Polydopamine and poly(dimethylsiloxane) modified superhydrophobic fiberglass membranes for efficient water-in-oil emulsions separation

Journal of Colloid and Interface Science 559 (2020) 178–185

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Polydopamine and poly(dimethylsiloxane) modified superhydrophobic fiberglass membranes for efficient water-in-oil emulsions separation Haixiao Kang a,c, Xiaoliang Zhang b, Lingxiao Li d, Baowei Zhao a,⇑, Fengfeng Ma a, Junping Zhang d,⇑ a

School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China Shandong Peninsula Engineering Research Center of Comprehensive Brine Utilization, Weifang University of Science and Technology, Shouguang, Weifang 262700, China c State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China d Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 17 August 2019 Revised 23 September 2019 Accepted 6 October 2019 Available online 9 October 2019 Keywords: Polydopamine Poly(dimethylsiloxane) Superhydrophobic Emulsion Oil/water separation

a b s t r a c t Separation of oil/water mixture using superwetting materials has received great interest in recent years. However, it is challenging to efficiently separate water-in-oil emulsions due to their high stability and complex structures in the presence of surfactants. Here, we report preparation of polydopamine (PDA) and poly(dimethylsiloxane) (PDMS) modified superhydrophobic fiberglass (FG) membranes for efficient separation of water-in-oil emulsions. The membranes were fabricated by in turn deposition of PDA and chemical vapor deposition of PDMS. In order to study the structure-performance relationship, the membranes were characterized using field emission scanning electron microscope, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, etc. The membranes are superhydrophobic with a water contact angle of 152° and meanwhile superoleophilic with an oil contact angle of 0°. Also, the membranes demonstrate excellent acid, alkali and fire resistance. The absorption capacity of the membranes for diverse oils is 5.3–14.0 g g
1. Moreover, the membranes can remove more than 98% of water from the surfactant-stabilized water-in-oil emulsions. It is expected that the superhydrophobic FG membranes can be used for effective separation of diverse water-in-oil emulsions. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding authors. E-mail addresses: [email protected] (B. Zhao), [email protected] (J. Zhang). https://doi.org/10.1016/j.jcis.2019.10.016 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Owing to the frequent occurrence of water pollution caused by chemical leakage and oil spill, oil/water separation has attracted widespread attention [1–3]. Oils and organic pollutants would lead to reduction of dissolved oxygen in water, which would signifi-

H. Kang et al. / Journal of Colloid and Interface Science 559 (2020) 178–185

cantly affect the normal growth of aquatic organisms and then destroy the water resources. The carcinogenic hydrocarbons in oily wastewater would be enriched by fishes and shellfishes, which would endanger human health through the food chain. On the contrary, water in oil would have a great impact on the physicochemical properties of the oil, resulting in decline in its quality. Furthermore, the existence of water in oil could easily form stubborn emulsions, which would accelerate oxidation and reduce lubricity of the oil. On the whole, polluted water by oil would cause serious damages to environment and ecology, and oil products contaminated by water would cause great losses of oil quality and machinery that used the oils. Therefore, oil/water separation is an important process in treating both immiscible oil/water mixtures and surfactant-stabilized emulsions. Various traditional methods have been used for oil/water separation, such as flotation, coagulation, biological treatment and membrane separation [4]. Although widely used in practical applications, these methods still have many disadvantages, such as environmental incompatibility, low absorption capacity and poor recyclability. Therefore, novel materials for efficient oil/water separation are in high demand. Recently, superhydrophobic surfaces with high water contact angle (CA) above 150° and low CA hysteresis [5,6] have attracted considerable attention in oil/water separation due to their totally different wettability towards water and oils [7–9], exhibiting high oil/water separation efficiency and selectivity [10,11]. Various approaches, such as chemical vapor deposition (CVD), selfassembly and electro-spinning, have been used for preparing superhydrophobic materials [12–15]. A lot of materials, such as meshes [16,17], nanowire membranes [18–20], activated carbon [21–23], sponges [24–26] and conjugated polymer [27], have been used for preparing superhydrophobic materials for oil/water separation. There are even some superhydrophobic meshes that can repel hot water and possess excellent mechanical stability [28,29]. However, practical applications for these materials were limited by poor performance in separation of surfactant-stabilized emulsions, complicated methods, low stability and reusability. Therefore, it is necessary to find a versatile method to prepare high-performance materials to overcome these limitations. Here, we report preparation of superhydrophobic fiberglass (FG) membranes by sequential modification of the FG membrane with polydopamine (PDA) and poly(dimethylsiloxane) (PDMS). The FG@PDA membrane was prepared by polymerization of dopamine on the surface of FG. Then, a thin layer of PDMS with low surface energy was coated on the surface of FG@PDA by CVD [30]. Compared with the previous report [31], an effective method for fabricating superhydrophobic FG membrane is reported. Moreover, the FG@PDA@PDMS membrane exhibits effective separation of both immiscible oil/water mixture and surfactant-stabilized water-in-oil emulsions.

2. Experimental section 2.1. Materials The FG membrane was obtained from Shanghai Xingya Purification Material Factory, China. Dopamine hydrochloride (98%) was obtained from Shanghai DEMO Medical Tech Co., China. PDMS prepolymer (Sylgard 184A) was purchased from Dow Corning. Tris(hydroxymethyl)amino methane hydrochloride (Tris-HCl, AR), sodium hydroxide (NaOH, AR), hydrochloric acid (HCl, AR), anhydrous ethanol (AR), n-hexane (AR), cyclohexane (AR), toluene (AR), tetrachloromethane (AR), trichloromethane (TCM, AR), dichloromethane (DCM, AR), o-xylene (AR), petroleum ether (AR), Span 80 (CP), methylene blue (MB, BS), and oil red O (BS) were obtained from China National Medicines Co., Ltd., China. Petrol

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and diesel were obtained from Sinopec, Lanzhou, China. All the chemicals were used as received without further purification. Deionized water was used throughout the experiments. 2.2. Fabrication of FG@PDA membranes The FG@PDA membranes were fabricated according to the following procedure. Typically, dopamine (1.5 mg mL
1) was dissolved in a 10 mM Tris-HCl aqueous solution, and the pH was adjusted to 8.5 using 2 M NaOH. The FG membrane with a diameter of 50 mm was immersed in 100 mL of the above solution. The solution was mechanically stirred at 25 °C for a period of time (0, 4, 12 and 24 h), during which the FG membrane was modified with PDA by polymerization of dopamine. Then, the as-prepared FG@PDAxh (the subscript  is time) membranes were washed with deionized water for several times and dried in an oven at 60 °C. 2.3. Fabrication of FG@PDA@PDMS membranes The superhydrophobic/superoleophilic FG@PDA@PDMS membranes were fabricated by CVD of PDMS onto the FG@PDA24h membranes. Typically, the as-prepared FG@PDA24h membrane was placed in a 500 mL chamber containing 0.2 g of PDMS. The temperature of the chamber was kept at 200 °C for a period of time (0, 0.5, 1, 2 and 3 h) for CVD of PDMS. The as-prepared FG@PDA24h@PDMSxh (the subscript  is time) membranes were washed by using anhydrous ethanol for several times, and then dried in an oven at 60 °C. 2.4. Measurement of oil absorption capacity The oil absorption capacity of the samples was determined by a gravimetric method according to the following procedure: (i) the FG@PDA@PDMS membrane was immersed in different oils at room temperature; (ii) the membrane was taken out of the oils after 1 min, drained for a few seconds, and wiped off the excess oil with a piece of filter paper; (iii) the weight of the membrane was measured, and the absorption capacity was calculated according to Eq. (1):

Absorption capacity ¼ ðM1
M0 Þ = M0  100

ð1Þ

where M1 (g) is the weight of wet membrane with absorbed oil and M0 (g) is the weight of the membrane. The measurement of membrane weight was completed within a short time after being taken out of oils in order to avoid the influence of oil evaporation on the accuracy of the results. 2.5. Oil/water separation The separation of surfactant-stabilized water-in-oil emulsions using the FG@PDA@PDMS membrane was carried out with a setup composed of a filter, the as-prepared membrane and a beaker. The FG@PDA@PDMS membrane 50 mm in diameter was fitted into the filter. When the water-in-oil emulsion was filtered through the membrane, water was removed and the cleaned oil was collected in the beaker. The initial concentration of water in the emulsion and the residual concentration of water in the collected oil were measured using a V20 Volumetric KF Titrator (Mettler Toledo, Switzerland). The removal efficiency (RE) of water was measured using Eq. (2):

RE ¼ ðC0
Cf Þ = C0  100

ð2Þ

where C0 (mg L
1) is the initial concentration of water in the emulsion, and Cf (mg L
1) is the concentration of water in the collected oil.

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For the preparation of the typical water-in-toluene emulsion (1:99, v/v), 99 mL of toluene and 1 mL of deionized water was mixed in the presence of Span 80 (1.0 mg mL
1). The mixture was stirred at 1000 rpm for 0.5 h to form a stable emulsion. The other emulsions were prepared according to the same procedure except for that toluene was replaced with other oils. 2.6. Characterization The water CA was measured by dropping 5 lL water drops onto the surface of the samples at ambient temperature. The reported CA was the average of the CA at six different positions recorded using a Contact Angle System OCA 20 (Dataphysics, Germany). The morphology of the samples was observed by a field emission scanning electron microscope (FE-SEM, HITACHI-SU8020). Before FE-SEM observation, all membranes were fixed on aluminum stubs and coated with gold (~7 nm). X-ray photoelectron spectroscopy (XPS) of the samples was obtained using a VG ESCALAB 250 Xi spectrometer equipped with a monochromated AlKa X-ray radiation source and a hemispherical electron analyzer. The spectra were recorded in the constant pass energy mode with a value of 100 eV, and all binding energies were calibrated using the C 1s peak at 284.6 eV as the reference. Depth profile XPS examination was operated by etching the surface step-by-step using Ar ion beam, and XPS spectra was taken after each etching step. Fourier transform infrared spectroscopy (FTIR) of the samples was obtained on a Nicolet NEXUS FTIR spectrometer using potassium bromide pellets. Thermogravimetric analysis (TGA) of the samples was carried out with a thermal gravimetric analyzer (STA 449 F3 Jupiter NETZSCH, Germany) under air atmosphere with the heating rate of 20 °C min
1 from 30 to 900 °C. 3. Results and discussion 3.1. Preparation of superhydrophobic FG@PDA@PDMS membranes Fig. 1a presents preparation procedures of the FG@PDA@PDMS membranes. For the pristine FG membrane (Fig. 1b), a large number of smooth microfibers with an average diameter at 1.5 mm mixed together and formed a fibrous network. This is favorable for the uniform deposition of PDA onto the surface of the FG membrane. Thus, PDA was deposited onto the FG membrane via oxida-

tive polymerization of dopamine in the Tris-HCl buffer solution, forming the FG@PDAxh membranes. The white FG became light gray after being modified with PDA. The deposition of PDA has small influence on surface morphology of FG (Fig. 1c). The surface of the FG@PDA24h membrane is smooth with a very small amount of PDA nanoparticles on the microfibers, indicating that the PDA layer is very thin and uniform. As is well-known, PDA is widely used for coating various materials and can be used as a platform for diverse secondary reactions [32]. Hence, a thin layer of PDMS was coated on the surface of the FG@PDA24h membrane to reduce the surface energy by a facile CVD method [11], forming the FG@PDA24h@PDMSxh membrane. The deposition of PDMS via the CVD process has negligible influence on surface morphology of the membrane (Fig. 1d), indicating that the PDMS layer is also very thin and uniform. Superhydrophobic FG@PDA24h@PDMS3h membranes were obtained after appropriate deposition of PDA on the surface of the FG microfibers followed by CVD of PDMS [33].

3.2. Chemical composition of FG@PDA@PDMS membranes To ascertain the chemical composition and chemical state of the as-prepared membranes, XPS analysis was conducted. For the FG membrane, the peaks corresponding to C 1s, O 1s, Na 1s, Si 2s and Si 2p are shown in Fig. S1a. After deposition of PDA on the FG membrane, the Na 1s, O 1s, Si 2s and Si 2p peaks became weak with the increment of deposition time, whereas the C 1s peak and the new N 1s peak became stronger. This means that PDA has been successfully deposited on the surface of the FG membrane and the PDA content increased with the increase of deposition time. After deposition of PDMS, significant enhancement of the Si 2s, Si 2p and O 1s signals, and reduction of the C 1s, N 1s and Na 1s signals were detected (Fig. S1b). These results show that the PDA and PDMS layers can significantly affect surface chemical composition of the FG membrane. The high resolution XPS spectra of the membranes are shown in Fig. 2. In the C 1s spectrum of the FG membrane, the peaks at 284.4 eV, 286.3 eV and 288.5 eV are assigned to the CAC/CAH, CAO, and C@O groups, respectively. After deposition of PDA and PDMS, a new C peak attributed to the CAN group was observed at 285.9 eV, which is originating from indolic compounds [34]. Moreover, the N 1s spectrum of the FG@PDA24h membrane was fit-

Fig. 1. (a) Schematic illustration for preparing the FG@PDA24h@PDMS3h membrane. SEM images of the (b) FG, (c) FG@PDA24h and (d) FG@PDA24h@PDMS3h membranes.

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Fig. 2. High-resolution (a) C 1s and (b) N 1s XPS spectra of the FG, FG@PDA24h and FG@PDA24h@PDMS3h membranes.

ted into three peaks, which are corresponding to the ANH2 (401.7 eV), ANAH (399.8 eV) and AN@ (398.5 eV) groups. The above changes in the C 1s and N 1s spectra indicate that PDA was formed on the surface of the FG membrane and dopamine was converted to indolic compounds [23,35]. Furthermore, no new peaks appeared after deposition of PDMS, but the N/C and N/O atomic ratios were dramatically changed, indicating that the PDMS layer was very thin [36]. The corresponding atomic percentages of the membranes are listed in Table 1. The N/C and N/O atomic ratios of the FG membrane were almost zero, as there was no N element on the FG membrane. Whereas the N/C ratio increased to 0.12 ± 0.1 and the N/O ratio increased to 0.32 ± 0.2 on the FG@PDA24h membrane owing to deposition of PDA. Then, the N/O ratio decreased to 0.13 ± 0.1 on the FG@PDA24h@PDMS3h membrane due to deposition of PDMS. An interesting finding is that the theoretical values for each element of dopamine are 62.7% for C, 9.2% for N and 23.5% for O, which are very closely to those of the FG@PDA24h membrane (Table 1). This means the surface of the FG@PDA24h membrane was uniformly covered by the PDA layer, which is consistent with the SEM observation (Fig. 1c). In order to further study the elemental composition of the asprepared membranes and elemental distribution near the heterophase surface, depth profile XPS examination was performed on

the FG@PDA24h@PDMS3h membrane with different etching time. Fig. 3 shows the XPS depth profiles for the FG@PDA24h@PDMS3h membrane with etching time of 0, 120, 370, 740, 1670 and 2590 s. The top surface (Fig. 3a, etching time = 0 s) is mainly composed of N, C, O, Si, and Na elements. With the increase of etching time, the peak intensity of C and N elements gradually decreased, whereas the peak intensity of O, Si, and Na elements gradually increased (Fig. 3a, etching time = 2590 s). Another interesting result revealed from Fig. 3 is that the N and C contents gradually decreased from the heterophase surface to the interior layer, rather than a sudden drop at the interface. The continuous concentration evolution across the interface indicates that polydopamine and PDMS were gradually deposited on the surface of the FG membrane [37]. In addition, EDS mapping of the FG@PDA24h@PDMS3h membrane shows that it is mainly composed of Si and O elements, whereas the contents of C and N elements are relatively low and evenly distributed throughout the surface (Fig. 3b). This is consistent with the XPS depth profiles. The interaction between PDA and FG was also studied by FTIR spectra of the membranes (Fig. S2). Compared with the FG membrane, significant changes have been observed in the spectrum of the FG@PDA24h membrane. A series of new absorption bands appeared at 3424 cm
1 (stretching vibrations of catechol OAH/

Table 1 Surface chemical composition of the FG, FG@PDAxh and FG@PDA24h@PDMS3h membranes determined by XPS (average of three spectrographic results).

C/% N/% O/% Si/% N/C atomic ratio N/O atomic ratio

FG

FG@PDA4h

FG@PDA12h

FG@PDA24h

FG@PDA24h@PDMS3h

18.4 ± 0.01 0.0 56.6 ± 0.1 25.0 ± 0.1 0.00 0.00

49.7 ± 0.4 6.7 ± 0.2 33.6 ± 3.7 10.1 ± 0.2 0.13 ± 0.5 0.20 ± 0.05

51.8 ± 1.2 6.1 ± 0.2 32.2 ± 1.6 9.9 ± 0.3 0.12 ± 0.2 0.19 ± 0.1

63.5 ± 3.1 7.7 ± 0.4 24.4 ± 2.6 4.4 ± 0.2 0.12 ± 0.1 0.32 ± 0.2

41.2 ± 6.4 4.8 ± 0.6 37.7 ± 4.3 16.2 ± 2.8 0.12 ± 0.1 0.13 ± 0.1

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that the CVD time has certain influence on wettability of the membrane. As shown in Fig. S3, with the increase of CVD time, the water CA gradually increased. The water CA of the FG@PDA24h@PDMS3h membrane was over 152°, showing good superhydrophobicity. Also, the FG@PDA24h@PDMS3h membrane was reflective in water and remained completely dry after being taken out of water. Water droplets could roll off from the surface of the tilted FG@PDA@PDMS membrane without leaving any trace (Movie S**1). Although the FG@PDA@PDMS membrane had excellent superhydrophobicity, it could be easily wetted by various oils. For example, the TCM drops could completely penetrate into the membrane in the blink of an eye (Fig. 4b). It is important to note that without the PDA layer, the PDMS layer itself is not sufficient to make the FG membrane superhydrophobic (CA = 145°). Water droplets are sticky on the surface and cannot roll off, which means the contact mode between the water droplets and the membrane is the Wenzel model [42,43]. This is because the surface of the FG membrane is very smooth. This method is different from the previous study [31], in which silica was used to increase the surface roughness, a very traditional method for preparation of superhydrophobic materials. In this study, FG was modified by the PDA layer because this is a relatively new method for coating various materials and the bioinspired PDA layer can be used as a platform for diverse secondary reactions. 3.4. Stability and fire resistance of FG@PDA@PDMS membranes

Fig. 3. (a) XPS depth profile results of the FG@PDA24h@PDMS3h membrane with different etching time and (b) EDS mapping of the FG@PDA24h@PDMS3h membrane.

NAH groups), 1625 cm
1 (shear vibration of catechol NAH groups) and 1056 cm
1 (stretching vibrations of catechol CAN groups) [34,38]. These absorption bands indicate that dopamine was converted to PDA with long repeated catechol chains via oxidative polymerization [39]. For the FG@PDA24h@PDMS3h membrane, no new absorption bands appeared after deposition of PDMS, but the intensity of the absorption band at 791 cm
1 (SiAC group) was enhanced. These results further demonstrate successful preparation of the FG@PDA24h@PDMS3h membrane [40]. 3.3. Wetting behaviors of FG@PDA@PDMS membranes The superhydrophobic and superoleophilic properties could be obtained by enhancing surface roughness and modifying with materials of low surface energy [41]. The FG and FG@PDA membranes were hydrophilic and oleophilic in air. Both water and oil could quickly penetrate into the FG and FG@PDA membranes. Interestingly, the FG@PDA membrane showed underwater superoleophobic and the CA of TCM underwater was over 162° (Fig. 4a). When the TCM droplet contacted with the FG@PDA membrane underwater, water was trapped in the membrane, forming an oil/water/solid composite interface. The trapped water greatly reduced the contact area between the TCM droplet and the membrane, forming an underwater superoleophobic surface [34]. Then, the FG@PDA membrane was treated by PDMS to make the membrane superhydrophobic. The surface energy of the PDMS layer is very low because it contains a lot of methyl groups, which is very important to achieve superhydrophobicity [5]. It is worth noting

Chemical stability and fire resistance are important properties for superwetting membranes employed in actual oil/water separation. Any damage of the coating by chemical reagents may result in complete loss of superhydrophobicity. The chemical composition of oily wastewater is complex, which is often corrosive to the materials used for treating oily wastewater. Hence, the chemical stability of the FG@PDA@PDMS membrane was evaluated by measuring the changes of the water CA under different corrosive conditions. As shown in Fig. S4, the FG@PDA@PDMS membranes were immersed in various corrosive aqueous liquids including acidic (pH 3), neutral (pH 7) and alkaline (pH 11) solutions for a period of time (1– 15 days). In the cases of acidic and neutral solutions, the water CA did not show obvious change and was above 150°, indicating that the superhydrophobic FG@PDA@PDMS membrane has excellent water and acid resistance. Nevertheless, the water CA decreased to about 140° after being immersed in the pH 11 solution for 15 days. This is owing to the low stability of PDMS towards alkaline and is consistent with our previous results [11]. Mechanical stability of superhydrophobic materials is also important for their applications. In order to test the mechanical stability, the abrasion tests were performed by using sandpaper (2000 meshes) as the abrasion partner. The FG@PDA@PDMS membrane was fixed onto the stainless steel column and moved repeatedly (20 cm per cycle) on the abrasion partner at 3 kPa. As shown in Fig. S5, abrasion against sandpaper has obvious influence the FG@PDA@PDMS membrane. The membrane was broken after 2 cycles, which is owing to the low mechanical stability of the FG membrane. Meanwhile, the water CA only slightly decreased to 141 ± 1.6°. The results indicate that the membrane remained highly hydrophobic although being seriously damaged by abrasion. This is because of self-similarity of the FG@PDA@PDMS membrane. As an excellent material for selective oil absorption or oil/water separation, it is important to have good fire resistance in case of fire or explosion when dealing with oil spills or organic pollutants. However, most of the reported superhydrophobic materials did not show fire resistance. As shown in Movie S**2, the FG@PDA@PDMS membrane cannot be ignited. When the membrane was kept contacting with a flame, the flame on the membrane was weak. When the membrane was removed from the flame of the alcohol burner,

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Fig. 4. (a) Still images of 5 lL drops of water and TCM in air, and TCM underwater on the FG@PDA24h membrane, (b) image of the FG@PDA24h@PDMS3h membrane with spherical water drops on the surface but completely wetted by oil, and still images of 5 lL drops of TCM and water in air on the FG@PDA24h@PDMS3h membrane. Water was colored with MB and oil was colored with oil red O in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

there was no residual flame on the membrane and it could still maintain 90% of the original weight after being exposed to the flame for more than 20 s. The water CA of the membrane decreased to 146.5 ± 2.1° after being exposed to the flame, indicating that the membrane was still hydrophobic. These results prove that the FG@PDA@PDMS membrane has good thermal stability. The fire resistance is consistent with the TGA curve of the FG@PDA@PDMS membrane in air (Fig. S6). The excellent fire resistance is owing to the fact that the superhydrophobic membrane was prepared using inorganic substrate and only a very small fraction of organic components (PDA and PDMS) were introduced. 3.5. Removal of free oils and organic solvents from water The FG@PDA@PDMS membrane could be easily wetted by organic liquids with low surface tension even though it is superhydrophobic. The CA of TCM was ~0° and the TCM drops could instantly penetrate into the membrane. These properties make the FG@PDA@PDMS membrane an ideal material for removal of free oils and organic solvents from water. In order to verify feasibility of the FG@PDA@PDMS membrane for oil/water separation, TCM dyed with oil red O was used as the probing oil. As shown in Fig. S7a, when the FG@PDA@PDMS membrane came into contact with TCM under water, the absorption process was rapid and took only a few seconds to complete. No TCM was observed in the remaining water after removing the membrane with absorbed oil from the beaker. The water in the beaker was clear and transparent. In order to further evaluate oil absorption capacity of the FG@PD@PDMS membrane, the absorption tests were carried out for various organic solvents and oils. The FG@PDA@PDMS membrane has an absorption capacity of 5.3–14.0 g g
1 (Fig. 5a), relying on density and viscosity of the oils and organic solvents [44]. When the density of oil is small, the absorption capacity is small and vice versa. Also, the viscosity of the oils affects the time to reach the absorption equilibrium. It takes a longer time to reach absorption equilibrium when the oil is more viscous. The oil absorption performance of the membrane was better than those reported in other literatures [45,46]. The recyclability of the FG@PDA@PDMS membrane for oil absorption was also studied. Toluene, petroleum ether and DCM were used as the representative oils to evaluate recyclability of the membrane via cyclic absorption-desorption. The recyclability of the membrane for other oils was also investigated. It could be seen that the absorption capacity did not change significantly after 10 cycles (Fig. 5b and S8).

Fig. 5. (a) Absorption capacity of the FG@PDA24h@PDMS3h membrane for different oils and organic solvents, and (b) variation of absorption capacity of the FG@PDA24h@PDMS3h membrane for different oils with absorption-desorption cycles. The error bars represent the average deviation.

3.6. Separation of water-in-oil emulsions Surfactant-stabilized water-in-oil emulsions have serious influences on oil quality, which are much more difficult to treat than immiscible oil/water mixtures. Here, we demonstrate good performance of the FG@PDA@PDMS membrane in treating this kind of

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emulsion. A Span 80 stabilized water-in-toluene emulsion (1:99, v/ v) was taken as an example. The emulsion was stable at room temperature for at least 24 h without demulsification or precipitation. As shown in Fig. S7b and Movie S**3, the gravity-driven filtration device is composed of a funnel fitted with the FG@PDA@PDMS membrane and a beaker for collecting the separated oil. Once the emulsion came into contact with the FG@PDA@PDMS membrane, the water-in-oil emulsion was broken by the fiberglass. Toluene immediately permeated through the membrane and was collected in the flask, while water in the emulsion was separated and intercepted because of superhydrophobicity and superoleophilicity of the membrane. The flux of the membrane is 734 L m
2 h
1 according to the flow volume per unit time and per unit area of the membrane. The emulsion was observed using an optical microscope before and after filtration in order to study the separation efficiency. Numerous water microdroplets were detected in the water-in-toluene emulsion (Fig. S9a). After separation, the milky emulsion became clear and transparent oil was collected in the beaker. No water microdroplet was detected in the filtrate, indicating very high separation efficiency (Fig. S9b). The water concentration in the filtrates and the RE of water were further quantified. The photographs of the filtrates with continuously increasing the volume of the water-in-toluene emulsion are shown in Fig. 6a, the RE of water was calculated according to Eq. (2) and the curve is shown in Fig. 6b. The filtrate was still clear and transparent when the emulsion volume increased to 160 mL. The initial concentration of water in the water-in-toluene emulsion was 10000 ppm. When the filtrate in the collecting beaker was 20 mL, the water concentration in the filtrate was about 23.4 ppm and the corresponding RE of water was 99.8%. With the increase of the filtrate volume, the water concentration in the filtrate gradually increased. When the filtrate volume increased to 100 mL, the water content

reached about 158.9 ppm and the corresponding RE of water was 98.4%. The water content in toluene is similar to that in the commercial reagent grade toluene, demonstrating excellent separation performance of the membrane for the water-in-oil emulsion. The separation efficiency of the FG@PDA@PDMS membrane for different water-in-oil emulsions was also studied, such as water-intrichloromethane, water-in-hexane, water-in-gasoline and waterin-diesel. The FG@PDA@PDMS membrane showed high separation efficiency for various water-in-oil emulsions (Fig. S10). This is owing to superhydrophobicity, superoleophilicity and fibrous microstructure of the membrane, which resulted in efficient demulsification and selective oil/water separation when the emulsion came into contact with the membrane [33]. The separation performance of the FG@PDA@PDMS membrane for the water-inoil emulsion is better than many previously reported results [31,44]. 4. Conclusions In summary, superhydrophobic FG@PDA@PDMS membranes were prepared by self-oxidative polymerization of dopamine followed by CVD of PDMS. The FG@PDA@PDMS membranes displayed superhydrophobicity with a water CA of above 152° and also showed superoleophilicity in air. The method is facile, environment friendly, and cost-effective, and will be a plausible method for constructing superhydrophobic materials, as the PDA layer could be easily applied onto diverse substrates. The FG@PDA@PDMS membrane exhibited excellent absorption selectivity and absorption capacity up to 5.3–14.0 g g
1, relying on density and viscosity of the oils and organic solvents. Furthermore, the membrane showed very high efficiency for gravity-driven separation of various surfactant-stabilized water-in-oil emulsions compared with previous studies. After separation, the water content in the filtrate, e.g., toluene, is comparable to that in reagent grade toluene, demonstrating high performance of the membrane in separating water from the water-in-toluene emulsion. We believe that the superhydrophobic/superoleophilic membrane is a very promising candidate for practical separation of water-in-oil emulsions because of simplicity of the method and high stability of the membrane. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgements This work is supported by the National Natural Science Foundation of China (51873220 and 41261077), and the Funds for Creative Research Groups of Gansu, China (17JR5RA306). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.016. References

Fig. 6. (a) Photograph of the filtrates with continuously increasing volume of the water-in-toluene emulsion. (b) Variation of water concentration in toluene and the RE of water with increasing the permeation volume. The error bars in (b) represent the average deviation.

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