 0.4 V in the third CV curve (Figure 1a). After 5-times CV, PPy is completely coated on the SSM surface, forming a rough micro- and nano-structural surface (Figure 1b and 1c). Compared to original smooth SSM (see Figure S1 in the Supporting Information), the pore size of SSM5-Cl slightly changes


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(5 µm). As the CV number is increased, the oxidation and reduction current intensities at 0.1~0.5 V are gradually increased (Figure 1d), which is attributed to the increase of PPy on the SSM surface. As for SSM25-Cl (Figure 1e and 1f), a large number of PPy nanoparticles form an interconnected network and fully cover the SSM. The controllable pore size of the obtained PPycoated meshes via the CV numbers can be clearly verified by the analysis of element distribution maps (Figure 2 and 3). The main elements such as C, N, Cl of SSM25-Cl are obviously more than that of SSM5-Cl, except for Fe. From Cl distribution map, it is noted that the pore size of SSM25-Cl is much less than 1 µm. XPS and FTIR spectra were used to analyze the chemical compositions of SSMs before and after coating PPy (Figure S2). The original mesh has three characteristic XPS peaks corresponding to Fe, C and O. After the modification of PPy, the Fe peak vanishes and new peaks of N and Cl appear. In addition, FTIR spectrum of SSM5-Cl has the peaks at 776, 913, 1036, 1169, 1286 and 1459 cm-1, which are assigned to C–H and C–N vibration, respectively. The characteristic peaks at 1566 and 1672 cm-1 are corresponding to C–C and C=C vibration of the pyrrole ring. Therefore, the XPS and FTIR results further confirm the successful modification of PPy on the SSM surface. Note that Cl ions and amine groups are hydrophilic and C–H, C–C and C=C units are hydrophobic. In other word, PPy with high conjugated structures would be amphiphilic. Besides HCl, other inorganic acids as electrolyte and doped acid were also selected to prepare PPy-coated SSMs by CV method. It is found that the type of inorganic acids has great impact on the electrochemical polymerization of PPy on the SSM surface. Figure S3 and S4 show SEM images of SSM5-S, SSM25-S, SSM5-P, and SSM25-P. Differently, the surfaces of PPy-coated SSMs using H2SO4 and H3PO4 are smoother than that of SSM-Cl. The electrochemical growth of


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PPy on the SSM surface slows down in H2SO4 and H3PO4 aqueous solutions, leading to only a large porous structure. In detail, we investigated the compositions and structures of the PPycoated SSMs from HCl, H2SO4 and H3PO4 (Figure 2 and 3) by the element distribution maps. Obviously, Cl distribution density of SSM25-Cl is higher than S and P distribution density of SSM25-S and SSM25-P. From EDS surface analysis, the mass percentages of Cl, S and P in SSM25-Cl, SSM25-S and SSM25-P are 8.5%, 2.4% and 1.3%, respectively. The Fe mass percentage of SSM25-Cl (6.9%) is less than that of SSM25-S (17.4%) and SSM25-P (16.3%). Together, it can be drawn that HCl is beneficial to the construction of highly rough surface and controllable pore size rather than H2SO4 and H3PO4. Superwettability of interfacial materials is of great advantage to their environmental applications. Furthermore, we systematically studied the wetting properties of SSM0, SSM-Cl, SSM-S, and SSM-P. The original SSM is hydrophobic with a WCA in air of about 120º, showing underwater oleophobicity and underoil hydrophobicity with a high adhesion force (Figure S5). After the modification of PPy, the coated SSMs become more hydrophilic with WCAs less than 40º. Interestingly, superior to SSM-S and SSM-P, SSM-Cl exhibit underwater superoleophobic and underoil superhydrophobic properties (Figure 4 and 5). Figure 4 shows underwater OCAs and underoil WCAs of SSM-Cl, SSM-S, and SSM-P for a series of oils. It is found that both OCAs and WCAs of SSM-Cl are above 150º. On the contrary, OCAs and WCAs of SSM-S and SSM-P are all below 150º in spite of the increase of CV numbers. In addition, dynamic adhesion measurements were used to demonstrate the superwetting properties of SSMCl rather than SSM-S and SSM-P (Figure S6-S9). When a water droplet in oil or an oil droplet in water contacts the surface of SSM-Cl, SSM-S and SSM-P under external pressure, the droplets are distorted and easily detach only from the surfaces of SSM5-Cl and SSM25-Cl without any


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residues after lifting up. The droplets can effortlessly roll off with oil and water sliding angles of about 12º and 6º (Movie S1), respectively, indicating a low adhesion force of SSM-Cl. The wetting properties of interfacial materials in the solid/oil/water three-phase system are determined by the surface chemical compositions and topographical roughness. We suggest that the PPy coatings play a decisive role in the underwater superoleophobicity and underoil superhydrophobicity of SSM-Cl. The chemical compositions of SSM-S and SSM-P should be similar to that of SSM-Cl. To further investigate the relationship between topographical roughness and wettability, the PPy coatings were prepared on conductive glasses as flat substrates by using the aforementioned electrochemical method. After 25-times CV, it is obviously seen that the PPy coating prepared in HCl has higher roughness than that in H3PO4 (Figure S10). Similar to SSM, the PPy coating (HCl) on the conductive glass possesses underwater superoleophobic and underoil superhydrophobic properties. As expected, the PPy coating (H3PO4) is not superwetting (Figure S11). As shown in Figure 5, the increased roughness of SSM-Cl can efficiently enhance the surface wettability, leading to superwetting properties and a low adhesion force. Next, we used SSM-Cl as interfacial materials for oily wastewater treatment. There are two dilemmas for the high-efficiency oil-water separation. First, larger pore size of filtration membrane gives a higher flux, but could result in lower separation efficiency, especially for emulsions. Second, superhydrophobic filtration membrane is suitable to separate immiscible heavy oil-water mixtures or water-in-oil emulsions, and vice versa. Facing a variety of oily wastewater, here SSM as an only robust substrate was selected and a facile electrochemical method was adopted to control pore size. Without any external stimulus, the obtained PPy-coated SSMs can be used for the on-demand separation of oil-water mixtures,


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including immiscible light/heavy oil-water mixtures (SSM5-Cl), oil-in-water and water-in-oil emulsions (SSM25-Cl). In contrast, the unmodified SSM cannot perform the on-demand separation (Figure S12). Figure 6 shows the separation of immiscible oil-water mixtures using SSM5-Cl. For example, a mixture of water and n-hexadecane dyed by Sudan Red (50%, v/v) was poured on the prewetted surface of SSM5-Cl that was fixed between two glass tubes. Note that gravity is the force for the oil-water separation. Due to the excellent underwater superoleophobicity and low adhesion force of SSM5-Cl, n-hexadecane is blocked over SSM5-Cl (Figure S13). On the contrary, water with a higher density quickly permeates through the superhydrophilic mesh and drops into the beaker below. As shown in Figure 6c, no visible n-hexadecane can be observed in the collected water. As a result, the high-efficiency separation of water and light oils can be achieved using PPy-coated mesh with large pore size, showing high water fluxes (nearly 5000 L m-2 h-1) and low COD values in the collected water (less than 50 mg/L). The “water removing” filtration membranes are not an optimal choice for heave oil-water separation, because heave oils will accumulate and form a barrier layer to prevent water permeation. Interestingly, the PPy-coated mesh not only show underwater superoleophobic but underoil superhydrophobic. Chloroform with a higher density (50%, v/v) can rapidly and selectively pass through SSM5-Cl rather than water dyed by Methylene Blue (Figure 6d). The fluxes and purities of the collected heavy oils exceed 10000 L m-2 h-1 and 99.9%, respectively. However, SSM5-Cl with large pore size cannot separate emulsions, especially surfactantstabilized emulsions. We prepared three kinds of oil-in-water and water-in-oil emulsions, respectively. Droplet sizes were analyzed by microscope images (Figure S14 and S15) and DLS measurements (Figure


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S16). The sizes are mainly 1-5 µm for oil-in-water emulsions and 0.2-2 µm for water-in-oil emulsions, which are smaller than the pore size of SSM5-Cl. Therefore, SSM5-Cl is not available for the emulsion separation. To effectively treat emulsified wastewater, the pore size of filtration membranes should be decreased despite sacrificing flux. With only increasing CV number, the obtained PPy-coated mesh (SSM25-Cl) can be used for on-demand emulsion separation just under gravity. Clearly, all turbid emulsions including oil-in-water and water-in-oil emulsions become transparent after separation using SSM25-Cl. From the microscope images, plenty of emulsion droplets intensively distribute in the visual field, whereas no droplets are observed in the corresponding filtrates. Moreover, we used DLS to analyze the collected water or oils. No DLS signals can be observed, indicating the complete removal of emulsion droplets. The fluxes of oil-in-water and water-in-oil emulsions are decreased to over 300 and 500 L m-2 h-1 (Figure 7), respectively, which are higher than those of conventional microfiltration and ultrafiltration membranes under high pressure (more than 1 bar).41-45 The COD values of the collected water are in the range of 300~500 mg/L, which mainly contain the emulsifier residues in the filtrates. In addition, the oil purities after separating water-in-oil emulsions are over 99.9%. In a word, under only gravity, oily wastewater can be economically and effectively separated by using our PPy-coated meshes with appropriate pore sizes regardless of oil density (immiscible oil-water mixtures) or the type of disperse phase (emulsions). For high-viscous oils such as silicone oil, it is found that SSM-Cl also exhibits underwater superoleophobic property with underwater OCA above 150º and low oil-adhesion characteristic (Figure S17). SSM5-Cl and SSM25-Cl can efficiently separate immiscible silicone oil-water mixture and silicone oil-in-water emulsion (Figure S18, S19, and Movie S2), respectively. However, the filtration technology using superwetting membranes is only suitable to selectively


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pass water. We suggest that selective water adsorption would be optimal to separate the mixtures containing water and high-viscous oils, especially water-in-oil emulsions.46 To further examine the recyclability of SSM-Cl for on-demand separation, we chose nhexadecane-in-water emulsion stabilized by 0.1 g/L Tween 80 and water-in-chloroform emulsion stabilized by 2 g/L Span 80 as model feed emulsions. In each cycle, SSM25-Cl was used to separate the n-hexadecane-in-water and water-in-chloroform emulsions in alternation for three times under gravity, followed by washing with ethanol. After each cycle, underwater OCAs and underoil WCAs were measured. Although both water and oil fluxes gradually decrease as separation time is increased in one cycle, the fluxes can recover its initial level in the next cycle (Figure 8a). Moreover, the CAs of SSM25-Cl are kept more than 150º, showing highly stable underwater superoleophobic and underoil superhydrophobic properties without apparent change of surface morphology (Figure 8b-8e). It is suggested that the PPy-coated meshes has excellent stability and antifouling performance. The stability under harsh conditions is important for the practical applications of filtration membranes. Hence, we systematically studied the resistibility of SSM-Cl to various severe environments by immersing the membrane into various corrosive solutions for one month, such as high temperature (80 ºC), low temperature (0 ºC), acid (1 M HCl), alkali (1 M NaOH) and salt (0.5 M NaCl). It is found that after one-month immersion, SSM25-Cl seems to be no change. However, the alkaline aqueous solution changes from colorless transparent to yellow brown, indicating that the PPy coatings are degraded in the strong alkaline solution over time (Figure S20). Furthermore, we used CA measurements to study the change of their wettability. As shown in Figure S21, the underwater OCAs and underoil WCAs of SSM25-Cl are all over 150º after immersing in the aforementioned corrosive solutions for one month, except for the strong


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alkaline solution. The underoil WCA changes from 160º to zero. Figure 9 shows SEM images of SSM25-Cl after immersing in the aforementioned corrosive solutions for one month. It can be seen that the surface structures of SSM25-Cl remain unchanged under harsh conditions including high temperature, low temperature, acid, and salt. In contrast, the PPy coatings have been seriously destroyed after immersing in the strong alkaline solution for one month. These data illuminate that the PPy-coated SSMs have excellent stability under various harsh conditions except for strong alkaline solution. Finally, we measured the emulsion separation capacity of SSM25-Cl after immersing in the four corrosive solutions for one month (Figure 10). The treated SSM25-Cl can effectively separate oil-in-water and water-in-oil emulsions. For n-hexadecane-inwater and water-in-dichloroethane emulsions, COD values and oil purities of the collected water and oils are 500~600 mg/L and over 99.9%, respectively, which are close to the separation capacity of the original SSM25-Cl. Hence, the PPy-coated meshes with outstanding stability are greatly beneficial to the treatment of actual oily wastewater.

CONCLUSIONS In summary, a facile electrochemical method has been adopted to prepare PPy-coated meshes with controllable pore size, which possess underwater superoleophobic and underoil superhydrophobic properties. Under only gravity, oily wastewater can be economically and effectively separated by using the PPy-coated meshes with appropriate pore sizes regardless of oil density (immiscible oil-water mixtures) or the type of disperse phase (emulsions). Even though the PPy-coated meshes have been immersed in various harsh solutions for one month, such as high temperature, low temperature, acid, and salt, the amphiphilic filtration membranes


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still maintain excellent separation performance. Wishfully, the PPy coatings can be practically applied in on-demand treatment of oily wastewater.


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Figure 1. (a, d) CV curves of electrochemical preparation of SSM5-Cl and SSM25-Cl. (b, c, e, f) SEM images of SSM5-Cl (b, c) and SSM25-Cl (e, f).


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Figure 2. SEM images (a-c) and element distribution maps (d-f) of SSM5-Cl (a, d), SSM5-S (b, e) and SSM5-P (c, f).


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Figure 3. SEM images (a-c) and element distribution maps (d-f) of SSM25-Cl (a, d), SSM25-S (b, e) and SSM25-P (c, f).


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Figure 4. Underwater OCAs (a, c) and underoil WCAs (b, d) of SSM5 (a, b) and SSM25 (c, d). 1, 2, 3, 4, 5 represent hexane, diesel, n-hexadecane, 1,2-dichloroethane and chloroform, respectively.


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Figure 5. (a) Photographs of a water droplet in oil and an oil droplet in water on the surfaces of SSM-Cl, SSM-S and SSM-P, respectively. (b) Schematic illustration of the wetting properties of SSM-Cl, SSM-S and SSM-P.


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Figure 6. Separation of immiscible oil-water mixtures using SSM5-Cl. (a, c) Water selectively permeates through SSM5-Cl. (b, d) Oils selectively pass through SSM5-Cl. (a) Fluxes and COD values of the collected water; oils include hexane (1), n-hexadecane (2) and diesel (3). (b) Fluxes and purities of the collected oils; oils include 1,2-dicloroethane (1), chloroform (2) and diesel (3). (c, d) Photographs of oil-water separation. Oil was dyed by Sudan Red in (c) and water was dyed by Methylene Blue in (d).


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Figure 7. Emulsion separation using SSM25-Cl. (a) Fluxes and COD values in the collected filtrates of oil-in-water emulsions; oils include hexane (1), n-hexadecane (2) and diesel (3). (b) Fluxes and oil purities in the collected filtrates of water-in-oil emulsions; oils include 1,2dicloroethane (1), chloroform (2) and diesel (3). (c) Photographs of n-hexadecane-in-water emulsion (left) and the collected filtrate (right). (d) Photographs of water-in-chloroform emulsion (left) and the collected filtrate (right).


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Figure 8. (a) Cycling performance of SSM25-Cl in the alternative separation of n-hexadecanein-water (blue) and water-in-chloroform (red) emulsions. In each cycle, underwater OCA (blue) and underoil WCA (red) were measured. (b-e) SEM images of SSM25-Cl before (b, c) and after (d, e) the cycling tests.


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Figure 9. SEM images of SSM25-Cl before and after immersing in various harsh solutions for one month. Harsh solutions include high temperature (1), low temperature (2), acid (3), alkali (4) and salt (5).


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Figure 10. COD values and oil purities in the collected filtrates after the separation of nhexadecane-in-water and water-in-dichloroethane emulsions using the treated SSM25-Cl. SSM25-Cl was immersed in various harsh solutions for one month. Harsh solutions include high temperature (1), low temperature (2), acid (3) and salt (4).


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ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, XPS, and FTIR of original and the prepared PPy-coated SSMs, mass percentage of each element from EDS analysis, adhesion characteristic of the PPy coatings, SEM images and CAs of the PPy coatings on the conductive glasses, oil-water separation using unmodified SSM, emulsion separation and stability of SSM25-Cl, Movie S1 and S2. AUTHOR INFORMATION Corresponding Author *(J.L.) Phone: 86-931-4968173; E-mail: [email protected]. *(Z.G.) Phone: 86-931-4968105; E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China (no. 51522510 and 51675513). REFERENCES (1) Xue, Z.; Cao, Y.; Liu, N.; Feng, L.; Lei, J. Special Wettable Materials for Oil/Water Separation. J. Mater. Chem. A 2014, 2, 2445–2460.


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