Research Article pubs.acs.org/journal/ascecg
Cobweb-Inspired Superhydrophobic Multiscaled Gating Membrane with Embedded Network Structure for Robust Water-in-Oil Emulsion Separation Xiangde Lin,† Moonhyun Choi,† Jiwoong Heo,† Hyejoong Jeong,† Sohyeon Park,† and Jinkee Hong*,† †
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School of Chemical Engineering and Material Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea ABSTRACT: The separation of oil−water mixtures using superwetting membranes is increasingly desired, particularly for the practical processes of environmental protection and industrial production. However, achieving durability and multifunction in current separation systems, among other issues, remains challenging. Herein, a cobwebinspired gating multiscale pore-based membrane has been created as the framework system for removing emulsified water from an oil phase. This membrane was assembled using macroscale chemically etched stainless steel mesh (ESSM), a microscale network of carbon nanofibers (CNFs), and a nanoscale network of single-walled carbon nanotubes (SWCNTs). Superhydrophobic and superoleophilic interfaces were then fabricated on the ESSM/CNFs−SWCNTs gating membrane using a polydimethylsiloxane (PDMS) coating. The ability of this membrane with a discrete water-repellent property to resist mechanical damage was demonstrated in gravity-driven waterin-oil emulsion separation with high performance; this behavior was attributed to the protective metal mesh and different pore scales resulting from the embedded dual-scale network structure. As a result, this smart superwetting membrane structure can serve as a novel platform for constructing a multifunctional emulsified oil−water separation system with high robustness. Moreover, on the basis of the findings in this study, current filter membranes fabricated using a fibrous network can be improved to achieve higher durability. KEYWORDS: Cobweb-inspired structure, Superhydrophobic property, Network gating membrane, High durability, Emulsion separation
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INTRODUCTION Recently, researchers have paid significant attention regarding the effective handling of oil−water mixtures, especially the emulsified type, given the increasing concerns about oil leakage destroying the ocean ecosystem, oily wastewater from industrial processing, energy demands from exploitation and recycling, and sustainable oil utilization by removing undesired water.1,2 Interfacial superwettable membranes for oil−water separation, which are solely driven by self-gravity, have been of considerable interest since a highly water-repellent metal mesh with macroscale pores was first prepared by Jiang and co-workers.3 Since then, porous superwetting membranes with a de-emulsification property and further decreased pore size as a sieving effect have been significantly developed, especially for separating surfactant-free or surfactant-stabilized oil−water emulsions.4 Meanwhile, higher performance involving oil− water separation selectivity and solvent permeability has been pursued using different strategies, such as high interfacial superwettability,5 ultrathin membranes,6 smart liquid-gating membranes,7 surfaces with extremely low emulsion adhesion forces,8 and other unique membrane structures.9 In the present work, a smart superhydrophobic multiscaled gating membrane with embedded network binary structure was subtly designed © 2017 American Chemical Society
for robust separation. In addition, as such, significant progress has been made in current membranes, and an oil−water emulsion separation system with high durability was highly desired. In the first place, to adapt well to practical demand, superwetting one-dimensional-material-based fibrous network membranes assembled by diverse prevailing strategies such as vacuum suction,10 electrospinning,11 and the freeze-drying technique12 have been the focus of research on oil−water emulsion separation. To adequately address time and advanced facility requirement concerns, vacuum suction as a means to simply prepare the desired membranes was focused on and improved in the present work. However, conventional fibrous membranes are generally designed with a single micro/ nanoscale structure, which is detrimental to achieving sufficient roughness for the superwetting interface and enhancing the molecular transport flux for gravity-driven oil−water emulsion separation, assuming that the membranes are the same thickness. For instance, even though a high flux was achieved Received: January 13, 2017 Revised: March 1, 2017 Published: March 2, 2017 3448
DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
Research Article
ACS Sustainable Chemistry & Engineering
water once the SSM was exactly bent because of the etching process. Then, CNFs−SWCNTs (weight ratio is 10:1) dispersion was prepared by mixing CNFs powder (10 mg), SWCNTs powder (1 mg), deionized (DI) water (100 mL), and SDBS (100 mg). The mixture was then treated by 40 kHz sonication for 1 h. The SDBS molecules can easily adhere to the nanotube surface, and charged SWCNTs were separated from each other under a sonication effect. After that, the resulting dispersion was filtered by 635 sized SSM to remove the unwanted bundled nanofibers and nanotubes. The CNFs−SWCNTs dispersion for the following membrane preparation was diluted 10 times. Then, the ESSM/CNFs−SWCNTs membrane was assembled by a vacuum suction system, including ESSM (top position) and PES membrane (middle position) placed on the ceramic support (bottom position). The resulting CNFs−SWCNTs dispersion (80 mL) was filtrated through the PES and ESSM under 0.6 bar pump-based pressure. The placement of the ESSM was regulated randomly for the next filtration of the CNFs−SWCNTs dispersion (80 mL), which ensured uniform CNFs−SWCNTs network coverage. Finally, the ESSM/CNFs−SWCNTs membrane was formed by filtering 320 mL of CNFs−SWCNTs dispersion. For removal of residual surfactant on the membrane surface, 100 mL of DI water was used for rinsing by the same suction process. Uncured 1 wt % PDMS (weight ratio of prepolymer:curing agent = 10:1) was completely dissolved in toluene. Then, the ESSM/CNFs−SWCNTs membrane was dipped into PDMS solution for 3 min. After evaporation of toluene in PDMS coating, the uncured ESSM/CNFs−SWCNTs−PDMS membrane was processed by pump suction for 5 min to make a homogeneous PDMS layer. After that, the ESSM/CNFs−SWCNTs−PDMS membrane was cured under a 100 °C environment for 10 min. Mechanical Durability Test. The mechanical durability of the asprepared membrane was examined through an abrasion test. An external force by 50 g weight was applied to the ESSM/CNFs− SWCNTs−PDMS test membrane (1 cm × 3 cm), followed by 3 cm sliding with a constant velocity along the sandpaper, and this was defined as one cycle. The durability by measuring static water contact angle was monitored after various numbers of abrasion cycles. Water-in-Oil Emulsion Separation Test. Water-in-toluene emulsion mixture (volume ratio is 1:99) was prepared under 40 kHz sonication for 1 h. Then, the emulsified surfactant-free water-in-oil mixture was collected for the subsequent test while the residual water that was not emulsified was discarded. Prior to the separation test, the ESSM edge without CNFs−SWCNTs−PDMS coverage was sealed by cured PDMS. Then the membrane was installed well in a filtration system without ceramic support while the separation area is 6.25 cm2. It should be noted that there is no external pressure being applied to the membrane during the separation test. Characterization. Field-emission scanning electron microscopy (FE-SEM; Carl Zeiss) was used for analyzing morphologies of the ESSM/CNFs−SWCNTs−PDMS membranes. X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific) measurements were conducted for chemical composition analysis. A homemade contact angle goniometer was employed to measure static water contact angle (4 μL of water), and the final value of the water contact angle was obtained by 3 measurements at different positions. In addition, a charge coupled device (CCD) camera (IMT 3, IMT solutions) accompanies the contact angle goniometer. The final value was acquired with the help of the LB-ADSA methods in IMAGE J software. Dynamic light scattering (DLS) measurements were carried out for the size distribution of the water-in-toluene emulsions.
for separating water-in-oil microemulsions using an ultrathin freestanding single-walled carbon nanotubes (SWCNTs) network membrane, the membrane did not exhibit superhydrophobicity and mechanical durability.13 A certain pressure was also applied to this separation system. Moreover, although many hierarchical architectures have been previously reported, most of them were intended for regular oil−water separation by creating surface superwettability via a single macroscale pore size rather than for removing smaller oil−water emulsions.14−16 As a result, the ability to deal with oil−water emulsions is still a severe challenge by superwetting thin membranes.17 Here, a superhydrophobic multiscale gating membrane assembled using macrowires, nanofibers, and nanotubes has been realized to retain different types of water droplets, considering the integrated state of real oil−water mixtures. On the other hand, the durability issue of current highperformance superwetting membranes, especially against mechanical force effects, has not yet been adequately resolved; these mechanical effects include surface abrasion due to manmade operation, shape deformation due to applied pressure or fluid shear, and easily broken structures due to the ultrathin membrane thickness. The durability issues result from vulnerable superwetting interfaces depending on a rough structure and specific modification of the surface, which are extremely susceptible to external environments.18,19 Recently, a few studies on the preparation of robust superwetting coatings or membranes have been conducted, which are represented by protective three-dimensional supports and durable coatings characterized by highly adhesive effects.14,20−23 However, only regular oil−water separation was achieved using these mechanically durable membranes, which nearly do not work for micro/nanoscale oil−water emulsions because of the size sieving effect. Therefore, to our knowledge, promising results regarding the construction of mechanically robust emulsion separation systems have rarely been reported. In view of the two aforementioned drawbacks, different types of materials, including chemically etched stainless steel mesh (ESSM), carbon nanofibers (CNFs), single-walled carbon nanotubes (SWCNTs), and polydimethylsiloxane (PDMS), were simply assembled into a superhydrophobic multiscale pore-based gating membrane using an improved vacuum suction technique; this structure is briefly described as ESSM/CNFs−SWCNTs−PDMS. Thanks in large part to the protective three-dimensional ESSM support, external damage to the embedded network binary membrane can be adequately avoided, leading to high robustness for oil−water emulsion separation.
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EXPERIMENTAL SECTION
Materials. The SSM (1000 sized mesh, Guangdong, China), CNFs (diameter, 100 nm; length, 20−200 μm; Sigma−Aldrich), SWCNTs (diameter, 1−2 nm; length, 5−30 μm; purity, > 95%; Chengdu Organic Chemicals Co. Ltd., China), sandpaper (320 grit, Deerfos, South Korea), PDMS (Sigma-Aldrich), poly(ether sulfone) (PES) membrane (0.45 μm pore diameter, Sigma-Aldrich), and sodium dodecylbenzenesulfonate (SDBS, Sigma-Aldrich) were available commercially and used as received. Preparation of the ESSM/CNFs−SWCNTs−PDMS Membrane. First, the 1000 sized SSM was cleaned with aqueous ethanol 50% (v/ v) with 10 min of 40 kHz sonication treatment and dried at room temperature. After that, it was immersed into 2 N sulfuric acid for 5 min of etching treatment to randomly remove metal oxide films and inner metal components, leading to the ESSM with higher surface roughness. The etching process was terminated by rinsing in deionized
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RESULTS AND DISCUSSION Nature always offers remarkably simple ideas for building special material structures. To catch prey, a cobweb made of proteinaceous silk is usually created by a spider. During the assembly of a cobweb, the silks are first intertwined with surrounding supports, such as tree branches and building walls. Subsequently, the fixed web framework and smaller pore-sized structure are gradually established for intercepting desired preys. Inspired by this design, a fibrous network gating 3449
DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
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Figure 1. Schematic illustration of water-in-oil emulsion separation by bioinspired superhydrophobic multiscaled gating ESSM/CNFs−SWCNTs− PDMS membrane with embedded network structure.
Figure 2. Characterization of resulting superhydrophobic ESSM/CNFs−SWCNTs−PDMS gating membrane. (a) Digital photograph of welldispersed mixture with CNFs and SWCNTs, further supported by FE-SEM images. (b) XPS-spectra-assisted elemental composition analysis on ESSM support and ESSM/CNFs−SWCNTs−PDMS membrane. (c) FE-SEM image of as-prepared superhydrophobic ESSM/CNFs−SWCNTs− PDMS gating membrane with embedded network structure. (d) Magnified FE-SEM image of the embedded dual-scale material-based CNFs− SWCNTs−PDMS network. (e) Magnified FE-SEM image of ESSM/PDMS surface with hierarchical structure. (f) Pore size distribution of ESSM/ CNFs−SWCNTs−PDMS and ESSM/CNFs−PDMS membranes.
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DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
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Figure 3. Analysis of morphology and wettability with respect to different amounts of CNFs used in the suction process. (a) Pristine protective support of ESSM/PDMS without CNFs. Inset: pristine SSM without etching treatment. (b−d) ESSM/CNFs−SWCNTs−PDMS membrane with (b) 0.8, (c) 2.4, and (d) 4 mg of CNFs. (e) Relationship between static WCA and amount of CNFs. Note that the ratio of CNFs to SWCNTs was fixed at 10:1 in all the ESSM/CNFs−SWCNTs−PDMS membranes.
with the usage of single-scale CNFs or SWCNTs. Considering the superior mechanical and anticorrosive performance of SSM, the 1000 sized SSM was determined to be a promising protective support. Moreover, the membrane preparation method was further modified on the basis of traditional vacuum suction. Generally, CNFs or SWCNTs cannot be directly deposited into large pores of ESSM to form a uniform network structure, which was overcome by the PES supports underneath the ESSM and by repeating the filtration process. To confirm that the PDMS coating was successfully introduced and modified as the outmost layer, surface element analysis using X-ray photoelectron spectroscopy (XPS) was performed, mainly identifying peaks with binding energies at approximately 532 eV (O 1s), 285 eV (C 1s), 102 eV (Si 2p), and 154 eV (Si 2s) in the XPS wide-scan spectrum (Figure 2b). The emerging core-level signals of Si 2p and Si 2s were demonstrated to originate from the silicone-containing PDMS layer. As observed in Figure 2c, the fibrous CNFs−SWCNTs network as a gating membrane was uniformly inserted into the mesh, and the ESSM backbone can be clearly recognized as resisting the external mechanical damage in the following durability test. As a key component of a water-in-oil emulsion separation device, the embedded dual-scale CNFs and SWCNTs were intertwined with each other and cross-linked to become a superhydrophobic CNFs−SWCNTs−PDMS network after introducing cured PDMS (Figure 2d). Moreover, it should be noted that the coated PDMS layer can affect the roughness of the membrane to some degree. Therefore, the uncured membrane can be adequately processed by pump suction to ensure the required roughness and porous structure for superwetting membrane. Moreover, with the help of the strong sulfuric acid etching process, the surface morphology of the ESSM wires at the macroscale was demonstrated to be a hierarchical structure, thereby forming a superhydrophobic interface for the entire ESSM/CNFs−SWCNTs−PDMS membrane (Figure 2e). Macroscale water droplets were expected to be effectively retained by the resulting PDMS-coated ESSM as the first barrier during water-in-oil emulsion separation. In addition, with the fibrous network pores with diameters of approximately 500 nm in by the cross-linked CNFs network as a secondary barrier, water microemulsions could be intercepted accordingly due to the size sieving effect. With the consideration of the
membrane can be embedded into a macroscale mesh support to create a mechanically durable superhydrophobic device for oil− water emulsion separation, as illustrated in Figure 1. Specifically, the SSM as a protective support was first etched by sulfuric acid to form an ESSM with a highly rough surface, which is required to create a superhydrophobic structure.24 A mixing dispersion of nearly 100-nm-diameter CNFs and sub-2nm-diameter SWCNTs was sucked by pump-based vacuum. The filtration process was repeated for several cycles to ensure a homogeneous ESSM/CNFs−SWCNTs membrane. As a result, a membrane with a highly rough interface was formed thanks in large part to the etched SSM and fibrous CNFs− SWCNTs network. The uncured PDMS, as a nonfluorine silicone-containing finishing agent, was poured onto the asprepared porous ESSM/CNFs−SWCNTs membrane to make it superhydrophobic. In addition, PDMS coating was also helpful for enhancing membrane stability due to the crosslinked fibrous structure. Because the transmembrane pressure of water increases as a result of the superhydrophobic property, the water-in-oil emulsion separation can be conducted under self-gravity. In fact, although the overall ESSM/CNFs− SWCNTs−PDMS membrane was designed to be superhydrophobic, it mainly included a superhydrophobic CNFs− SWCNTs−PDMS network and hydrophobic impermeable ESSM/PDMS support, as shown in the illustration. Surface roughness plays an important role for creating desired hydrophobicity. The ESSM with supplementary roughness was critical for the final water-repellent membrane, because the roughness from the CNFs−SWCNTs network is not enough for superhydrophobicity. Recently, research interest has increasingly focused on molecular separation based on carbon materials with nanoscale diameters or thicknesses, such as carbon nanofibers, carbon nanotubes, and graphene.25−27 For instance, the superhydrophobic and superoleophilic membrane caused by growing a vertically aligned multiwalled carbon nanotube on SSM by thermal chemical vapor deposition can be used for addressing large-scale oil−water emulsions.28 In the present work, onedimensional carbon materials with dual-scale diameters were selected as the building blocks of the membrane (Figure 2a), which can guarantee both higher roughness because of the CNFs and smaller pore size because of the SWCNTs compared 3451
DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
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Figure 4. Mechanical durability test involving abrasion of superhydrophobic ESSM/CNFs−SWCNTs−PDMS membrane with sandpaper. (a) Illustration of detailed abrasion test by applying constant pressure. (b) Digital photographs of static WCA at point 2 measured after various numbers of abrasion cycles (n). (c) Static WCA variation under a different number of abrasion cycles. (d) FE-SEM image of the ESSM/CNFs−SWCNTs− PDMS membrane after 30 abrasion cycles. (e) Magnified FE-SEM image of abraded ESSM/CNFs−SWCNTs−PDMS membrane. (f) FE-SEM image of the PES/CNFs−SWCNTs−PDMS membrane after 30 abrasion cycles. (g) Magnified FE-SEM image of abraded PES/CNFs−SWCNTs− PDMS membrane. Inset: FE-SEM image of the pristine PES/CNFs−SWCNTs−PDMS membrane.
nanoscale diameter of the SWCNTs and long membrane path, the embedded SWCNTs network with a pore size of approximately 200 nm served as the last barrier for separating water-in-oil nanoemulsions, which is primarily attributed to the combined effects of de-emulsification, size sieving, and deep filtration. The pore size distributions of the ESSM/CNFs− SWCNTs−PDMS and ESSM/CNFs−PDMS membranes were determined and are presented in Figure 2f. The as-prepared superhydrophobic ESSM/CNFs−SWCNTs−PDMS gating membrane for tailored oil−water separation was estimated to remove multiscale water droplets with diameters ranging from the macro- to the nanoscale, simultaneously displaying superior mechanical and chemical durability. Surface roughness plays a key role in creating a waterrepellent interface, especially to obtain superhydrophobicity.29,30 From both thermodynamic and hydrodynamic aspects, increased surface roughness or CNFs−SWCNTs coverage in the present research can obviously change the static contact angle of the surface and enable the membrane to be highly waterproof, which can significantly contribute to water droplets rolling off due to the low water-adhesive force during oil−water separation. Therefore, different amounts of CNFs and SWCNTs were applied to prepare ESSM/CNFs−SWCNTs− PDMS membranes with various fibrous network coverage, as shown in Figure 3. Briefly, the CNFs−SWCNTs−PDMS surface area on ESSM was gradually increased with an increasing amount of CNFs: 0 mg (Figure 3a), 0.8 mg (Figure 3b), 2.4 mg (Figure 3c), and 4 mg (Figure 3d). Meanwhile, the superwettability represented by the WCA also changed remarkably with respect to the amount of CNFs, as observed in Figure 3e. Herein, the superhydrophobicity of the PES membrane with the CNFs−SWCNTs−PDMS network was first investigated, which indirectly demonstrates the embedded superhydrophobic CNFs−SWCNTs−PDMS network regardless of the ESSM. This result is observed because the completely covered PES membrane support has no effect on the superhydrophobic property of the deposited network with high roughness. Accordingly, the superhydrophobicity of the ESSM/CNFs−SWCNTs−PDMS membrane including the embedded superhydrophobic network and hydrophobic etched SSM was attained when more than 3.2 mg of CNFs was used. Although a hydrophobic ESSM had a detrimental effect on the
overall superwettability, it can be relieved with increased proportions of the CNFs−SWCNTs−PDMS network. Water droplets impacting a superhydrophobic interface can be easily repelled because of trapped air holes. In addition, a transition state of superhydrophobicity usually occurs once the surface structure breaks down, leading to strong competition from the fully wetting state.31 Therefore, superhydrophobicity, which strongly depends on high roughness and surface chemistry, is readily affected by external stimuli, such as chemical erosion,32 mechanical damage,33 and biological fouling.34 In particular, a superhydrophobic structure with excellent ability to withstand physical forces has not been addressed well, which has dramatically inhibited its use in realworld applications.35 For instance, current superhydrophobic membranes with high oil−water separation performance have faced similar issues because external forces caused by applied pressure, man-made damage, and mass flow impact on the membranes cannot be avoided during the separation process. In this work, a PDMS-assisted cross-linked, superhydrophobic network was subtly embedded into a protective ESSM support, which can prevent mechanical damage from destroying the core membrane structure. As illustrated in Figure 4a, the as-prepared ESSM/CNFs−SWCNTs−PDMS membrane was placed between a 50 g weight and sandpaper, and constant movement under a certain force was performed. An abrasion cycle was defined as 3 cm with the sandpaper. Considering the nonuniform surface after a number of abrasion cycles, three arbitrary points on the membrane surface were examined to show the superwettability variation. The WCA at position 2 was measured after a designated number of abrasion cycles from 0 through 30 with an interval of 5 cycles, as shown in Figure 4b. The superhydrophobicity gradually broke down under the mechanical abrasion effect. Once more than 20 abrasion cycles were applied, the superhydrophobicity of the overall ESSM/ CNFs−SWCNTs−PDMS membrane was lost, as demonstrated in Figure 4c. In addition, the WCA fluctuated approximately 145°, which is mainly attributed to the destroyed ESSM with smooth surface and lost PDMS layer, which can be directly observed in Figure 4d. However, the core CNFs−SWCNTs− PDMS network with superhydrophobicity was not broken and was well-preserved, which was expected not to obviously affect the water-in-oil emulsion separation performance (Figure 4e). The only effect of losing the overall superhydrophobicity was 3452
DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
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Figure 5. Water-in-oil emulsion separation performance of as-prepared and mechanically abraded ESSM/CNFs−SWCNTs−PDMS membranes. (a) Size distribution of pristine surfactant-free water-in-toluene emulsions. (b) Size distribution of emulsions separated by the pristine ESSM/CNFs− SWCNTs−PDMS membrane. (c) Size distribution of emulsions separated by the ESSM/CNFs−SWCNTs−PDMS membrane abraded for 30 cycles. (d) Emulsion size variation before and after separation. Inset shows digital photographs of original emulsion seeds and separated emulsions by two types of membranes. (e) Corresponding separation fluxes obtained by the pristine and abraded ESSM/CNFs−SWCNTs−PDMS membranes.
emulsions could be successfully separated by the superhydrophobic ESSM/CNFs−SWCNTs−PDMS membrane with a multiscaled gating structure, as demonstrated in Figure 5b. The resulting emulsion size was 76 ± 17 nm, which indicates that nearly more than 100-nm-sized emulsions can be effectively separated by the superhydrophobic ESSM/CNFs− SWCNTs−PDMS membrane. Moreover, as mentioned above, the mechanically abraded ESSM/CNFs−SWCNTs−PDMS membrane was expected to separate water-in-oil emulsions without obvious effects, as demonstrated in Figure 5c. The emulsions with sizes of 184 ± 15 nm were obtained by the abraded membrane after 30 cycles. The emulsion size variation is directly reflected in Figure 5d. In addition, digital photographs of the original emulsion seeds (milky) and emulsions (clear) separated by two types of membranes are also displayed in the inset of Figure 5d. In addition to measuring the separation selectivity, the permeability was calculated on the basis of the collected oil volume in a unit time for a membrane with fixed size. As summarized in Figure 5e, fluxes of 430 and 470 L/m2 h were obtained for the pristine and abraded ESSM/CNFs−SWCNTs−PDMS membranes.
an increase in the adhesive force of water droplets at more than the macroscale size onto the impermeable ESSM. The original ESSM simply exhibited hydrophobicity, which is similar to the behavior of the abraded ESSM. Therefore, the micro- or nanoemulsions can be effectively separated as normal, and larger emulsions can also be retained by the abraded ESSM because of the combined effects of the ESSM impermeability and superhydrophobic network. For comparison, a PES/ CNFs−SWCNTs−PDMS membrane without the protective support was assembled and abraded. The basic surface structure was completely destroyed as compared with the pristine superhydrophobic membrane, as observed in Figure 4f,g, and the inset of Figure 4g. Before examination of the separation performance, the border of the as-prepared superhydrophobic ESSM/CNFs− SWCNTs−PDMS membrane was first sealed with cured PDMS. Then, the membrane was well-sealed in a normal filtration system without a pump because of separation driven by self-gravity of the water-in-toluene emulsion solution. The surfactant-free water-in-toluene mixture with an emulsion size of 3.4 ± 1.1 μm was first prepared for the following separation test, which was monitored by DLS in Figure 5a. During the separation process, the toluene can spread out quickly on the membrane surface because of the superoleophilicity; meanwhile, the water droplets remained on the top glass vessel or were trapped in the CNFs−SWCNTs−PDMS network because of the superhydrophobicity. With the consideration that the required transmembrane pressure of water droplets was much greater than the applied pressure caused by self-gravity, the water droplets could not pass through the membrane. However, the required transmembrane pressure of toluene was much smaller than the applied pressure; therefore, toluene could penetrate the membrane in a high flux. This explanation for separating water−oil emulsions is also supported by previous research.36 Consequently, the water-in-toluene
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CONCLUSIONS In summary, to address the mechanical durability issue concerning water−oil emulsion separation using superwetting membranes, a bioinspired superhydrophobic multiscaled gating membrane with a CNFs−SWCNTs−PDMS network embedded into a protective ESSM support was developed to effectively separate nanometer-sized water droplets. The resulting superhydrophobic ESSM/CNFs−SWCNTs−PDMS gating membrane exhibited superior robustness to mechanical abrasion damage because of the network inlay-gated structure. Also, the ESSM/CNFs−SWCNTs−PDMS membrane was demonstrated to exhibit good separation performance in terms of selectivity and permeability. First, considering that the gating 3453
DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
Research Article
ACS Sustainable Chemistry & Engineering
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membrane was assembled by multiscale materials including ESSM, CNFs, and SWCNTs, the formed pores can be used to retain different-sized water emulsions for tailored water-in-oil emulsion separation. Second, more than 100-nm-sized water emulsions can also be effectively intercepted, and a high separation flux of 430 L/m2 h was attained, driven only by selfgravity. Moreover, the separation performance of the membrane remained nearly unchanged even after 30 abrasion cycles. Inspired by our work, a durable water-in-oil emulsion separation system with a higher flux is expected in the near future.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +82 02 824 3495. Phone: +82 2 820 5561. ORCID
Jinkee Hong: 0000-0003-3243-8536 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Korean government (MSIP) (Nos. 2012M3A9C6050104, 2016M3A9C6917405). Additionally, this research was also supported by a grant from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (Nos. HI14C-3266 and HI15C-1653).
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DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
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DOI: 10.1021/acssuschemeng.7b00124 ACS Sustainable Chem. Eng. 2017, 5, 3448−3455
