Janus Membranes with Charged CNT Coatings for Deemulsification

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Janus Membranes with Charged CNT Coatings for Deemulsification and Separation of Oil-in-Water Emulsions Yun-Peng An, Jing Yang, Hao-Cheng Yang, Ming-Bang Wu, and Zhi-Kang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19700 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Applied Materials & Interfaces

[Revised as an article for publication in ACS Appl. Mater. & Interfaces]

Janus Membranes with Charged CNT Coatings for Deemulsification and Separation of Oil-in-Water Emulsions Yun-Peng An, Jing Yang*, Hao-Cheng Yang, Ming-Bang Wu, Zhi-Kang Xu* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China.

ABSTRACT: Oil-water separation, especially for those surfactant stabilized oil-in-water emulsions, is urgent to protect our ecological environment free from destruction. Janus membranes with a function of deemulsification appear as a kind of efficient materials for the separation of oil-in-water emulsions due to a precise adjustment of the surface nature for the hydrophilic and hydrophobic layers. However, existing strategies of membrane preparation suffer from complicated multi-steps, leading to uncontrolled thickness of the hydrophilic deemulsification layer. Herein, we present a facile and tunable method to prepare a series of Janus membranes consisting of negatively or positively charged carbon nanotubes (CNTs) and hydrophobic microfiltration membranes by vacuum filtration. The thickness of the hydrophilic CNTs coating is thus well controlled by engineering the 1

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amount of CNTs deposited on the substrate membrane. The prepared Janus membranes are effective for the separation of both heavy oil and light oil from oil-in-water emulsions through deemulsification owing to the charge-screening effect. It is very interesting that those membranes displaying a combination of water contact angle and underwater oil contact angle both above 90o have unique oil delivery behavior and thus high separation performance of oil from oil-in-water emulsions. Such Janus membranes can retrieve 89% of oil in 40 min from the 1,2-dichloroethane/water emulsions with the droplet size of 19 µm. This easy-to-prepare and easy-to-tune strategy provides feasibilities for practical applications of Janus membranes to the deemulsification and separation of oil-in-water emulsions. KEYWORDS: Janus membrane; carbon nanotubes; oil-in-water emulsion; oil/water separation; deemulsification.

1. Introduction In recent years, oily waste water from industrial emissions and oil spill accidents have caused huge damages to the human ecological environment. It is thus urgent to develop efficient

separation

strategies

for

oil/water

mixtures,

especially

for

those

surfactant-stabilized oil-in-water emulsions.1 Among the traditional separation approaches, membrane technologies1-3 are superior to others such as flotation,4 sedimentation,5 and centrifugation,5 due to their high consecutiveness and low energy consumption .1 It has been universally acknowledged that those membranes with specific surface wettability can effectively realize oil/water separation. Taking oil-in-water (O/W) emulsion as an example, 2


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the continuous water phase can permeate through a hydrophilic membrane whereas the dispersed oil phase is retained by hydrophilic-hydrophobic repulsion.6-14 In contrast, a hydrophobic membrane can filtrate oil but intercept water from water-in-oil (W/O) emulsions.15-18 However, all these membranes can only effectively separate one certain type of emulsion, i.e., the hydrophilic membranes can merely be used to separate O/W emulsions, while the hydrophobic ones are limited to dispose W/O emulsions. In order to address this issue, Janus membranes, two-dimensional materials with asymmetric properties on each side,1,19-23 have recently shown as a kind of competitive candidate to separate both O/W and W/O emulsions based on “sieving effect”.24-26 When the hydrophilic side is set towards O/W emulsions, water can permeate through the hydrophilic layer, and the other hydrophobic layer is equally able to promote the water permeation flux via the facilitated water detachment.24 For separating W/O emulsions, one can simply flip over the Janus membrane, making the hydrophobic side face the feed.24 In this case, water droplets are retained but oil permeates through the membrane. Nevertheless, the separation mechanism is ascribed to the “sieving effect” of the porous membranes; the pore size thus must be uniform and smaller than the diameter of the emulsion droplets,27,28 which is obviously at the expense of high permeation flux. On the other hand, for O/W emulsions, if the density of oil is higher than that of water, the rejected oil droplets are prone to accumulate and form a barrier layer on the membrane surface, hence impeding the further separation.3,25 A feasible solution towards the issues mentioned above is to endow Janus membranes with a function of deemulsification which is a reverse process of emulsification.29 Taking 3



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O/W emulsion as an example, when the feed emulsion contacts the hydrophilic side of the Janus membrane, the continuous water phase will spread on the membrane surface to generate a thin water layer, and the hydrophobic side will resist water to further penetrate if this layer is thick enough. After the deemulsification of the dispersed oil droplets by the functionalized hydrophilic side, it is expected that the accumulated oil phase can touch on the hydrophobic layer of the Janus membrane if the hydrophilic layer is thin enough.19 Subsequently, the oil phase is promoted to permeate through the membrane due to the strong hydrophobic-hydrophobic interactions.19,30 Consequently, the continuous water phase is retained, and the dispersed oil phase passes through the membrane. This separation mechanism is quite different from the simple “sieving effect” as discussed above in which continuous water permeates across the membrane. Thus, the pore size of the Janus membranes can be tuned as much larger than the diameter of the emulsion droplets, which is beneficial to increase the oil permeation flux.30,31 Currently, Liu and co-workers, developed a series of Janus cotton fabric membranes with the deemulsification ability for the separation of O/W emulsions. The hydrophobic surface was poly(dimethylsiloxane) (PDMS)

whereas

the

hydrophilic

layer

was

formed

by

either

ionized

poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA)30,31 or nonionized polysoap bearing oligo(ethylene glycol) monolaurate chains.32 They demonstrated that not only ionic30,31 but also nonionic surfactant32 stabilized O/W emulsions could be separated by their Janus membranes. Thanks to the deemulsification function of the hydrophilic layers, the coalesced oil can fill into the fabric pores on the hydrophobic layer and then selectively permeate through the membrane, resulting in a highly efficient and rapid oil separation. 4


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Although the deemulsification and separation have been successfully integrated, multiple reaction steps and complicated preparation methods lead to uncontrolled thickness of the hydrophilic layer for these Janus membranes. It is obvious that the thickness of the hydrophilic layer must be precisely managed for the oil phase to be able to touch the hydrophobic layer. Meanwhile, the hydrophobic layer should be thick enough to hold back the water phase for permeation through the membranes. Therefore, it is required to develop a more facile and thickness-controlled way to prepare Janus membranes with the ability of deemulsification. The influence of the thickness on the emulsion separation needs to be further investigated to obtain an understanding of the separation mechanism in depth. Herein, a series of Janus membranes have been facilely fabricated by depositing hydrophilized and positively/negatively charged CNTs on hydrophobic microfiltration membranes via vacuum filtration (Figure 1). CNT network was previously used as support and grafted with hydrophilic and hydrophobic polymers on two sides to prepare Janus membrane for the separation of both W/O and O/W emulsions based on “sieving effect”.25 However, in our work, CNTs were applied as the hydrophilic layer to tune the surface wettability and the deemulsification functions simultaneously for the Janus membranes. They have feasibilities to be functionalized with positive or negative charges33 as well as to create a network of capillaries allowing almost frictionless mass transport (water or oil).25,34 The positively charged CNTs layer is able to promote the deemulsification of emulsions stabilized by negatively charged surfactants before oil penetration, and vice versa (Figure 1). The thickness of the hydrophilic CNTs coating can also be tailored by simply regulating the concentration of CNTs in aqueous dispersion. The effect of CNTs coating thickness on the 5



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surface wettability and the oil permeation are investigated in detail. Moreover, we demonstrate that the separation efficiency reaches its maximum when the water contact angle (WCA) and the underwater oil contact angle (UOCA) are both around 90° on the CNTs-coated membrane surface. The as-prepared Janus membranes can effectively separate surfactant stabilized O/W emulsions, thereby potentially opening a new way to practical oil/water separation.

Figure 1. Schematic illustration of the preparation of CNTs coated Janus membranes and the deemulsification and separation of O/W emulsions process.

2. Experimental Section 2.1 Materials

Microfiltration polypropylene membranes (MPPMs) (thickness = 200 µm)

with a mean pore size of 0.2 µm and a diameter of 2.5 cm were purchased from Membrana GmbH (Germany). The membrane samples were pretreated by washing with acetone for 30 min to remove adsorbed impurities and then dried in a vacuum oven at room temperature. Multi-walled carbon nanotubes (CNTs) were provided by Chengdu Organic Chemicals Co., Ltd. (China). Dodecyl trimethyl ammonium chloride (DTAC) and polyethyleneimine (PEI, Mw = 1,000 g/mol) were procured from Aladdin Co., Ltd. (China). Other chemicals, 6


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including ethanol, acetone, nitric acid, sulfuric acid, Oil Red O, sodium dodecyl sulfate (SDS) and 1,2-dichloroethane, were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were used as received without further purification. Water was deionized and ultrafiltrated to 18.2 ΩM by an ELGA LabWater system (France). 2.2 Surface charged modification of CNTs

For negatively charged modification, CNTs

(12 g) were dispersed in a mixture of concentrated nitric acid (100 mL) and concentrated sulfuric acid (300 mL) by sonication for 1 h at room temperature. Then, the mixture was heated to 120 ºC and refluxed under vigorous stirring for 6 h. After cooling to room temperature, the crude products were rinsed by ultrapure water with 3 cycles of centrifugation/decanting/dispersion until the pH was approximately 5. The carboxylated CNTs with negative charges were obtained after drying at 100 ºC overnight.35 For positively charged modification, PEI (0.4 g) with opposite charges was added into 20 mL aqueous dispersion of the as-prepared carboxylated CNTs (0.147 mg/mL). The mixture was stirred at room temperature for 5 h, and the aminated CNTs with positively charged surface were obtained after the similar rinsing/drying processes above.36 2.3 Preparation and mechanical stability of CNTs/MPPM Janus membranes

In a

typical preparation, the aqueous suspensions of the modified CNTs (carboxylated or aminated ones) were prepared as the mother liquors (C = 0.147 mg/mL). A certain amount of the mother liquor was taken out and diluted to 20 mL by ultrapure water. Then, the carboxylated CNTs or aminated CNTs aqueous dispersions with different concentrations were filtered through MPPM samples using a vacuum filtration device to prepare the CNTs/MPPM composite Janus membranes. The resulting Janus membranes were then dried 7



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in a vacuum oven at room temperature overnight. Note that the as-prepared CNTs coatings were too thin to obtain the absolute thickness. Thus, the mass thickness (in g/cm2)37,38 determined by the mass of CNTs coating per effective area of the MPPM sample was used to investigate the CNTs deposition in this study. The mass thickness of the CNTs coatings were controlled by varying the concentration of the CNTs dispersions (Table S1 and S2 in Supporting Information). To evaluate the mechanical stability of the as-prepared Janus membranes, carboxylated CNTs (mass thickness = 2.9×10-3 g/m2) and aminated CNTs (mass thickness = 12×10-3 g/m2) coated membranes were immersed into 20 mL of deionized water for ultrasonic treatment under 100 Hz for 20, 40 and 60 min. After ultrasonication, the aqueous solution was taken out and tested by UV-vis spectroscopy for probing the CNT residues detached from the membranes. 2.4 Characterization

The UV-vis spectra were recorded by a UV-vis spectrophotometer

(UV-2450, Shimadzu Inc., Japan). Copper meshes (200 mesh) were put into the CNTs aqueous dispersions and then picked up, dried under an infra-red lamp. The morphologies of the carboxylated and aminated CNTs were then observed using transmission electron microscopy (TEM, Hitachi H7650, Japan) and the accelerating voltage was set as 120 kV. Janus membranes were cut into small pieces and attached to the specimen stage with double-faced conductive tape for the surface microscopy. The specimen stage was then placed in a high-vacuum metal-spraying apparatus to form a gold-plated layer on the surfaces of samples. A field emission scanning electron microscope (FESEM, Hitachi S4800, Japan) was then used to characterize the surface morphologies of the membranes. X-ray photoelectron spectra were employed to measure the chemical composition of the 8


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coatings using a spectrometer (XPS, PerkinElmer, USA) with Al Kα excitation radiation (1486.6 eV). The charge potentials of the CNTs and emulsions were detected by a ZetasizerNano (Malvern ZCEC, UK). The surface wettabilities of the membranes were evaluated by the detection of water contact angles (3 µL of water droplets as the probe liquid) using a DropMeter A-200 contact angle system (MAIST VisionInspection & Measurement Co. Ltd., China), and the drop contact diameter (Drop CD), i.e., the diameter of the circular contact interface between droplet and membrane surface, was also captured and given at the same time. For measuring UOCA, a small piece of Janus membrane was first placed into a transparent glass container filled with ultrapure water. An oil droplet of 1,2-dichloroethane (3 µL) was then dropped onto the surface of the CNTs coatings using a 5 µL micro injector. 2.5 Oil/water separation of the Janus membranes

The O/W emulsions were prepared by

adding different surfactants to an oil/water mixture and then stirred at 200 rpm for 6 h. DTAC and SDS were selected to evaluate the influence of different surfactants. 1,2-Dichloroethane was first used as the heavy oil phase and was dyed with Oil Red O before mixing with ultrapure water to enhance the visibility of the experiments. The oil/water ratios were set as 20/80, 15/85, 10/90 and 5/95 (v/v) to prepare a series of O/W emulsions. All emulsions were observed using an optical microscopy (BX51, Olympus), and ImageJ software was applied for measuring the droplet size (more than 100 droplets were taken and analyzed for measuring the average value of each sample). The as-prepared Janus membranes were clipped into a homemade glass device with the CNTs coating side facing the emulsion feed. For each membrane, 40 mL of emulsion was poured into the left 9



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cell of the device, and the process of emulsion separation by the Janus membrane was carried out under magnetic stirring at 200 rpm. Hexadecane was also used to prepare emulsions with an oil/water ratio of 20/80 (v/v). The as-prepared carboxylated CNTs (mass thickness = 2.9×10-3 g/m2) and aminated CNTs (mass thickness = 12×10-3 g/m2) coated Janus membranes were chosen to evaluate the separation performance via the similar test above. The recovery of the oil (Roil) was calculated by Equation (1): ௠

ܴ௢௜௟ = ௠ × 100%

(1)

బ

where m is the mass of obtained oil after separation, m0 is the feed mass of oil in the emulsion. In order to determine whether CNTs were detached from the membranes during the separation, both filtrates and feed emulsions were probed by UV-vis spectroscopy afterward.

3. Results and Discussion 3.1 Structures and mechanical stability of the CNTs/MPPM Janus membranes The oxidation process of CNTs by refluxing with strong oxidants has been widely used to functionalize the surface of CNTs, endowing them with covalently bonded carboxylic (-COOH), carbonyl (-C=O) and hydroxyl (-OH) groups.35 Especially, the HNO3/H2SO4 mixture is able to induce a high content of -COOH groups,39 leading to negatively charged CNTs when the pH is higher than the pKa of -COOH. Moreover, positively charged CNTs can be continuously functionalized by immobilizing PEI, an amino-rich cationic polyelectrolyte, via the physisorption and electrostatic interaction with the -COOH groups.36 Both the negatively and the positively charged CNTs show an excellent hydrophilicity and can be well dispersed in water as stable colloid solutions due to the 10


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charge repulsion effect. Their surface charges are reflected by Zeta potentials of dilute aqueous dispersions: -26.8 mV for the carboxylated CNTs and 22.5 mV for the aminated ones (Table S3 in Supporting Information). The surface chemical compositions were determined by XPS analyses (Figure S1 and Table S4 in Supporting Information). After acid oxidation, the O/C ratio of the carboxylated CNTs is 1.3 times than that of the neat ones, indicating an increased oxygen percentage due to the oxygen-containing carboxyl groups on the surfaces. Moreover, it is clear to see an O 1s peak at 532.4 eV assigned to carboxyl groups after peak deconvolution via Gaussian fitting (Figure S1(b) in Supporting Information). For the aminated CNTs, a new N 1s peak at 398.9 eV proves the existence of amine groups thanks to the immobilization of PEI on the surface of the carboxylated CNTs. The surface morphologies after functionalization were visualized by TEM (Figure S2 in Supporting Information). As similar as the neat CNTs, the carboxylated CNTs shows a clear outer edge because only small molecular groups are covalently bonded on the surface after acid oxidation. However, the aminated CNTs show a blurred outer edge due to the polymeric feature of PEI. Vacuum filtration was subsequently used to construct uniform CNTs coating on the substrate membrane with tunable deposition amount.40,41 Figure 2a and 2b show that the coating thickness linearly rises with increasing the concentration of CNTs dispersion. Therefore, we can simply and accurately control the CNTs deposition on the substrate membrane by adjusting the concentration of CNTs suspension. Figure 2c and 2d indicate the charged CNTs are uniformly deposited on the membrane surface. The micropores on the membrane surface are not fully covered at a relatively low deposition amount. With the 11



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increase of CNTs dispersion concentration, the membrane surface becomes more compact and the exposed area is reduced due to the gradual deposition and covering of CNTs layer. This process enables us to prepare composite Janus membranes with a tunable thickness of charged CNTs coatings (12×10-3 ~ 120 ×10-3 g/m2 for carboxylated CNTs coating and 12×10-3 ~ 180 ×10-3 g/m2 for aminated CNTs coating). Since the CNTs coatings were deposited on the surface of MPPM, it is necessary to investigate the mechanical stability of the membranes. A drastic ultrasonication was performed for the Janus membranes immersed in water. It was found no cracks or damage for our Janus membranes and no obvious CNTs absorption at 253 nm42 in the UV-vis spectra (Figure S3 in Supporting Information). These results prove that the detachment of the CNTs from the MPPM layer is negligible, indicating a good mechanical stability of the CNTs coatings as well as a robust interface adhesion between the CNTs layer and the MPPM membrane. It is thus reasonable to predict that the mechanically stable and charged coatings can attract oil droplets emulsified by oppositely charged surfactants and demulsify them more easily and quickly.

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Figure 2. Mass thickness evolutions of a) the carboxylated CNTs coatings and b) the aminated CNTs coatings as a function of the concentration of the CNTs dispersions. FE-SEM images of the substrate membrane surfaces deposited by c) the carboxylated CNTs coatings and d) the aminated CNTs coatings with different mass thicknesses (scale bar = 50 µm). 3.2 Surface wettability and permeation property of the Janus membranes

In view of the

O/W emulsion separation, WCA and UOCA were measured to characterize the surface wettability by hydrophilicity in air and oleophobicity in water for the as-prepared Janus membranes (Figure 3). The surface wettability is crucial for these Janus membranes to the morphology of water or oil droplets on the membrane surface, which dominates the permeation behavior of water or oil and subsequent separation performance.43 The neat substrate membrane has a WCA of 158° and a UOCA of 15°, suggesting superhydrophobic in air and superoleophilic underwater. It was observed that the oil droplets rapidly infiltrated into the membrane and those air bubbles were exhausted quickly from the 13



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membrane pores. This behavior is attributed to the strong affinity between the substrate membrane and the oil (1,2-dichloroethane) through hydrophobic-hydrophobic interaction, facilitating the fast oil spreading and occupation into the membrane pores as well as followed release of air bubbles. On the other hand, either the negatively or the positively charged modification endows CNTs with superhydrophilic and underwater oleophobic features.44 Therefore, it is clear to see a decline of WCA and an ascent of UOCA with raising the mass thickness of CNTs coating on the substrate membrane, indicating a simultaneous enhancement of hydrophilicity and underwater oleophobicity of the formed Janus membranes. It is very interesting that this tunable surface wettability can be divided into three zones, and the values of 90o are the borderline for the Janus membranes with distinguishing lyophilic/lyophobic property. It means the mass thickness has a great effect on the surface wettability of Janus membranes either from the carboxylated CNTs (Figure 3a) or the aminated CNTs (Figure 3b) layer. Taking the carboxylated CNTs coating as an example, a tiny deposition amount with a mass thickness of 1.8×10-3 g/m2 displays almost no change for the surface wettability in which the WCA and UOCA values are similar to the neat substrate (Figure 3a). In this low-deposition zone, the CNTs layer is too thin to cover all the area of substrate, thus the surface wettability is mainly governed by the exposed hydrophobic microfiltration membrane. The low-deposition zone is similarly observed in the case of Janus membranes deposited by the aminated CNTs with a mass thickness range from 4.7×10-3 g/m2 to 9.4 g/m2 (Figure 3b).

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Figure 3. WCA and UOCA of a) the carboxylated CNTs and b) the aminated CNTs coated Janus membranes with different mass thicknesses. A medium-deposition zone can be found with increasing the mass thickness of CNTs layers from either the carboxylated CNTs (mass thickness = 2.9×10-3 ~ 15×10-3 g/m2) or the aminated CNTs coatings (mass thickness = 12×10-3 ~ 59×10-3 g/m2). Both WCA and UOCA in this zone are above 90°, suggesting a combination of hydrophobicity in air and oleophobicity underwater. Although the mass thickness of the hydrophilic CNTs layer increases, we can still find uncovered areas of the hydrophobic substrate surface shown in Figure 2c and 2d, impeding water wetting in air. However, the as-deposited CNTs are hydrophilic enough for embedding water molecules into the rough micro-/nano-structures of the CNTs coatings, offering the Janus membranes with an oil-repellent capability underwater.43,45 The Janus membrane surface is hydrophilic in air and superoleophobic underwater with further increasing the mass thickness of CNTs in the high-deposition zone. It can be seen that WCA is 51° in air for the Janus membrane with a mass thickness of 120×10-3 g/m2 from the carboxylated CNTs layer. However, the oil droplets could not escape from the injector and stay on the surface of the CNTs coating for measuring UOCA, indicating a superior oil resistance underwater.43 This behavior is due to the fully covered CNTs coating 15



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on the substrate membrane with a sufficient high mass thickness reflected by the surface morphology in Figure 2c and 2d. The hydrophilic charged CNTs layer benefits the affinity with water molecules, resulting in embedded water in the micro-/nano-structural groove of CNTs, thus hinders the penetration of oil droplet.43,45 The surface wettability is extremely significant to the permeation property of the Janus membranes.41 The carboxylated CNTs coating with different mass thicknesses are compared to study the effect of deposition amount on the permeation of oil droplets through the membrane. For the high thickness of 47×10-3 g/m2, the oil droplet on the membrane surface maintains the initial UOCA after 300 s (Figure 4a, 4c) and no permeation phenomenon occurs after 300 s. The coating surface is completely free of oil traces after removing the oil droplet. However, for the Janus membrane with a mass thickness of 2.9×10-3 g/m2 in the medium-deposition zone, the UOCA of oil droplet rapidly decreases from 90° to 0° within 4 s (Figure 4b and 4c). It is notable that the value of drop contact diameter on these two membranes remains unchanged with decreasing UOCA (Figure 4d), indicating that the oil globules penetrate through the Janus membranes rather than spread on the hydrophilic side surface. Analogous results were also observed for those aminated CNTs coated Janus membranes. (Figure S4 in Supporting Information).

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Figure 4. Digital images of underwater oil permeation process on the carboxylated CNTs coated Janus membranes with the mass thickness of a) 47×10-3 g/m2 and b) 2.9×10-3 g/m2. c) Dynamic UOCA and d) oil Drop CD of the carboxylated CNTs coated Janus membranes with different mass thicknesses within 300 s underwater. The difference in the oil permeation through the Janus membranes originates from the mass thickness of CNTs coatings. For the carboxylated CNTs layer with a high mass thickness of 47×10-3 g/m2, almost all areas of the hydrophobic substrate membrane are blanketed (Figure 2c). Oil droplets therefore cannot get in touch with the hydrophobic substrate, prohibiting the permeation through the membrane. In contrast to the mass thickness in the high-deposition zone, the relatively thin CNTs coating bearing a mass thickness of 2.9×10-3 g/m2 in the medium-deposition zone shows favored oil permeation. On the one hand, the hydrophilic layer with such suitable thickness gives rise to proper oil wettability, ensuring an intact globular drop shape without spreading on the membrane 17



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surfaces. However, such moderate hydrophilic layer is not able to block the contact between the oil drop and the hydrophobic substrate. The properly bare area of the hydrophobic substrate can draw and deliver the oil across the membranes through hydrophobic-hydrophobic interaction. Therefore, the penetration property can be well tuned by the thickness control of the CNTs coatings. It is inferred that those CNTs coated Janus membranes with the mass thickness in the medium-deposition zone are propitious to the oil permeation and contribute to an effective O/W separation afterwards. 3.3 Performances of the Janus membranes for the separation of O/W emulsions The separation performances were investigated for the Janus membranes with WCA and UOCA both above 90° via a home-made device with an effective contacting area of 4.91 cm2 for O/W emulsions. In our cases, the Janus membranes were vertically contacted with the O/W emulsions to avoid dead-end filtration (Figure 5). This set-up is suitable to scale-up into crossflow filtration modules for reducing membrane fouling. It was applied for light oils (e.g., hexadecane in this work) while the dead-end one was usually used for heavy oils to increase the permeation probability of oils through the membranes by gravity.30,31 Nevertheless, the surfactant-stabilized O/W emulsions are able to touch with the CNTs coatings although 1,2-dichloroethane was used as a heavy oil in our experiments. Then, the CNTs coatings trigger the deemulsification of the O/W emulsions via charge-screening effect.30,31 These CNTs-coated Janus membranes show specific and distinguished oil delivery properties as mentioned above. Therefore, the demulsified oil can be strongly absorbed by the hydrophobic substrate membrane and permeate through the Janus membranes spontaneously. At the same time, water has a low solubility in the oil phase and 18


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it cannot enter into the membrane pores.31 Based on this separation mechanism, the CNTs coated Janus membranes achieve an effective separation performance of O/W emulsions, yielding a clear and transparent filtrate without any water droplets inside observed by optical microscopy (Figure 5). In addition, no obvious UV absorption due to CNTs is observed in both feed emulsions and filtrates after separation (Figure S5 in Supporting Information). This behavior indicates that the CNTs coatings are stable during the membrane service process, which is beneficial to ecological environment in real applications because CNTs are recognized to have possible hazard to the environment.46

Figure 5. Optical microscope photographs of the feed emulsion before separation (left) and the obtained filtrate after separation (right) achieved by a home-made device (middle) with the carboxylated CNTs coated Janus membrane (mass thickness = 2.9×10-3 g/m2). The scale bar is 500 µm. To study the charge-screening effect on the deemulsification, two types of O/W emulsions containing 1,2-dichloroethane were prepared with different surface charge profiles using DTAC and SDS as surfactants. The measured Zeta potentials are 102 and -53 mV for the DTAC and SDS stabilized emulsions, respectively, due to the difference in charge nature of each surfactant (Table S3 in Supporting Information). The evolutions of Roil as a function of time are compared for the carboxylated CNTs (Figure 6a) and the aminated CNTs (Figure 6b) coated Janus membranes. For the negatively charged Janus

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membrane constructed from the carboxylated CNTs coating, it is clear to see the highest separation recovery (54%) is for those emulsions prepared with positively charged DTAC, while the negatively charged emulsions by SDS displays almost no separation performance (Roil = 9%). On the contrary, the positively charged Janus membrane fabricated from the aminated CNTs coating can separate SDS stabilized emulsions rather than DTAC ones. This behavior is due to the charge-screening effect: the electrostatic attraction between unlike charges facilitates the deemulsification on the membrane surfaces, whereas the electrostatic repulsion between like charges blocks the approach of emulsion droplets to the membrane surfaces. Therefore, the oppositely charged CNTs coating is favorable to accelerate an effective separation according to the requirement of O/W emulsions with different surface charge features.

Figure 6. Roil evolutions of O/W emulsion containing 1,2-dichloroethane separation as a function of time using a) the carboxylated CNTs coated Janus membrane (mass thickness = 2.9×10-3 g/m2) and b) the aminated CNTs coated Janus membrane (mass thickness of 12×10-3 g/m2) towards the emulsions stabilized by different surfactants. The emulsions were stirred for 6 h with an oil/water ratio of 20/80 and the concentration of the surfactants was 3 mg/mL. After deemulsification, oil permeation will take place if the transmembrane pressure is high enough.1 When the oil droplet contacts the Janus membrane, the driving force for oil permeation is ∆Pdrop, calculated by Equation (2),48 leading to a quick penetration from the 20


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hydrophilic side to the hydrophobic side across the Janus membrane:

∆ܲௗ௥௢௣ =

ଶఊ ୱ୧୬ ఏ ௥೏ೝ೚೛

(2)

where rdrop is the radius of the oil droplet, γ is the surface tension of oil and θ is the UOCA of oil droplet on the hydrophilic surface. In view of an ideal separation effect, the larger the ∆Pdrop value is, the higher the final Roil is. Therefore, we can promote the separation efficiency by adjusting the UOCA value which can be tuned by engineering the thickness of CNTs coating as verified above. According to Equation (2), ∆Pdrop has a maximum value when UOCA is 90° for given rdrop and γ values (Figure S6 in Supporting Information). The CNTs coated Janus membrane with a UOCA around 90° is thus anticipated to have the highest ∆Pdrop. As shown in Figure 7(a), the Janus membrane coated by carboxylated CNTs with a mass thickness of 2.9×10-3 g/m2 achieves a Roil of 57% within 40 min, while either thinner (mass thickness = 1.5×10-3 g/m2, UOCA = 78°) or thicker (mass thickness = 5.9×10-3 g/m2, UOCA = 99°) coated membrane exhibits a mitigated separation performance due to a deviation of UOCA value from 90°. Similar results were also observed in the cases of the Janus membranes with aminated CNTs coatings (Figure 7b). The final recovery rises to its maximum when the mass thickness is 12×10-3 g/m2 with a UOCA of 92°.

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Figure 7. Roil evolutions of O/W emulsions containing 1,2-dichloroethane separation as a function of time using a) the carboxylated CNTs and b) the aminated CNTs coated Janus membranes with different coating mass thicknesses. The emulsions are stirred for 6 h with an oil/water ratio of 20/80. The DTAC concentration is 10 mg/mL in a) and the SDS concentration is 3 mg/mL in b).

Furthermore, the emulsions containing hexadecane were prepared to evaluate the universality of Janus membranes concerning the separation of various oils with different densities in practical applications. The carboxylated CNTs coated Janus membranes can separate the light oil from O/W emulsions to achieve a Roil of 52% in 40 min (Figure S7(a) in Supporting Information). The reason of the lower Roil of hexadecane than 1,2-dichloroethane can be explained as follows. The oil droplets of hexadecane after deemulsification tended to float upwards and aggregate in the left neck of the device due to its low density, leading to an inefficient contact with the membranes and thus a low recovery efficiency. Similarly, the aminated CNTs coated membranes are able to recover 44% of hexadecane in 40 min, which is also a little lower than that of 1,2-dichloroethane (Figure S7(b) in Supporting Information). Droplet radius (rdrop) is the second factor to influence ∆Pdrop in Equation (2). We evaluated it by using the charged CNTs coated Janus membranes with the highest O/W separation performances mentioned above. A series of O/W emulsions containing 22


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1,2-dichloroethane with different droplet sizes were prepared by varying the oil/water ratio and the surfactant amount. It can be seen that the larger the droplet is, the slower the separation is (Figure S8 in Supporting Information). This behavior is ascribed to the reduced ΔPdrop, giving rise to a decrease of the final oil recovery accordingly.

Conclusion A series of Janus membranes were simply prepared from negatively or positively charged CNTs coating and hydrophobic polypropylene microfiltration membrane by vacuum filtration. The as-prepared Janus membranes can effectively demulsify and separate surfactant stabilized O/W emulsions. On the one hand, the charged CNTs coatings can attract oil droplets with the opposite charged nature, thus dramatically boosting the deemulsification. On the other hand, the surface wettability can be facilely tuned through elaborating the mass thickness of the CNTs layer to endow the Janus membranes with an integration of hydrophobic in air and oleophobic underwater. Such Janus membranes with special wettability possess distinguished oil permeation properties and subsequent O/W separation performance. This work provides the basic understanding of the influencing factors of Janus membranes on the O/W emulsion separation and therefore paves a solid way for practical applications for this kind of membrane.

ASSOCIATED CONTENT Supporting Information. Mass thicknesses of the coatings with different CNTs amounts taken out from the carboxylated and the aminated CNTs mother liquor, Zeta potentials of CNTs in dilute aqueous dispersions and O/W emulsions stabilized by differently charged surfactants, XPS spectra of CNTs, chemical compositions from XPS data, TEM images of CNTs, digital images of underwater oil permeation process on the aminated CNTs coated Janus membranes, dynamic UOCA and Oil Drop CD of the aminated CNTs coated Janus 23



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membranes during the permeation process underwater, the ∆Pdrop-UOCA curve based on Equation (2), Roil of O/W separation for emulsions with different oil/water ratios and different DTAC and SDS amounts, effect of varying oil type in O/W separation, digital pictures of membranes and UV-vis spectra of the liquid after 20, 40, and 60 min of ultrasonication, UV-vis spectra of the feeds and filtrates after separation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (J. Yang); [email protected] (Z.-K. Xu)

Acknowledgement This work is financially supported by the Zhejiang Provincial Natural Science Foundation of China (Grant no. LZ15E030001) and the National Natural Science Foundation of China (Grant no. 21534009).

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Table of Content Graphic Janus Membranes with Charged CNT Coatings for Deemulsification and Separation of Oil-in-Water Emulsions Yun-Peng An, Jing Yang*, Hao-Cheng Yang, Ming-Bang Wu, Zhi-Kang Xu*

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Janus Membranes with Charged CNT Coatings for Deemulsification

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