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A Lotus- and Mussel-inspired PDA-PET/PTFE Janus Membrane: towards Integrated Separation of Light Oil and Heavy Oil from Water Ya'nan Liu, Ruixiang Qu, Weifeng Zhang, Xiangyu Li, Yen Wei, and Lin Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04775 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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ACS Applied Materials & Interfaces

A Lotus- and Mussel-inspired PDA-PET/PTFE Janus

Membrane:

towards

Integrated

Separation of Light Oil and Heavy Oil from Water

Ya’nan Liu, Ruixiang Qu, Weifeng Zhang, Xiangyu Li, Yen Wei and Lin Feng*

Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China.

E-mail: [email protected]

KEYWORDS: Janus membrane, light and heavy oil/water separation, acetone drainage ABSTRACT: Current special wetting materials designed for use with oily wastewater are usually classified as either the oil-removing type or the water-removing type, which are

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unifunctional and limited by the oil density. Inspired by the integrated Janus system of the lotus leaf as well as the mussel-like mollusks adhered to the lotus, we fabricate a Janus PDA-PET/PTFE membrane by simple immersion and tape-peeling. This membrane shows a lotus-like Janus wettability, self-cleaning effect and lotus floating property. What’s more, the Janus membrane can separate light oil (ρoil < ρwater)/water mixtures with the superhydrophilic side facing upwards, while, in contrast, heavy oil (ρoil > ρwater)/water mixtures with the hydrophobic side facing upwards. The separation efficiency is outstanding even after ten repeats (> 99.10%). By aid of drainage of acetone, the separation process has avoided the use of external pressure. Moreover, integrated separation of oil-in-water and water-in-oil emulsions were achieved with high efficiency. This simply prepared PDA-PET/PTFE Janus membrane has realized an integrated separation system, overcoming the monotony of traditional special wettability separation membrane and extending the bionics field into oily wastewater treatment.

1. INTRODUCTION


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Efficient oil-water separation has long been focused, because of the threat to the environment and human health posed by oily wastewater, usually from chemical industry discharge or oil spill accidents.1-4 Due to their outstanding properties of high efficiency and low cost, materials with special surface wettability have been widely applied to treat oily waste water.5-9 Traditionally, special wetting materials are classified into two types: oil-removing and water-removing. Oil-removing materials are superhydrophilic in air and superoleophobic in water, which can enable the removal of oil from oil/water mixtures through the discharge of water as well as the inhibition of oil permeation.10-14 Water-removing materials are superoleophilic and superhydrophobic, and can separate water from oil/water mixtures by their unidirectional oil transportation while preventing water from passing through.15-18 Though these two types of materials have been used to achieve oil/water mixtures separation to some extent, they are unilateral and limited, heavily dependent on the oil density, and thus cannot be used to meet different needs. In this case, materials that can be used to integrally achieve the separation of both light (ρoil < ρwater) and heavy oil (ρoil > ρwater) from water are strongly desired. 3 ACS Paragon Plus Environment


Nature often offers us inspiration for designing materials with needed functions; the lotus leaf is a very typical example in bionics for its ‘rising unsullied from mud’: superhydrophobicity and self-cleaning effect. The upper side of the lotus leaf typically has hierarchical micropapillae and nanoepicuticular wax; as a result, water droplets remain spherical and roll off easily with dirt. Based on this knowledge, many superhydrophobic and self-cleaning materials have been studied.19-21 In contrast, the superhydrophilic and underwater superoleophobic properties of its lower side are often neglected; however, these properties are important for revealing the Janus interfacial characteristic of the lotus leaf.22 During the observation of lotus leaf, we have accidentally found a mussel-like creature adhered to the hydrophilic side of a lotus, which reminded us of another bioinspiration by polydopamine (PDA) with its strong adhesion and good hydrophilicity. PDA is a mussel-inspired high molecule due to its high concentrations of catechol and amine functional groups, which are critical for mussels to adhere to most organic and inorganic surfaces.23-26 As reported, in a weak alkaline environment, dopamine can robustly coat onto various substrates through selfpolymerization and can then impart hydrophilicity or act as a secondary reaction 4 ACS Paragon Plus Environment

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platform. This simple but universal modification has enlightened many studies, especially in application for oily wastewater treatment,27-30 which has also inspired us to fabricate a Janus membrane for universal oil/water separation. Herein, inspired by the cooperative binary wettability of a lotus leaf as well as the strong adhesion of mussels, we modified a PET/PTFE composite substrate with PDA by simple immersion. Taking advantage of the inherent hydrophobicity of the PET/PTFE composite membrane, we successfully fabricated a Janus PDA-PET/PTFE membrane by peeling off the hydrophilic PDA layer from the PTFE side. This bionic Janus membrane showed one side high-hydrophobicity and self-cleaning (PTFE side), and the other side superhydrophilicity and underwater superoleophobicity (PDA-PET side), having successfully integrated the separation of light oil and heavy oil from water (Scheme 1). Fortunately, such a membrane also has high efficiency and good reusability for separating various kinds of oil/water mixtures. The as-prepared Janus PDA-PET/PTFE membrane has solved the severe dependence on oil density of traditional special wetting materials, while achieving an integrated system for application to universal types of oil/water mixtures. This work may open up new insight in bionics 5 ACS Paragon Plus Environment


that we combined lotus-and-mussel effects and subtly applied them to oil/water separation.

Scheme 1. Inspired by the lotus Janus system and a mussel adhered to the leaf, we used a mussel-inspired molecule, polydopamine (PDA), to modify a PET/PTFE composite substrate. After polydopamine modification, the pristine hydrophobic substrate turned superhydrophilic on both sides. When the PDA-PTFE layer was peeled off, the pristine hydrophobic PTFE side was exposed, and thus a Janus membrane was fabricated. This bionic Janus PDA-PET/PTFE membrane was used to successfully separate both light oil (ρoil < ρwater) and heavy oil (ρoil > ρwater) from water. Here we used khaki to demonstrate the layer modified PDA (the real color of PDA modified layer was close to muddy color). Lavender was used to demonstrate color of the initial membrane surface, which was white in fact.


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2. RESULTS AND DISCUSSION 2.1 Morphology and chemical composition of the Janus membrane

2.1.1 Micro-nano structures of the Janus membrane. Currently, methods for Janus membrane fabrication, such as sequential electrospinning, sequential surface modification, single-faced photocrosslinking or photodegradation, single-faced coating, usually have the disadvantages of complex operation, the use of environmentendangering reagents and high cost.31-37 Here, we only used one hydrophilic modification agent, dopamine, combined with a hydrophobic PET/PTFE composite membrane to fabricate a Janus membrane by a simple immersion and subtle peeling-off process. The PET/PTFE composite membrane consists of a thick, rough PET supporting layer and a thin, smooth PTFE layer (illustrations in Figure 1a and 1c, respectively). By immersing the white PET/PTFE composite substrate into a dopaminetris solution for 48 h, a membrane with both sides turning a dark color was generated (illustrations in Figure 1b and 1d, respectively). After peeling off the dark layer on the PTFE side (Movie S1), a new white smooth face was exposed (illustration in Figure 1e). As the field emission scanning electron microscopy (FESEM) images showed, the 7 ACS Paragon Plus Environment


original PET side was interlaced with fibers (Figure 1a), while the PTFE side was composed of hairlike silks (Figure 1c). After immersion in dopamine, some particles can be observed on both sides. On the PET side (Figure 1b), the particles were small and fine, distributed evenly and densely, and some micro scale aggregation could be seen in the magnified picture. The microscale aggregates were further observed with higher magnification (Figure S1, a1-a3), where we observed that those microscale aggregates were composed of nanoscale sphere-like particles. On the PTFE side (Figure 1d), the initial fibrous face was almost totally covered by concentrated modifiers, with only tiny amounts of silk observed in the cracks. The dense layer may be generated from the more rapid growth of polydopamine molecules: they aggregated and formed a lamellar structure, on which new globular particles sequentially forming. Moreover, magnified FESEM images were presented in Figure S1 (b1-b3) and we could also observe the micro-nano hierarchical structure of modifiers. After peeling off the modified PTFE layer (Figure 1e), almost the same morphology as seen on the initial PTFE side was observed. However, it should be mentioned that by comparing the magnified images of Figure 1c and 1e, the hairlike silks on the modified layer were more ‘connected’ than the 8 ACS Paragon Plus Environment

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original fractured silks. In other words, some broken silks seemingly appeared to have been ‘repaired’, which might be due to the PDA residue. Sectional FESEM images were also characterized for pristine PET/PTFE substrate and modified-peeled PET/PTFE membrane, to observe the thickness of PET layer and PTFE layer before and after fabrication (Figure S2). As a result, the thickness of pristine PET layer was about 136.1 µm and the thickness of pristine PTFE layer was 22.4 µm. After modification and peeling-off process, the thickness of PDA treated PET layer turned 142.9 µm and the thickness of peeled off PTFE layer was 11.9 µm.



Figure 1. FESEM images are shown with membrane photos (top right corner) and magnified FESEM images (lower left corner); EDX results are also shown. (a) PET side of the original PET/PTFE composite membrane. (b) PDA-PET face of PDA-modified PET/ PTFE membrane. (c) PTFE side of the original PET/PTFE composite membrane. (d) PDA-PTFE face of PDA-modified PET/PTFE membrane. (e) PTFE face of the PDAmodified PET/PTFE membrane after the PDA-PTFE layer was peeled off.

2.1.2 Elements analysis of the Janus membrane. To confirm the successful modification of PDA, energy-dispersive X-ray spectroscopy (EDX) was used for 10 ACS Paragon Plus Environment

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preliminary characterization. Dopamine, a catechol structure with an amino group, selfpolymerizes into polydopamine through Michael addition and the Schiff base reaction while connecting with the substrate.38 By comparing the EDX results for the pristine PET side (Figure 1a) and the modified PET side (Figure 1c), the increase in N element strongly suggested PDA formation (in dopamine, the percentage of N content is approximately 0.1%). The increase in O element, from 3.5% to 9.5%, might result from the hydroxyls and quinones of polydopamine. In similar manner, it is not surprising to find the appearance of N and O elements on the PTFE modified side (Figure 1d). Furthermore, the decreased content of F element (from 68.4% to 47.1%) might be attributed to increases in other elements such as C (from 30.8% to 45.2%) and O (from 0% to 7.6%). After peeling off the PDA layer (Figure 1e), element ratios were similar to those of the initial PTFE layer, with only slight differences for the emergence of N and O elements and a higher content of C element, which is in accordance with the ‘repair’ phenomena observed via FESEM, indicating the existence of residual PDA.

2.1.3 Chemical components of the Janus membrane. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results have 11 ACS Paragon Plus Environment


further indicated the modification of PDA. For convenience, we labeled the PET face as the down side (d-side) and the PTFE face as the up side (u-side). It can be confirmed that the constitution of the u-PDA-PET/PTFE is almost the same as the original sample (u-PET/PTFE), with their mimic C 1s and F 1s peaks in XPS (Figure 2b) and the resembled 1400-950 cm-1 adsorption peaks, which was typical of the PTFE substrate (C-F bonds, 1200 cm-1 and C-C bonds, 1150 cm-1) (Figure 2d). Except for slight differences in the range 3000-2800 cm-1, which might be the effects of the remaining PDA on the u-PDA-PET/PTFE, coincident with those obtained from FESEM and EDX. For the d-side, distinctive changes appeared. The obvious emergence of N 1s and O 1s peaks of d-PDA-PET/PTFE strongly suggested the existence of -OH and -NH2 groups or other possible intermediates such as dopamine quinone and pyrrole derivatives (Figure 2a). In the FTIR spectrum (Figure 2c), d-PDA-PET/PTFE showed the broad absorption band in the range 3700-3000 cm-1, indicating combined effects of -OH, -NH2 groups, as well as the resultant intra and inter molecular hydrogen bonds. Besides, the sharp band in the range of 3100-2700 cm-1 was representative of saturated C-H bonds, while the enhancement of d-PDA-PET/PTFE was contributed by -CH2 groups in 12 ACS Paragon Plus Environment

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dopamine (2900 cm-1). In addition, the small wide band in the range of 1750-1500 cm-1 was the typical stretching vibration region of unsaturated double bonds, such as C=C, C=N, σC=C (benzene ring skeleton) and C=O (1600 cm-1), consistent with the catechol structures in PDA and the polymerization processes of Michael addition and Schiff base reaction. Finally, bands in the 800-600 cm-1 range of the fingerprint region might be the disubstitution of the benzene ring (ortho-, meta-, para-). As the catechol groups are easily oxidized into highly reactive quinones, the ortho-, meta-, and para- sites are activated to different degrees.39-40 Therefore, it can be concluded that the PET side of the membrane was successfully modified by the PDA via simple immersion for 48 h, and meanwhile the other face remained almost the same as the original PTFE side, except with little residual PDA after peeling off the dark PDA-modified layer.



Figure 2. XPS results for the d-side (a), u-side (b) for both the initial PET/PTFE membrane and PDA-PET/PTFE Janus membrane, respectively, and FTIR pictures of the d-side (c) and u-side (d) for both the initial PET/PTFE membrane and PDAPET/PTFE Janus membrane, respectively. Water contact angles in air for the u-side of the lotus leaf (side usually facing upwards in a pond) (e1), u-side of the PDA-PET/PTFE membrane (e2), d-side of the lotus leaf (side usually in contact with water) (e3), d-side of the PDA-PET/PTFE membrane (e4). Oil contact angles underwater for the d-lotus (e5) and d-PDA-PET/PTFE membrane (e6).

2.1.4 Formation mechanism of micro-nano hierarchical particles. The formation mechanism of micro-nano hierarchical structured particles on two sides of the Janus membrane was explored as following. Three groups of PET/PTFE membrane were treated by dopamine with different concentrations and immersion time: (Ⅰ) 2 g/L dopamine solution with immersion 24 h; (Ⅱ) 2 g/L dopamine solution with immersion 48


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h; (Ⅲ) 10 g/L dopamine solution with immersion 48 h. From the FESEM images, we found that the modification of polydopamine could be divided into two parts (Figure 3). First polydopamine was inclined to form a layer covered on the substrate. Then particles were inclined to aggregate where the growth rate was fast. For PDA-PET side, the aggregates in 2 g/L PDA-24 h group were scarce. Large areas seemed smooth without particles (a1) and only small amounts of aggregates could be found (a2). To verify the smooth layer was polydopamine, EDX was characterized for (a1) (Figure S3). The appearance of N elements indicated the formation of PDA layer. For 2 g/L PDA-48 h group, the microscale aggregates were increased and from (b3) we could find that nanoscale sphere-like particles stacked on each other and composed the microscale aggregates. Besides, the scale of nanoparticles in (b3) was larger than that in (a3), demonstrating that along with the aggregation phenomena the scale of particles was increased as well. Furthermore, the accumulation of particles was more obvious in 10 g/L PDA-48 h group. The PDA layer covered on substrate was inclined to split and curl up (c2), which might be ascribed to overquick growth rate. From (c2) and (c3), we could observe that both the diameters and the amount of particles were increased, more 15 ACS Paragon Plus Environment


micro-scale spheres emerged. Situations were similar for PTFE side (Figure S4). With the increasement of dopamine concentration and immersion time, the covered areas of PDA on PTFE surface were enlarged and aggregated particles were emerged. At low dopamine concentration and short immersion time (2 g/L PDA-24 h), the cover of PDA was insufficient and large areas of PTFE fibers were exposed out. Only tiny amounts of aggregates in (a3) could be found. With the increasement of immersion time (2 g/L PDA48 h), the PDA layer was denser and more micro/nano scale particles showed up. From the inserted image of (b2), it could be clearly seen that the PDA layer with curling edges. If the concentration of dopamine was further increased (10 g/L PDA-48 h), the PDA layer was getting denser and more large-scale particles showed up. Therefore, the fabrication mechanism of PDA was generated as first layer formation on the substrate and then particles aggregation at rapid-growth area. We ascribed the growth of nanoscale PDA in this work to the appropriate dopamine concentration and immersion time.


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Figure 3. (a1), (a2), (a3) are FESEM images of PET side after 2 g/L PDA treatment for 24 h in different magnification. (b1), (b2), (b3) are FESEM images of PET side after 2 g/L PDA treatment for 48 h in different magnification. (c1), (c2), (c3) are FESEM images of PET side after 10 g/L PDA treatment for 48 h in different magnification. The inserted image of (c3) is nanoscale modifiers on the membrane. It can be observed that with the increasement of dopamine concentration and immersion time, both the amounts and diameters of modified particles are increased. The modification mechanism of polydopamine on the substrate can be proposed that first PDA layer is formed on the membrane surface and then particles are aggregated and further grown at the place with faster growth rate.

2.2 Lotus-like property of the Janus membrane



2.2.1 Lotus-like Janus wettability. It can be seen that the lotus leaf has Janus wettability on two sides: the superhydrophilic side is close to the water surface (water contact angle, WCA = 0°, Figure 2e3), while the superhydrophobic side is exposed to the air (WCA = 153.8°, Figure 2e1). Besides, the original PET/PTFE membrane is hydrophobic on both sides, with a water contact angle of 127.8° for the PET side (Figure S5a) and 135.1° for the PTFE side (Figure S5b). After PDA modification, both the PET (Figure 2e4) and PTFE (Figure S6a) side became superhydrophilic (WCA = 0°), due to the quantities of hydrophilic hydroxy and amino groups in PDA. In addition, the underwater superoleophobicity (oil contact angle, OCA = 151.5° for the PDA-PET side, Figure 2e6; OCA = 150.1° for the PDA-PTFE side, Figure S6b) was also in accordance with the lotus leaf (OCA = 150.9°, Figure 2e5). When the hydrophilic PDA-PTFE layer was peeled off by tape, a new PTFE surface (u-PDA-PET/PTFE) was exposed, which naturally showed a hydrophobic character (Figure 2e2). The improvement in the WCA from 135.1° to 145.2°, which could be explained by the ‘repair’ function due to the PDA, should be noted. Some polymerized dopamines connected the fractured silks, resulting in more intact structures to support the higher hydrophobicity. Therefore, a biomimetic 18 ACS Paragon Plus Environment

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PDA-PET/PTFE membrane with Janus wettability was achieved by simple immersion and tape-peeling.

2.2.2 Lotus-like self-cleaning effect. In addition to the Janus wettability, self-cleaning is a notable trait of the lotus, described in poetry as ‘come out of mud unsoiled’, which is due to the micro/nanocomposite structure and low surface energy of the wax. Here, methylene blue was used to contaminate the PTFE surface, and remarkably, blue marks can be easily washed away by water, leaving no prints left (Figure 4a1 and 4a2; Movie S2), demonstrating a lotus leaf-like self-cleaning effect. In this way, dust particles residing on the membrane surface can be removed easily, and naturally good separation performance can be maintained.



Figure 4. Membrane dyed by methylene blue (a1), with the membrane easily cleaned by water leaving nothing behind on the surface (a2). (b1) and (b2) are the start and end moments for the ‘floating lotus test’. The membrane was put into water vertically, and it can return to the floating ‘lotus’ state spontaneously, with the hydrophobic side facing upwards and hydrophilic side facing downwards. (c) Mechanism for the ‘floating lotus’ property. The hydrophilic face was permeated by water in a wetting Wenzel state, while the hydrophobic side exist in a nonwetting Cassie state; therefore, the gravity center of 20 ACS Paragon Plus Environment

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the membrane is reduced to the hydrophilic side, which finally faces downwards. In addition, the surface tension generated on both sides contributes to the ‘rising’ and ‘floating lotus’ phenomena.

2.2.3 Lotus-like floating phenomenon. Due to its special constitution, the as-prepared PDA-PET/PTFE membrane has a ‘lotus-like floating’ property (Figure 4b1 and 4b2). When the membrane was placed vertically at the bottom of a beaker, it can automatically return to the ‘floating lotus’ state (Movie S3). In particular, the membrane floated with the hydrophobic PTFE-side facing upwards, while the superhydrophilic PDA-PET side contacted the water surface, exactly the same as a floating lotus in the pond (Figure S7). Furthermore, the ‘lotus-like floating’ state was relatively stable: though disturbed by external forces, it can still recover (Movie S4). This behavior can be explained by the mechanism that complete wetting has taken place for the hydrophilic PDA-PET side, with a Wenzel state formed. In contrast, a nonwetting Cassie state formed for the hydrophobic PTFE face (Figure 4c). Accordingly, the mass of the waterwetted hydrophilic side was larger than that of the hydrophobic side and the barycenter was reduced to the hydrophilic side; therefore, the hydrophobic side was inclined to face



upwards. As for the rising behavior and final floating on the water/air interface, it should be attributed to the stronger impetus generated from the resistance of water towards the hydrophobic surface. Besides, the wetting Wenzel state of the hydrophilic face contributed to the stability of the ‘floating lotus’ state, as the connecting force between the membrane and water interface was strengthened.41 2.3 Integrated separation system for oil/water mixtures

2.3.1 Force balance for water permeation with the Janus membrane. Since the original membrane possesses hydrophobicity and fairly small pore sizes, two upward forces: surface tension and pore resistance will form accordingly during the infiltration, causing somewhat difficult water permeation. After PDA modification, though the dPDA-PET/PTFE side turned superhydrophilic, accompanied by a downward surface tension (Fphilic), water still cannot penetrate from the d-side to the u-side simply by gravity, until the water column height was larger than 37.5 cm (Figure 5a). In other words, the critical value of external pressure that permitted the water permeation corresponded to a water column height of 37.5 cm. This phenomenon can be ascribed to the rather thick hydrophobic layer (Fphobic) of the PET/PTFE composite substrate and 22 ACS Paragon Plus Environment

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the smaller membrane pore size (Fresistance, noted as Fr) after the polymerization of dopamine. Therefore, the intrinsic Fphilic was limited by Fphobic and Fr, unable to guarantee the water permeation (Figure 5Sa). Contrarily, when u-side faced upwards (Figure 5b, Figure 5Sb), Fphilic disappeared because water cannot penetrate the thick hydrophobic layer to contact the hydrophilic layer, result in the critical water column height higher than the whole tube (> 60 cm). Therefore, it can be seen water permeation is not easy with the as-prepared PDA-PET/PTFE Janus membrane.



Figure 5. Critical hydrostatic pressure generated by a water column allowing the membrane to stand. (a) Height of 37.5 cm for d-side upward, (b) Height of > 60 cm for u-side upward. The schematics on the sides (Sa) and (Sb) are force analyses for two separate conditions: d-side facing upward and u-side facing upward, respectively.

2.3.2 Guiding agent of acetone. To solve the difficulty of the water permeation, two basic principles were put forward. According to the force balance, we can either offer an external downward force or increase the Fphilic. Unlike other reported works, which exerted external pressure to force water to permeate,42-43 we took the strategy of Fphilic increasement, where a thimbleful of a ‘guiding agent’ was used for drainage. Here, acetone was chosen as the guiding agent for its relatively small surface tension, high solvent polarity and volatility (Figure 6). The hydrophobic layer presented a Cassie state initially, because air could fill into the membrane surface and formed an “air cushion” to support water droplets (Figure 6a). Then d-side was on top and 0.5 mL acetone was used to moisten the bottom hydrophobic layer, as a result the air cushion in hydrophobic surface was replaced by acetone (Figure 6b). Ensuing it, the surface energy of u-side increased sharply and the u-side became hydrophilic due to the similar polarity of acetone to that of water; hence, the downward Fphilic has increased successfully. In this


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way, water droplets were inclined to spread on the membrane surface, where Cassie state turned into a hydrophilic Wenzel state. The whole Janus membrane became entirely superhydrophilic and naturally water could infiltrate it (Figure 6c). During the permeation of water, the acetone in the membrane was gradually replaced (Figure 6d). Finally, a water layer was formed and a steady ‘tractive force’ (Fphilic indeed) between water was emerged, where infiltrated water below the membrane continuously acted as a guiding agent for uninfiltrated water above the membrane. Despite the small amount of acetone volatilized or washed away by water, the permeation can continue until all the water above the membrane flowed into the beaker below. Because during the whole process, there was no chance for air to be trapped into the membrane. The room occupied by acetone was replaced only by water so that the membrane could remain the wetting Wenzel state until all water ran out.



Figure 6. Illustrations of how water infiltrates the membrane and flows continually after the drainage of acetone. After acetone rinsing, the membrane is changed from hydrophobic Cassie state to hydrophilic Wenzel state. The membrane surface is filled with air initially (a) and then is replaced by acetone after the drainage (b). Then water can infiltrate the membrane (c) and the acetone is replaced by water gradually (d). No


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air can insert into the surface during the whole process. Therefore, the water layer on membrane surface can continually provide tractive forces for water above the membrane to permeate through the membrane.

2.3.3 Light oil/water separation. Based on these studies, the separation of universal oil and water mixtures can be achieved with the Janus PDA-PET/PTFE membrane. In accordance with the wettability performance, light oil (ρoil < ρwater) can be removed from water with the d-side facing upwards, while in contrast, heavy oil (ρoil > ρwater) and water mixtures can be separated with the u-side facing upwards. Here, a fixture was used for separating the mixture of toluene (dyed by oil red, ρ = 0.866 g/cm3) and water, as shown in Figure 7a: the Janus PDA-PET/PTFE membrane was fixed between two Teflon clamping pieces with the hydrophilic d-side facing up. With 0.5 mL acetone used for drainage, the entire membrane became momentarily superhydrophilic, permitting water to trickle through the membrane until the water ran out. Besides, a water layer has formed on the membrane surface as water infiltrated, thus oil leaking can be impeded. Finally, the removal of toluene from the toluene/water mixture was achieved



(Figure 7b), with red dyed toluene blocked above and clean water remained in the beaker below. In addition to toluene, various kinds of light oil were tested, all with satisfactory efficiency (all higher than 99.92%) (Figure 7f). The flow rate of light oil/water separation was tested with three mixtures: toluene/water, n-octane/water, cyclohexane/water (all 30 mL: 30 mL) and the resultant flow rate was 1.899×103 L·m-2·h-1, 1.661×103 L·m-2·h-1, 1.790×103 L·m-2·h-1, separately (Figure S8). The slight differences of flow rates between three light oil/water mixtures could be attributed to the different density of light oils. Moreover, the separation efficiency was still higher than 99.97% after ten times of toluene removal (Figure 7e). The surface wettability (Figure S9a and S9b) and micro morphology (Figure S9c and S9d) remained unchanged, and typical peaks in FTIR spectra were highly coincident with those of d-side on the pristine membrane (Figure S10a), proving the membrane a good reusability.


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Figure 7. Photos for the separation of toluene/water before (a) and after (b) separation with the d-side facing upward, as well as for the separation of CCl4/water before (c) and after (d) separation with the u-side facing upward, where the oils were all dyed by oil red for better observation. (e) Ten separation efficiency tests for toluene/water (all higher than 99.97 %) and (f) separation efficiency for different types of light oil/water mixtures (all higher than 99.92 %). Subfigure (g) shows the cycling ability test for ten iterations of heavy oil/water separation (all higher than 99.12 %).

2.3.4 Heavy oil/water separation. On the other hand, CCl4 (dyed by oil red, ρ = 1.595 g/cm3) was chosen as representative of heavy oil, and its mixture with water was separated by simply reversing the previously mentioned fixture, thus, the hydrophobic u-



side on top (Figure 7c). Due to the greater density of CCl4 and the high hydrophobicity/superoleophilicity of the u-side, upon the mixture contacted the membrane, CCl4 could flow through the membrane fluently while water was obstructed. Finally, the separation was achieved with isolated CCl4 in the bottom beaker and clean water above the membrane (Figure 7d). Besides, the separation efficiency was excellent (> 99.90%) and still higher than 99.10% even after ten repeats of separation (Figure 7g). It should be mentioned that between switching the u-side and d-side upwards for different kinds of oil/water mixtures separation, a simple washing step by acetone or ethanol was desired. This could protect the membrane from being trapped by water or oil and influencing the following separation (Figure S11). Moreover, there was no need to disassemble the fixture during the washing process, which could be finished in situ with a few seconds. Meanwhile, the recycling use had almost no effect on the micromorphology (Figure S12b, S12c), the high hydrophobicity (WCA = 145.2°) of the u-side (Figure S12a) and the functional groups on surface (Figure S10b). The flow rates of this heavy oil/water mixtures were tested as well. Two mixtures including CCl4/water and chloroform/water (both 30 mL: 30 mL) were tested, and the flow rate was 2.865×103 30 ACS Paragon Plus Environment

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L·m-2·h-1 and 3.243×103 L·m-2·h-1, respectively (Figure S8). The differences of flow rates between heavy/oil mixtures might be ascribed to their different polarity.

2.3.5 Emulsion separation. To apply this PDA-PET/PTFE Janus membrane into more practical situation, the ability for separating emulsion was further explored. According to the asymmetric wettability of the two sides of the Janus membrane, hydrophobic u-side was faced upwards for water-in-oil emulsion separation so that the oil droplet could penetrate the membrane while the water droplet would be blocked above the PTFE layer. As a result, the water-in-toluene emulsion separation was achieved with efficiency of 99.75%. The optical microscope photos of water-in-toluene emulsion before and after separation were shown in Figure S13c and S13d. It could be clearly seen that after separation, the water droplet dispersed in oil were removed sufficiently. The water-in-oil emulsion was prepared by adding 1 mL H2O into 100 mL toluene with 0.1 g Span 80 and stirring at 1000 rpm for 48 h. Furthermore, superhydrophilic d-side was faced upwards for separating oil-in-water emulsion, which was prepared by adding 1mL toluene into 100 mL H2O with 0.1 g Tween 20 and stirring at 1000 rpm for 48 h. Small amounts of acetone were used as drainage, as a result, the toluene-in-water emulsion 31 ACS Paragon Plus Environment


was separated with 99.85% efficiency. Because water can infiltrate the membrane by aid of acetone, once the water layer was formed the oil was blocked above the PDAPET layer. The optical microscope photos of oil-in-water emulsion before and after separation were also presented in Figure S13a and S13b. The oil droplets dispersed in water were successfully blocked by this Janus membrane. Therefore, oil/water emulsion could also be treated with this PDA-PET/PTFE Janus membrane efficiently.

2.3.6 Durability of this Janus membrane towards harsh environments. The durability of this Janus PDA-PET/PTFE membrane towards harsh environments like strong acid, alkaline, salty solutions were firstly experimented, as a result the membrane performed good resistance towards these harsh environments. After immersing the membrane into three different kinds of solutions for 24 h (pH = 2, Figure S14; pH = 10, Figure S15; 3.5 wt% NaCl, Figure S16; respectively), the micromorphology and contact angle of the samples showed unchanged. Furthermore, due to the frequent uses of acetone in the light oil/water separation, the effect of acetone on the reusability was also tested. The Janus membrane was immersed in acetone for 24 h and then tested for its reusability


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performances for separating oil/water mixtures. As a result, the efficiency for separating toluene/water (light oil) and CCl4/water (heavy oil) was still satisfying (Figure S17). The efficiency was all higher than 99.96% for light oil/water mixtures and higher than 99.42% for heavy oil/water mixtures. FESEM images of Janus membrane after acetone immersion was presented as well (Figure S18). The intact structure indicated acetone had no effects on the reusability of the membrane. Therefore, this Janus PDAPET/PTFE

membrane

can

perform

outstanding

durability

towards

complex

environments and keep good separation ability for both light oil/water and heavy oil/water mixtures.

2.3.7 Mechanism for oil/water separation of this Janus membrane. It was accidental that we found the separating performance and mechanism in our work were completely opposite to those reported in previous Janus studies, who separated light oil/water with the hydrophobic side facing up while heavy oil/water with the hydrophilic side on top. The mechanism for the previously reported Janus membrane is as follows: when the hydrophobic side faced up, the WCA remained large; then, water would be pulled to the



bottom by the hydrophilic layer, while conversely, when the hydrophilic side faced up, water spread onto the hydrophilic side and cannot penetrate the whole membrane. However, to achieve this kind of Janus membrane, a critical factor needed to be required that the hydrophobic layer should be thin enough. In this way, the water droplet can contact the hydrophilic layer under gravity or external pressure and realize the continuous infiltration. Correspondingly, the preparation required a strict control, and the separation sometimes needed assistance from an external force.44-46 In contrast, our Janus membrane was designed to form a hydrophilic PDA layer on a hydrophobic substrate. Due to the inherent hydrophobicity and thickness of the PET/PTFE composite substrate, the resultant Janus PDA-PET/PTFE membrane was generated with a fairly thick hydrophobic layer (exposed PTFE layer and the unhydrophilized PET layer) but a rather thin hydrophilic layer (PDA modified PET layer); therefore, the performance and mechanism were totally different from those mentioned above (Figure 8). Owing to the prewetting role of acetone, the thick hydrophobic layer temporarily became superhydrophilic; as a result, the upward hydrophobic resistance disappeared, and a new downward hydrophilic dragging force appeared. Following this 34 ACS Paragon Plus Environment

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change, water drops penetrated steadily and acted as new guiding agents for uninfiltrated water. In addition, a water layer formed on the hydrophilic surface during the infiltration, which, as a result, offered assistance for the water phase and formed a barrier towards the oil phase. Therefore, water permeated more readily, while the oil above was blocked. For heavy oil/water mixtures, the thick hydrophobic layer prevented water from infiltrating, and its superoleophilicity enabled oil to filtrate, hence achieving a successful separation.

Figure 8. Mechanism for the separation of light oil/water by virtue of acetone drainage. The acetone-infiltrated hydrophobic layer became superhydrophilic, therefore enabling water to permeate. Permeated water sequentially imparted a dragging force to water 35 ACS Paragon Plus Environment


above the membrane until all the water ran out. In addition, water can form a layer immediately during its contact with the membrane, and thus inhibit oil infiltration. As a result, the separation of light oil/water can be achieved.

Table S1 has listed the differences with other reported Janus works, from which the advantages of our Janus PDA-PET/PTFE membrane can be clearly seen. (i) Compared with other preparation methods, like photopolymerization, electrospinning, fluorine and oxygen functionalization, laser-induced surface modification and so on, our immersion and tape peeling method performed facile and environmentally friendly. (ii) the subtle use of acetone has avoided the application of extra pressure. (iii) This Janus membrane presented a thick hydrophobic layer and a thin hydrophilic layer, no strict requirements for the layer thickness ratios. (iv) This Janus membrane was inspired from lotus leaf and possessed lotus-like properties: Janus wettability, self-cleaning and lotus-like floating. Besides, compared with traditional superwetting materials, this Janus membranes could integrate the separation light oil/water and heavy oil/water mixtures, oil-in-water emulsion and water-in-oil emulsion, enormously simplifying the separation process and avoiding extra consumption for fabricating different single functional


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membranes. Therefore, this PDA-PET/PTFE membrane has conducted a highly efficient removal of both light and heavy oil from water and performed good tolerance towards tough environments and repeated uses. 3. CONCULSIONS In conclusion, we fabricated a Janus PDA-PET/PTFE membrane with a simple immersion and peeling-off method, inspired by the floating Janus lotus leaf and the adherence of mussels. The resulting membrane showed lotus-like Janus wettability, self-cleaning and floating properties. Moreover, the Janus membrane can separate varieties of oils with a density lower or higher than that of water by simply reversing the fixture so that the hydrophilic or hydrophobic side faced upwards accordingly. When separating light oil/water mixtures, d-side faced up and small amounts of acetone were used as a guiding agent to avoid the use of extra pressure. The separation efficiency was satisfactory, and good reusability was confirmed: after ten tests, the efficiency remained higher than 99.97%. When separating heavy oil/water mixtures, u-side faced up and the separation performance was similarly outstanding with 99.12% efficiency after ten repeats. Besides, integrated separation of water-in-oil and oil-in-water 37 ACS Paragon Plus Environment


emulsions were also achieved with high efficiency. Even immersed in harsh environments such as acidic, basic and salty environments, the micromorphology and surface wettability of the membrane performed unchanged, proving the excellent resistance and durability. Overall, this bioinspired, simply prepared Janus membrane can allow the integrated separation of both light oil/heavy oil from water and emulsions successfully overcoming the monotony and limitation of traditional special wettability separation membranes, and meanwhile extending the application of the lotus and mussels in bionics to the oil-water separation. 4. EXPERIMENTAL SECTION 4.1. Materials. Dopamine chloride was purchased from J&K Scientific, and tris (hydroxymethyl) aminomethane (Tris) was purchased from Heowns Scientific Ltd. PET/PTFE composite membrane was purchased from Membrane Solutions. All chemicals were used without purification. 4.2. Membrane modifications. A PET/PTFE membrane was used as the substrate, which was ultrasonically cleaned in acetone to remove the surface dirt. To a 150 mL beaker with 100 mL deionized water, 0.2 g dopamine chloride and 1 mL Tris (1 mol/L) 38 ACS Paragon Plus Environment

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was added before the composite membrane was added, and the beaker was sealed for 48 h at room temperature. The resulting membrane was taken out and washed with ethanol and water. After drying at 60 °C for 24 h, the PTFE face was peeled off by tape, and thus the Janus PDA-PET/PTFE membrane was fabricated. 4.3. Materials characterization. Field emission scanning electron microscope (FESEM) images were obtained via a field emission scanning electron microscope (SU-8010, Hitachi Limited, Japan). Energy-dispersive X-ray (EDX) images were measured using EDX analysis (Horiba, Ltd., Japan). Fourier Transform infrared spectroscopy (FTIR spectra) were recorded using a Fourier Infrared Spectrometer (NICOLET6700, Thermo Scientific, USA). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Escalab 250Xi spectrometer using an Al Kα X-ray source (1486.6 eV). Contact angles were measured with a contact angle measurement machine (OCA 15 machine, Data-Physics, Germany). The oil content in the filtrate was measured with an infrared spectrometer oil content analyzer (Oil 480, Beijing Chinainvent Instrument Tech. Co., Ltd, China). The water content in the oil filtrate was measured by a Karl Fischer titrator (Cou-Lo Aquamax KF Moisture Meter, UK). Optical microscopic images 39 ACS Paragon Plus Environment


of the original emulsions and filtrates were captured using a Nikon ECLIPSE LV100POL polarizing optical microscope. 4.4. Oil/water separation experiment. The light oil/water mixtures (1:1 by volume) were poured onto the membrane, which was fixed between two Teflon clamping pieces, with the hydrophilic PDA-PET face facing upwards. Here, 0.5 mL acetone was injected onto the hydrophobic PTFE face by an injector and used for drainage; as a result, separation could be achieved. Before pouring the mixtures, a small amount of water was added onto the membrane to form a water layer. The separation efficiency was measured by the oil rejection coefficient (R) according to the following equation: R = (1 − Of/O0) × 100 %, where Of and O0 are the oil concentration of the collected filtrate and the original mixture, respectively. The flow rate of light oil/water separation tested by three mixtures including toluene/water, n-octane/water, cyclohexane/water, (all 30 mL: 30 mL). After prewetting PTFE side by small amounts of acetone, water started to penetrate the membrane and time was started. In contrast, for heavy oil/water mixtures, the hydrophobic PTFE side was placed facing up by simply reversing the fixture. The mixtures were then poured in and allowed to run solely by gravity. The separation efficiency was measured


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by the water rejection coefficient (R) according to the following equation: R = (1 − Wf/W0) × 100 %, where Wf and W0 are the water content in the collected filtrate and the original mixture, respectively. The flow rate of separating heavy oil/water mixtures were tested by two mixtures including CCl4/water and chloroform/water (both 30 mL: 30 mL). PTFE side was faced upwards and time was started once oil infiltrated the Janus membrane. The switchable separation of light oil/water and heavy oil/water mixtures with different faces upwards could be realized by simple reversing the fixture, without disassembling the fixture. However, a washing step was required during the switch to wash away the trapped water or oil in the membrane. This washing step could be realized in situ with some acetone or ethanol flushing the two sides of membrane. After blowing drying by dryer, the membrane could be used for switchable function with the desired side upwards. 4.5. Self-cleaning and floating lotus experiments. The self-cleaning property was verified by contaminating the membrane with methyl blue dye followed by washing the dye away with water. In addition, water flow can bounce off the membrane and go the other direction without infiltration. For the floating lotus experiment, the membrane was put into water vertically and allowed to float spontaneously. After floating on the water surface in the lotus state, despite the disturbance of the membrane by a tweezer to impart a downward force, the membrane returned to the lotus floating state.



4.6. Emulsion separation experiments. The fixture for emulsion separation was same with that for oil/water mixtures separation. First, oil-in-water emulsion was prepared by adding 1mL toluene into 100 mL H2O with 0.1 g Tween 20 and stirring at 1000 rpm for 48 h. Hydrophilic d-side was faced upwards and 0.5 mL acetone was injected onto the hydrophobic PTFE face by an injector and used for drainage; as a result, separation could be achieved. Before pouring the mixtures, a small amount of water was added onto the membrane to form a water layer. The separation efficiency was measured by the oil rejection coefficient (R) according to the following equation: R = (1 − Of/O0) × 100 %, where Of and O0 are the oil concentration of the collected filtrate and the original mixture, respectively. As for water-in-oil emulsion, which was prepared by adding 1 mL H2O into 100 mL toluene with 0.1 g Span 80 and stirring at 1000 rpm for 48 h. Hydrophobic u-side was faced upwards and the separation could be conducted. The separation efficiency was measured by the water rejection coefficient (R) according to the following equation: R = (1 − Wf/W0) × 100 %, where Wf and W0 are the water content in the collected filtrate and the original mixture, respectively. The switchable separation of water-in-oil emulsion and oilin-water emulsion with different faces upwards could be realized by simple reversing the fixture, without disassembling the fixture. However, a washing step was required during the switch to 42 ACS Paragon Plus Environment

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wash away the trapped water or oil in the membrane. This washing step could be realized in situ with some acetone or ethanol flushing the two sides of membrane. After blowing drying by dryer, the membrane could be used for switchable function with the desired side upwards.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. FESEM images of PDA-PET side or peeled-off PTFE side at high magnification, with sectional views, after repeated uses, after durability tests (acid, base, salt and acetone). Wettability performances (contact angles) of PDA-PET side or peeled-off PTFE side after repeated uses, after durability tests (acid, base, salt and acetone). Contact angles of two sides of original membrane. Optical images of emulsion before and after separation, flow rates of oil/water separation, exploration on PDA fabrication mechanism, FTIR spectra after repeated use, proof for the necessity of washing between switching faces upwards. Table of comparison with other reported Janus work. (PDF) Peeled-off process of PDA-PTFE layer. (MP4) 43 ACS Paragon Plus Environment


Self-cleaning property of PDA-PET/PTFE Janus membrane. (MP4) “Floating-lotus” state of PDA-PET/PTFE Janus membrane. (MP4) Lotus floating state remained though with disturbance. (MP4)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for financial support from the National Natural Science Foundation (51173099).

REFERENCES


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Superamphiphobic/Superhydrophobic

Oleophilic

Nanofibrous

Membranes

with

Unidirectional Oil-Transport Ability and Strengthened Oil-Water Separation Performance.

Adv. Mater. Interfaces 2015, 2, 1400506

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