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Interface-confined surface engineering constructing water-unidirectional Janus membrane
Author’s Accepted Manuscript Interface-confined surface engineering constructing water-unidirectional Janus membrane Xiaobin Yang, Linlin Yan, Feitian Ran, Avishek Pal, Jun Long, Lu Shao www.elsevier.com/locate/memsci
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S0376-7388(18)32764-9 https://doi.org/10.1016/j.memsci.2019.01.014 MEMSCI16784
To appear in: Journal of Membrane Science Received date: 4 October 2018 Revised date: 7 January 2019 Accepted date: 11 January 2019 Cite this article as: Xiaobin Yang, Linlin Yan, Feitian Ran, Avishek Pal, Jun Long and Lu Shao, Interface-confined surface engineering constructing waterunidirectional Janus membrane, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.01.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interface-confined surface engineering constructing water-unidirectional Janus membrane
Xiaobin Yang1, Linlin Yan1, Feitian Ran, Avishek, Pal, Jun Long and Lu Shao*
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, PR China.
*Corresponding Author: Tel. +86-451-86413711 ； fax:+86-451-86418270; Email address: [email protected] (L. Shao).
Abstract Janus membranes (JMs) have attracted increasing attention in the fields of liquid manipulation owing to its interesting liquid-unidirectional transportation feature. Nonetheless, most of the currently available methods to fabricate JMs suffer from specific or complicated operations, impeding practical applications of JMs. Herein, a facile liquid/liquid (immiscible oil/water) interface-confined surface engineering strategy was first applied on hydrophilic cotton fabric membrane to construct JM with the aid of mussel-inspired chemistry. The water-infused fabric channels and the unique distribution of amphiphilic C18-NH2 at the oil/water interface enable the unilateral hydrophobicity transformation on fabric. As a result, the as-prepared membranes exhibited asymmetric
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These authors contributed equally.
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chemistry, wettability, and surface morphology. The evaluation of water droplet behaviors across the membrane at the oil/water interface verified the water unidirectional transport feature of our JM. In addition, two collectors made by our JMs successfully demonstrated the excellent ability of water collection from both oil/water mixture and water-in-oil emulsion. The newly developed liquid/liquid interface-confined surface engineering strategy provides enormous potential for Janus membrane construction to manipulate liquid transportation towards smart applications. Graphical Abstract
Keywords: Janus membrane, asymmetric wettability, unidirectional transportation, surface engineering, water collection
1. Introduction Membrane technology plays an important role in practical applications, such as water treatment and pharmaceutical separations [1-6]. Most recently, there is an increasing demand for membranes with the ability for intelligent response to the environment or precise control of liquid transportation [7-11], which is difficult to be achieved by
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conventional fabrication techniques. Janus membrane (JM) as a new conceptual separation membrane has opposing properties on their two faces, such as hydrophilic/hydrophobic wettability, positive/negative charge, etc [12, 13]. In this study, JMs we focused on are membranes with asymmetric wettability and liquid-unidirectional transportation property owing to the integrated hydrophilic/hydrophobic structure [1215]. Due to the unique structure and properties, JM has gained enormous attention in the fields of liquid manipulation, fog collection, oil/water separation, etc [14-24]. However, most of the currently available methods to fabricate JMs are involved with specific instruments or fluorinated toxic chemicals and complicated operations which retard the practical usages of JMs [12]. Therefore, it is crucial to design facile strategies to fabricate JMs with excellent unidirectional transportation properties.
The conventional JM preparation strategy is based on the elaborate cementation of two membranes with opposite wettability or asymmetric surface modification. Jiang et al. successively electro-spinned the hydrophobic polyurethane and hydrophilic poly (vinyl alcohol) fibers into an integrated JM with the asymmetric wettability [25]. Lin et al. also fabricated oil-unidirectional JMs via a similar sequent electrospinning method [26]. These approaches could precisely tune the wetting gradient, but specific instruments are required. To fabricate JMs from one piece of substrate, asymmetric surface modification is generally required. Xin et al. and Lin et al. prepared a superhydrophobic layer containing photo-catalytic materials on substrates, and generated gradient wettability across the membrane via unilateral ultra-violet irradiation [27, 28]. Darling’s group unilaterally deposited hydrophilic metallic oxide on a hydrophobic polymer membrane
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via atomic layer deposition technology to obtain JM [29]. Liu et al. asymmetrically grafted two polymers with opposite wettability to obtain JM with oil-unidirectional transportation property [30]. Furthermore, one synthetic block co-polymer or incorporating moiety with emulsification were proposed to realize the in situ generation of similar JMs [31, 32]. Tian et al. prepared JMs via unilateral modification; perfluorooctyl-trichlorosilane vapor fumigation right below a hydrophilic membrane or plasma treatment on one side of the hydrophobic membrane [33]. These approaches are effective to construct asymmetric wettability but involved with some specific instruments and strict synthesis conditions. Generally, the interface engineering plays a prominent role in fabricating functional membranes towards efficient water-energy nexus optimization [34, 35]. Recently, our group [36] and Yang et al. [37, 38] fabricated JMs via floating coating; the hydrophobic membrane can float on the aqueous solution and realize its bottom layer to be modified by a thin hydrophilic layer, which could be considered as a kind of surface engineering at an air/liquid interface. However, such floating coating strategy cannot be extended to hydrophilic membranes because it will sink into the aqueous solution or other solvents. Thus, a new toolbox to construct JM on hydrophilic substrates needs to be urgently explored.
Interestingly, we recently found that the oil/water interface is a good space to realize specific surface engineering on opposite sides of the material settled there by means of its stratification feature of the immiscible oil and water. Herein, we designed a brand-new strategy, liquid/liquid interface-confined surface engineering, to realize the waterunidirectional JM. As a proof of concept, the hydrophilic cotton fabric had sunk and
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landed at the oil/water interface (the upper layer is water phase). Therein, the waterinfused porous substrate could validly prevent oil phase entering inside the bulk; the amphiphilic molecules waiting for hydrophobic surface engineering can be easily dissolved in the oil phase and grafted on the fabric surface toward oil side. The wettability, chemistry, and microscopic morphology on both surfaces of the resultant fabrics were determined by a series of physicochemical characterization. Measurements on the typical liquid transportation behaviours were conducted at the oil/water interface by squeezing water droplets (above or below) onto the resultant fabric. In addition, the water collection assays were demonstrated when treating both oil/water mixture and water-in-oil emulsion using two kinds of JM-assembled devices. The developed interface-confined surface engineering strategy exhibits great potential towards the fabrication of various JMs and liquid manipulation for advanced smart usages. Moreover, it may have some limitations; it is probably difficult to precisely tune the position of the Janus property barrier within the membrane using this approach.
2. Experimental 2.1. Materials Cotton fabric was purchased from a local store. Dopamine hydrochloride (DA) was obtained from Sigma-Aldrich. Octadecylamine (C18-NH2) and tris (hydroxymethyl) aminomethane (Tris) was supplied by Aladdin (China). Hydrochloric acid (HCl), ethanol, petroleum ether, 1,2-dichloroethane, and Oil red O were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (China). All the chemicals were used as received. Ultra-pure water was laboratory-made.
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2.2. Preparation of water-unidirectional JM The received cotton fabric was purified using ethanol and ultra-pure water three times, respectively. Then the fabric was incubated in a fresh dopamine solution (2 mg mL-1, Tris-HCl buffer, pH=8.5). The fabric after polydopamine (PDA) deposition was rinsed vigorously by water several times. Then, it was transferred into a beaker containing upper water phase and bottom oil phase (dichloroethane, containing 1 mg mL-1 C18-NH2) with same volumes. The fabric automatically landed and stayed at the water/oil interface during its sinking process in the beaker. After 10 h of incubation, the fabric was rinsed with ethanol and water, respectively (the optimization of incubation time is shown in Fig. A1). Then, the as-prepared fabric was dried in air.
2.3. Physicochemical characterizations The microscopic morphology variation of fabric surface before and after modification was collected by scanning electron microscopy (SEM, Hitachi S-4500). The surface chemistry evolution was analyzed according to attenuated total reflectance-Fourier transform infrared spectroscopy spectra (ATR-FTIR, Spectrum One instrument (PerkinElmer, USA)) and X-ray photoelectron spectroscopy data (XPS, gathered from Shimadzu AXIS Ultra DLD spectrometer with Al-Kα X-ray source). The detailed surface wettability was obtained from the static water contact angles (WCA) using an SL 200KB measuring system.
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2.4. Unidirectional transportation and water collection assays To demonstrate the Janus property of the as-prepared fabric, the liquid unidirectional transportation behaviors were investigated. The detailed measurement methods refer to our previous work [14]. As for water collection, a laboratory-made device was fabricated by wrapping as-prepared JM at the end of one tube with hydrophobic layer towards outside. Then the device was placed under oil surface to conduct the collection assay of water droplet (dyed by methyl blue). Another Janus fabric-sandwiched device was also used to gather water from water-in-oil emulsion. The adopted water-in-oil emulsion was prepared via stirring the mixture of 980 mL of petroleum ether, 20 mL of water, and 120 mg of Span-80 for 6 h. The hydrophilic and hydrophobic layers of JMs were wetted by water and the corresponding oil, respectively. The hydrophobic layer was tended towards emulsion feed during separation.
The permeation flux was obtained by Equation 1: (1) Where P, V, A, and t, denotes to the permeation flux (L m-2 h-1), filtrate volume (L), test area (m2), and test duration time (h), respectively. The initial transmembrane pressure is 10 cm height of emulsion column.
Rejection was deduced by Equation 2: (
)
(2)
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Where R denotes to oil rejection ratios, Cp and Cf to oil concentration (measured using a UV-vis Cintra20-GBC machine, oil dyed by Oil red) in permeation filtrate and feed, respectively.
3. Results and discussion 3.1. Fabrication and physicochemical characterization of JM Scheme 1 showed the interface-confined surface engineering strategy to fabricate JM. First, isopycnic oil phase (dichloroethane containing C18-NH2) and water were successively poured into one beaker. The adopted oil/water mixture exhibits the obvious immiscible two phases owing to the typical difference in their density and compatibility (Fig. A2). Then, the hydrophilic PDA-modified fabric membrane was transferred to the beaker and steadily landed at the oil/water interface without further sinking into the oil phase. It was ascribed to the water infused inside the fabric channels blocked the entrance of the underlying oil, preventing the occurrence of continuous sinking. At the oil/water interface, the terminated amino moiety from C18-NH2 molecule tends to orientate toward water phase (alkyl chains orientate to oil phase) owing to its amphiphilic property (the large figure see Fig. A3). Meanwhile, the amphiphilic octadecylamine (C18-NH2) molecules tend to distribute at the oil-water interface to reduce their own energy [39]. This feature provides sufficient opportunity for C18-NH2 molecules to access and graft at the bottom region of PDA-decorated fabric via the Michael addition/Schiff base reactions [40]. It led to local wettability transformation from pristine hydrophilicity to hydrophobicity at the lowermost fabric surface, while the remaining part of the resulting
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fabric membrane maintaining hydrophilic. The strategy can realize the integration of a thin hydrophobic layer on the hydrophilic substrate, resulting in water-unidirectional JM [14].
Scheme 1. The preparation schematic of Janus fabric membrane at the oil/water interface using liquid/liquid interface-confined surface engineering strategy and possible mechanism.
After incubation at the oil/water interface, the Janus fabric membrane was readily constructed. To demonstrate the difference of both sides on resultant fabric membrane, the detailed surface chemistry and wettability were measured. As for surface chemistry, the corresponding FTIR spectra of water side (the side orientated to water during modification) and oil side (another side orientated to oil during modification) located on as-prepared fabric were shown in Fig. 1. The spectrum of water side exhibited the typical peaks of aromatic C=C (stretching vibration, 1615 cm−1) and N−H bonds (shearing vibration, 1513 cm−1), also the broad peak of −OH/−NH2 moieties (stretching vibration, 3600-3200 cm−1), which reflects the feature of PDA coating [41]. Meanwhile, the new peaks arise at the oil side of the as-prepared fabric. Components at 2920 and 2854 cm−1
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were ascribed to the symmetric and asymmetric C-H stretching vibrations originated from methyl or methylene moieties. The components at 1459 and 717 cm−1 were attributed to CH2 deformation vibration and –(CH2)n− moiety bending vibration, respectively. These characteristics verified C18-NH2 molecules successfully grafted on the oil side of the fabric. The aforementioned analysis demonstrated the asymmetric chemistry of both sides of the as-prepared fabric. As for surface wettability, the corresponding water droplet shapes of both sides were displayed in the inset images. The WCA of oil side of the as-prepared fabric is ca. 119o (Fig. 1a), revealing the hydrophobicity characteristic. Meanwhile, WCA of another side (water side) is ca. 0o (Fig. 1b), demonstrating the hydrophilic feature. The difference of WCA demonstrated the asymmetric wettability of the as-obtained fabric. The results revealed that one side of fabric successfully transformed from hydrophilic to hydrophobic after oil/water interfaceconfined surface modification while preventing another side being modified by hydrophobic substances (maintaining hydrophilic).
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Fig. 1. FTIR spectra of both sides of the Janus fabric membrane, water side (the side of fabric membrane towards water when preparing it) and oil side of the fabric membrane. The inset images are corresponding water droplet shapes on the (a) oil side and (b) water side.
Furthermore, XPS analysis on both sides of the as-prepared Janus fabric membrane also verified the asymmetric surface chemistry (Fig. 2). The grafting of C18-NH2 molecule resulted in the improvement in C at.% on the oil side of the fabric compared to that of water side. The C 1s fitting analysis of water side and oil side of the as-prepared fabric indicated the realization of PDA coating and C18-NH2 molecule grafting; the C-O and CN peaks appeared in PDA coating and a new C=N peak (it is ascribed to the reaction product of PDA and amino-containing species via Schiff base reactions) appeared in subsequent C18-NH2 grafting [39, 42].
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Fig. 2. XPS analysis on both sides of the as-prepared Janus fabric membrane. (a) Widescan spectra and (b) surface element ratios, C 1s fitting analysis of (c) water side (the side orientated to water during modification) and (d) oil side (another side) of the as-prepared fabric.
In addition, surface morphology evolution was monitored by “eye-guided” SEM. As shown in Fig. 3, the pristine fabric displayed a smooth surface. After the interfaceconfined surface engineering, the fiber surface of the as-prepared fabric on the water side was obviously wrapped by a coating layer. It exhibited the typical microscopic “mountain-and-valley” morphology originated from the as-deposited PDA film [32], which is similar to that of PDA-treated fabric in the previous stage (more contrast data
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see Fig. A4). Unlike the fabric surface on the water side, the surface on another side orientated to the oil phase was relatively smooth. It was ascribed to the successful grafting of C18-NH2 molecules, which filled the “valley” to some extent, outside the predeposited PDA film. Meanwhile, the difference in microscopic morphology between opposite sides also confirmed water both from the top layer and the fabric channels can be used as a protective media to prevent hydrophobic reactions occurring on the water side in spite of the on-going hydrophobic modification on the oil side. The asymmetric surface chemistry and microstructure on the fabric generate the cross-sectional wetting chemical potentials, which could power the unidirectional liquid transportation.
Fig. 3. SEM images of surfaces on (a, d) the pristine fabric and both sides of the asprepared Janus fabric membrane, corresponding (b, e) water side, and (c, f) oil side
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located on fabric when preparing it. The images on the right side are corresponding magnified versions.
3.2. Water unidirectional transportation behaviours Water-infused fabric settled at the oil/water interface implies a large proportion of the fabric (settling in the upper water) will be protected and sealed by water. The amphiphilic C18-NH2 located at the oil/water interface ensured the oil side of fabric to realize the regional hydrophobic transformation while the water side maintaining hydrophilic. Thus, the as-prepared fabric would possess relatively a thin hydrophobic layer and a thick hydrophilic layer, demonstrating the feature of water unidirectional JMs [12, 31]. Water transportation behaviors across the fabric were observed at the oil/water interface. As shown in Fig. 4a, b, the fabric was placed at the oil/water interface steadily. The asprepared Janus fabric exhibited an obvious unidirectional transportation behavior to water droplets on the membrane surface with opposite placement ways. When a water droplet (blue color) was dropped on the hydrophobic side (Video 1), it exhibited and maintained a spherical state in the drop position and then penetrated the membrane, presenting a jellyfish shape. On the contrary, a water droplet was blocked when placing fabric upside down (Video 2). The different behaviors also appeared when supplying water droplets under the membrane (Fig. 5). In detail, water droplet penetrated fabric when the hydrophobic side is upward (Video 3) (the arrow and the inset magnified photo all points to the penetrated dyed water). The droplet was blocked when putting the fabric upside down (Video 4). All the aforementioned phenomena about water transportation behaviors exhibited the unidirectional transportation features of Janus fabric towards water. The
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underlying mechanism could be interpreted on the basis of the radius of curvature of membrane surface-contacted droplets and the hydrophilic/hydrophobic synergism. The wettability of the membrane surface (the water droplet contacting) determines the initial state of a water droplet on the membrane surface; spherical or spreading state. The spherical state of a water droplet on the hydrophobic side corresponds to a large curvature to generate a big Laplace pressure; spreading state on another side (hydrophilic side) corresponds to a negligible one. A large Laplace pressure derived from the spherical water droplet on the hydrophobic side and the synergistic hydrophilic/hydrophobic interactions from integrated asymmetric wettability powers the vertical penetration of water across the membrane (Fig. 4c). On the contrary, the small curvature of the spreading water state on the hydrophilic side cannot offer enough pressure to pass through the cross-section (Fig. 4d). The underlying driving force brought by the asymmetric wettability of the as-prepared JMs enables the great potential towards water collection.
Fig. 4. Water unidirectional transportation behaviors. The schematic and corresponding snapshots of water transportation across the as-prepared Janus fabric membrane at the oil/water interface; a water droplet (dyed by methylene blue) (a) penetrated the fabric when the hydrophobic side was upward and (b) was blocked when placing the fabric 15
upside down. The oil is petroleum ether (dyed by oil red O). The schematic illustration of difference in driving force (Laplace pressure, ΔP) generated by (c, d) Janus fabric membrane with different configuration modes on the oil/water interface (corresponds to the regions in the dashed box in Fig a, b).
Fig. 5. The schematic and snapshots of water droplets (supplied from below) on surfaces of Janus fabric at the oil/water interface. (a) A water droplet penetrated the fabric when the hydrophilic side was upward (the arrow points to the penetrated water, the inset is a locally magnified photo) and (b) was blocked when placing the fabric upside down. The oil is 1,2-dichloroethane (dyed by Oil red).
3.3. Water collection using laboratory-made collectors configured by Janus fabric membrane Water collection assays were unfolded using the as-prepared Janus fabric in virtue of its water-unidirectional transportation feature. As the demonstration, two collection devices were used to collect water. Therein, one collector was fabricated via capping a tube with Janus fabric and sealing it with an iron hoop. The fabric cap was fixed with the hydrophobic layer outside (Fig. 6a). The collector was transferred to a cup containing oil phase with some scattered water droplets on the bottom (Fig. 6b, Video 5). When the 16
fabric cap approached water droplets (dyed blue), it quickly sucked the water droplets around. All water droplets were sucked in along with moving the tube around, indicating as-prepared Janus fabric can realize water droplet collection below oil. It was attributed to the asymmetric wettability of fabric cap that only enables water to pass through it but blocks the oil phase from entering into the channels in the fabric. In detail, the external hydrophobic layer of the as-prepared Janus fabric enables the contacting water droplets to generate large Laplace pressure and to penetrate through the fabric via the synergistic incentive from the generated Laplace pressure and capillary action derived from the internal hydrophilic side. Meanwhile, the oil well wetted the external hydrophobic layer and the inner thicker hydrophilic layer effectively blocked the penetration of oil phase. The asymmetrical wettability of Janus fabric membrane endowed it with the selective transport capacity and helped it to realize the water collection under the oil.
Fig. 6. (a) The schematic and (b) snapshots of water collection (dyed by methylene blue) under the oil using a laboratory-made Janus membrane collector.
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In practice, sometimes we want to directly collect the water-soluble pharmaceutical ingredients from water-in-oil emulsions during the pharmaceutical production. Thus, researching how to efficiently collect water phase from water-in-oil emulsion is, in a sense, energy-saving and meaningful. In previous reports, the oil-unidirectional JM have successfully realized oil collection (the phase with a relatively low volume fraction in the emulsion) from oil-in-water emulsion, which is derived from the unilateral transport of oil phase [30,32,36]. Thus, we wonder whether we can extend this feature to the waterunidirectional transportation system. Theoretically, the water-unidirectional JM could also realize the separation of water phase from the water-in-oil emulsion in virtue of its “water-diode” character. Therefore, as proof of concept, the water-in-oil emulsion was prepared as the feed to conduct water collection using the Janus fabric. As shown in Fig. 7a, the Janus fabric-sandwiched device is used to conduct the water collection from water-in-oil emulsion. The emulsion feed exhibited an obviously pink milky trait, the filtrate is colorless and transparent (Fig. 7b). We further verified the filtrate is water when distinguishing it by oil-soluble dye Oil red. The permeation flux reached ca. 20.8 L m-2 h-1 with an oil rejection of 99.6 % during the gravity-driven emulsion separation. Furthermore, the optical microscopic views were determined (Fig. 7c,d); the feed exhibited a water droplet-scattered bi-phase trait, the filtrate was only one phase. Therefore, the water collection was realized from the water-in-oil emulsion feed using the as-prepared Janus fabric. Conventionally, the super-hydrophobic porous membranes are also applied to separate water-in-oil emulsions [43-54]. But the underlying mechanism is totally different; the super-hydrophobic membrane allows oil phase to pass through and defends water droplets outside owing to its super-hydrophobic and underwater super-
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oleophilic feature, whereas, the as-prepared JMs validly recover the water (the phase with a relatively low volume fraction in the emulsion) and defend oil effectively owing to its "water-diode" transport feature (Fig. A5).
Fig. 7. The schematic of particular water collection from water-in-oil emulsion using the as-prepared Janus fabric-sandwiched device and (b) photos of emulsion feed and filtrate, the optical images of (c) feed and (d) filtrate.
4. CONCLUSIONS A liquid/liquid interface-confined surface engineering strategy was developed to fabricate JM with the aid of mussel-inspired chemistry. The as-prepared fabric exhibited asymmetric chemistry, wettability, and surface morphology, indicating the successful fabrication of JM by our new strategy. Water-infused fabric membrane settled at the oil/water interface implies that the majority of fabric membrane on the top layer was protected and sealed by water. The immiscible oil below, containing amphiphilic C18-NH2,
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guarantees the realization of regional hydrophobicity transformation on the oil side of fabric membrane while the hydrophilicity of the water side maintaining unaffected. The as-prepared Janus fabric membrane exhibited the water unidirectional transportation feature across the membrane at the oil/water interface when supplying water droplets above or under the as-prepared JMs. Furthermore, two kinds of JM-assembled devices were successfully demonstrated for water collection both under oil and from the water-inoil emulsion, respectively. The interesting results indicate the enormous potential of liquid/liquid interface-confined surface engineering strategy for JM fabrication and liquid manipulation towards diverse advanced applications.
Acknowledgements The authors thank the financial support from National Natural Science Foundation of China (21878062, 21676063) and Open Project of State Key Laboratory of Urban Water Resource and Environment (Harbin Institute Technology) (No. HC201706).
Appendix A. Supplementary material The digital photos of the PDA-treated cotton fabric positioned at the immiscible oil/water interface, schematic of possible distribution feature of C18-NH2 (ODA) molecule in the oil/water mixture, SEM images of PDA-treated fabric surface and the water side of the as-prepared fabric.
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[33] X. Tian, H. Jin, J. Sainio, R.H.A. Ras, O. Ikkala, Droplet and fluid gating by biomimetic Janus membranes. Adv. Funct. Mater. 24 (2014) 6023-6028. [34] S. B. Darling, Perspective: Interfacial Materials at the Interface of Energy and Water. J. Appl. Phys. 124 (2018) 030901. [35] H. C. Yang, R. Waldman, Z. Chen, S. Darling, Atomic layer deposition for membrane interface engineering. Nanoscale 10 (2018) 20505–20513. [36] X. Yang, Z. Wang, L. Shao, Construction of oil-unidirectional membrane for integrated oil collection with lossless transportation and oil-in-water emulsion purification. J. Membr. Sci. 549 (2018) 67-74. [37] H.C. Yang, W. Zhong, J. Hou, V. Chen, Z.K. Xu, Janus hollow fiber membrane with a mussel-inspired coating on the lumen surface for direct contact membrane distillation. J. Membr. Sci. 523 (2017) 1-7. [38] H.C. Yang, M.B. Wu, J. Hou, S. Darling, Z.K. Xu, Nanofilms directly formed on macro-porous substrates for molecular and ionic sieving. J. Mater. Chem. A 6 (2018) 2908-2913. [39] H. C. Yang, K. J. Liao, H. Huang, Q. Y. Wu, L. S. Wan, Z. K. Xu, Mussel-inspired modification of a polymer membrane for ultra-high water permeability and oil-in-water emulsion separation. J. Mater. Chem. A, 2 (2014), 10225-10230. [40] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings. Science 318 (2007) 426-430. [41] X. Yang, H. Du, S. Li, Z. Wang, L. Shao, Codepositing mussel-inspired nanohybrids onto one-dimensional fibers under “green” conditions for significantly enhanced surface/interfacial properties. ACS Sustainable Chem. Eng. 6 (2018) 4412-4420.
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List of videos
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Video 1. A water droplet (blue color) dropped on hydrophobic side of as-prepared JM (the hydrophobic layer toward oil) at oil/water interface. Video 2. A water droplet (blue color) dropped on hydrophilic side of as-prepared JM (the hydrophobic layer toward water) at oil/water interface. Video 3. A water droplet (blue color, supplied from below) dropped on hydrophobic side of as-prepared JM (the hydrophobic layer toward oil) at oil/water interface. Video 4. A water droplet (blue color, supplied from below) dropped on hydrophilic side of as-prepared JM (the hydrophobic layer toward water) at oil/water interface. Video 5. Water collection (dyed by methylene blue) under the oil using a laboratorymade Janus collector.
HIGHLIGHTS
Interface-confined surface engineering realized asymmetric surface modification.
Asymmetric surface modification can construct brand-new Janus membrane (JM).
The as-prepared JM possesses water-unidirectional transportation ability.
The JM devices successfully collect water under oil or from water-in-oil emulsion.
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PII: DOI: Reference:
S0376-7388(18)32764-9 https://doi.org/10.1016/j.memsci.2019.01.014 MEMSCI16784
To appear in: Journal of Membrane Science Received date: 4 October 2018 Revised date: 7 January 2019 Accepted date: 11 January 2019 Cite this article as: Xiaobin Yang, Linlin Yan, Feitian Ran, Avishek Pal, Jun Long and Lu Shao, Interface-confined surface engineering constructing waterunidirectional Janus membrane, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.01.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interface-confined surface engineering constructing water-unidirectional Janus membrane
Xiaobin Yang1, Linlin Yan1, Feitian Ran, Avishek, Pal, Jun Long and Lu Shao*
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, PR China.
*Corresponding Author: Tel. +86-451-86413711 ； fax:+86-451-86418270; Email address: [email protected] (L. Shao).
Abstract Janus membranes (JMs) have attracted increasing attention in the fields of liquid manipulation owing to its interesting liquid-unidirectional transportation feature. Nonetheless, most of the currently available methods to fabricate JMs suffer from specific or complicated operations, impeding practical applications of JMs. Herein, a facile liquid/liquid (immiscible oil/water) interface-confined surface engineering strategy was first applied on hydrophilic cotton fabric membrane to construct JM with the aid of mussel-inspired chemistry. The water-infused fabric channels and the unique distribution of amphiphilic C18-NH2 at the oil/water interface enable the unilateral hydrophobicity transformation on fabric. As a result, the as-prepared membranes exhibited asymmetric
1
These authors contributed equally.
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chemistry, wettability, and surface morphology. The evaluation of water droplet behaviors across the membrane at the oil/water interface verified the water unidirectional transport feature of our JM. In addition, two collectors made by our JMs successfully demonstrated the excellent ability of water collection from both oil/water mixture and water-in-oil emulsion. The newly developed liquid/liquid interface-confined surface engineering strategy provides enormous potential for Janus membrane construction to manipulate liquid transportation towards smart applications. Graphical Abstract
Keywords: Janus membrane, asymmetric wettability, unidirectional transportation, surface engineering, water collection
1. Introduction Membrane technology plays an important role in practical applications, such as water treatment and pharmaceutical separations [1-6]. Most recently, there is an increasing demand for membranes with the ability for intelligent response to the environment or precise control of liquid transportation [7-11], which is difficult to be achieved by
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conventional fabrication techniques. Janus membrane (JM) as a new conceptual separation membrane has opposing properties on their two faces, such as hydrophilic/hydrophobic wettability, positive/negative charge, etc [12, 13]. In this study, JMs we focused on are membranes with asymmetric wettability and liquid-unidirectional transportation property owing to the integrated hydrophilic/hydrophobic structure [1215]. Due to the unique structure and properties, JM has gained enormous attention in the fields of liquid manipulation, fog collection, oil/water separation, etc [14-24]. However, most of the currently available methods to fabricate JMs are involved with specific instruments or fluorinated toxic chemicals and complicated operations which retard the practical usages of JMs [12]. Therefore, it is crucial to design facile strategies to fabricate JMs with excellent unidirectional transportation properties.
The conventional JM preparation strategy is based on the elaborate cementation of two membranes with opposite wettability or asymmetric surface modification. Jiang et al. successively electro-spinned the hydrophobic polyurethane and hydrophilic poly (vinyl alcohol) fibers into an integrated JM with the asymmetric wettability [25]. Lin et al. also fabricated oil-unidirectional JMs via a similar sequent electrospinning method [26]. These approaches could precisely tune the wetting gradient, but specific instruments are required. To fabricate JMs from one piece of substrate, asymmetric surface modification is generally required. Xin et al. and Lin et al. prepared a superhydrophobic layer containing photo-catalytic materials on substrates, and generated gradient wettability across the membrane via unilateral ultra-violet irradiation [27, 28]. Darling’s group unilaterally deposited hydrophilic metallic oxide on a hydrophobic polymer membrane
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via atomic layer deposition technology to obtain JM [29]. Liu et al. asymmetrically grafted two polymers with opposite wettability to obtain JM with oil-unidirectional transportation property [30]. Furthermore, one synthetic block co-polymer or incorporating moiety with emulsification were proposed to realize the in situ generation of similar JMs [31, 32]. Tian et al. prepared JMs via unilateral modification; perfluorooctyl-trichlorosilane vapor fumigation right below a hydrophilic membrane or plasma treatment on one side of the hydrophobic membrane [33]. These approaches are effective to construct asymmetric wettability but involved with some specific instruments and strict synthesis conditions. Generally, the interface engineering plays a prominent role in fabricating functional membranes towards efficient water-energy nexus optimization [34, 35]. Recently, our group [36] and Yang et al. [37, 38] fabricated JMs via floating coating; the hydrophobic membrane can float on the aqueous solution and realize its bottom layer to be modified by a thin hydrophilic layer, which could be considered as a kind of surface engineering at an air/liquid interface. However, such floating coating strategy cannot be extended to hydrophilic membranes because it will sink into the aqueous solution or other solvents. Thus, a new toolbox to construct JM on hydrophilic substrates needs to be urgently explored.
Interestingly, we recently found that the oil/water interface is a good space to realize specific surface engineering on opposite sides of the material settled there by means of its stratification feature of the immiscible oil and water. Herein, we designed a brand-new strategy, liquid/liquid interface-confined surface engineering, to realize the waterunidirectional JM. As a proof of concept, the hydrophilic cotton fabric had sunk and
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landed at the oil/water interface (the upper layer is water phase). Therein, the waterinfused porous substrate could validly prevent oil phase entering inside the bulk; the amphiphilic molecules waiting for hydrophobic surface engineering can be easily dissolved in the oil phase and grafted on the fabric surface toward oil side. The wettability, chemistry, and microscopic morphology on both surfaces of the resultant fabrics were determined by a series of physicochemical characterization. Measurements on the typical liquid transportation behaviours were conducted at the oil/water interface by squeezing water droplets (above or below) onto the resultant fabric. In addition, the water collection assays were demonstrated when treating both oil/water mixture and water-in-oil emulsion using two kinds of JM-assembled devices. The developed interface-confined surface engineering strategy exhibits great potential towards the fabrication of various JMs and liquid manipulation for advanced smart usages. Moreover, it may have some limitations; it is probably difficult to precisely tune the position of the Janus property barrier within the membrane using this approach.
2. Experimental 2.1. Materials Cotton fabric was purchased from a local store. Dopamine hydrochloride (DA) was obtained from Sigma-Aldrich. Octadecylamine (C18-NH2) and tris (hydroxymethyl) aminomethane (Tris) was supplied by Aladdin (China). Hydrochloric acid (HCl), ethanol, petroleum ether, 1,2-dichloroethane, and Oil red O were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (China). All the chemicals were used as received. Ultra-pure water was laboratory-made.
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2.2. Preparation of water-unidirectional JM The received cotton fabric was purified using ethanol and ultra-pure water three times, respectively. Then the fabric was incubated in a fresh dopamine solution (2 mg mL-1, Tris-HCl buffer, pH=8.5). The fabric after polydopamine (PDA) deposition was rinsed vigorously by water several times. Then, it was transferred into a beaker containing upper water phase and bottom oil phase (dichloroethane, containing 1 mg mL-1 C18-NH2) with same volumes. The fabric automatically landed and stayed at the water/oil interface during its sinking process in the beaker. After 10 h of incubation, the fabric was rinsed with ethanol and water, respectively (the optimization of incubation time is shown in Fig. A1). Then, the as-prepared fabric was dried in air.
2.3. Physicochemical characterizations The microscopic morphology variation of fabric surface before and after modification was collected by scanning electron microscopy (SEM, Hitachi S-4500). The surface chemistry evolution was analyzed according to attenuated total reflectance-Fourier transform infrared spectroscopy spectra (ATR-FTIR, Spectrum One instrument (PerkinElmer, USA)) and X-ray photoelectron spectroscopy data (XPS, gathered from Shimadzu AXIS Ultra DLD spectrometer with Al-Kα X-ray source). The detailed surface wettability was obtained from the static water contact angles (WCA) using an SL 200KB measuring system.
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2.4. Unidirectional transportation and water collection assays To demonstrate the Janus property of the as-prepared fabric, the liquid unidirectional transportation behaviors were investigated. The detailed measurement methods refer to our previous work [14]. As for water collection, a laboratory-made device was fabricated by wrapping as-prepared JM at the end of one tube with hydrophobic layer towards outside. Then the device was placed under oil surface to conduct the collection assay of water droplet (dyed by methyl blue). Another Janus fabric-sandwiched device was also used to gather water from water-in-oil emulsion. The adopted water-in-oil emulsion was prepared via stirring the mixture of 980 mL of petroleum ether, 20 mL of water, and 120 mg of Span-80 for 6 h. The hydrophilic and hydrophobic layers of JMs were wetted by water and the corresponding oil, respectively. The hydrophobic layer was tended towards emulsion feed during separation.
The permeation flux was obtained by Equation 1: (1) Where P, V, A, and t, denotes to the permeation flux (L m-2 h-1), filtrate volume (L), test area (m2), and test duration time (h), respectively. The initial transmembrane pressure is 10 cm height of emulsion column.
Rejection was deduced by Equation 2: (
)
(2)
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Where R denotes to oil rejection ratios, Cp and Cf to oil concentration (measured using a UV-vis Cintra20-GBC machine, oil dyed by Oil red) in permeation filtrate and feed, respectively.
3. Results and discussion 3.1. Fabrication and physicochemical characterization of JM Scheme 1 showed the interface-confined surface engineering strategy to fabricate JM. First, isopycnic oil phase (dichloroethane containing C18-NH2) and water were successively poured into one beaker. The adopted oil/water mixture exhibits the obvious immiscible two phases owing to the typical difference in their density and compatibility (Fig. A2). Then, the hydrophilic PDA-modified fabric membrane was transferred to the beaker and steadily landed at the oil/water interface without further sinking into the oil phase. It was ascribed to the water infused inside the fabric channels blocked the entrance of the underlying oil, preventing the occurrence of continuous sinking. At the oil/water interface, the terminated amino moiety from C18-NH2 molecule tends to orientate toward water phase (alkyl chains orientate to oil phase) owing to its amphiphilic property (the large figure see Fig. A3). Meanwhile, the amphiphilic octadecylamine (C18-NH2) molecules tend to distribute at the oil-water interface to reduce their own energy [39]. This feature provides sufficient opportunity for C18-NH2 molecules to access and graft at the bottom region of PDA-decorated fabric via the Michael addition/Schiff base reactions [40]. It led to local wettability transformation from pristine hydrophilicity to hydrophobicity at the lowermost fabric surface, while the remaining part of the resulting
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fabric membrane maintaining hydrophilic. The strategy can realize the integration of a thin hydrophobic layer on the hydrophilic substrate, resulting in water-unidirectional JM [14].
Scheme 1. The preparation schematic of Janus fabric membrane at the oil/water interface using liquid/liquid interface-confined surface engineering strategy and possible mechanism.
After incubation at the oil/water interface, the Janus fabric membrane was readily constructed. To demonstrate the difference of both sides on resultant fabric membrane, the detailed surface chemistry and wettability were measured. As for surface chemistry, the corresponding FTIR spectra of water side (the side orientated to water during modification) and oil side (another side orientated to oil during modification) located on as-prepared fabric were shown in Fig. 1. The spectrum of water side exhibited the typical peaks of aromatic C=C (stretching vibration, 1615 cm−1) and N−H bonds (shearing vibration, 1513 cm−1), also the broad peak of −OH/−NH2 moieties (stretching vibration, 3600-3200 cm−1), which reflects the feature of PDA coating [41]. Meanwhile, the new peaks arise at the oil side of the as-prepared fabric. Components at 2920 and 2854 cm−1
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were ascribed to the symmetric and asymmetric C-H stretching vibrations originated from methyl or methylene moieties. The components at 1459 and 717 cm−1 were attributed to CH2 deformation vibration and –(CH2)n− moiety bending vibration, respectively. These characteristics verified C18-NH2 molecules successfully grafted on the oil side of the fabric. The aforementioned analysis demonstrated the asymmetric chemistry of both sides of the as-prepared fabric. As for surface wettability, the corresponding water droplet shapes of both sides were displayed in the inset images. The WCA of oil side of the as-prepared fabric is ca. 119o (Fig. 1a), revealing the hydrophobicity characteristic. Meanwhile, WCA of another side (water side) is ca. 0o (Fig. 1b), demonstrating the hydrophilic feature. The difference of WCA demonstrated the asymmetric wettability of the as-obtained fabric. The results revealed that one side of fabric successfully transformed from hydrophilic to hydrophobic after oil/water interfaceconfined surface modification while preventing another side being modified by hydrophobic substances (maintaining hydrophilic).
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Fig. 1. FTIR spectra of both sides of the Janus fabric membrane, water side (the side of fabric membrane towards water when preparing it) and oil side of the fabric membrane. The inset images are corresponding water droplet shapes on the (a) oil side and (b) water side.
Furthermore, XPS analysis on both sides of the as-prepared Janus fabric membrane also verified the asymmetric surface chemistry (Fig. 2). The grafting of C18-NH2 molecule resulted in the improvement in C at.% on the oil side of the fabric compared to that of water side. The C 1s fitting analysis of water side and oil side of the as-prepared fabric indicated the realization of PDA coating and C18-NH2 molecule grafting; the C-O and CN peaks appeared in PDA coating and a new C=N peak (it is ascribed to the reaction product of PDA and amino-containing species via Schiff base reactions) appeared in subsequent C18-NH2 grafting [39, 42].
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Fig. 2. XPS analysis on both sides of the as-prepared Janus fabric membrane. (a) Widescan spectra and (b) surface element ratios, C 1s fitting analysis of (c) water side (the side orientated to water during modification) and (d) oil side (another side) of the as-prepared fabric.
In addition, surface morphology evolution was monitored by “eye-guided” SEM. As shown in Fig. 3, the pristine fabric displayed a smooth surface. After the interfaceconfined surface engineering, the fiber surface of the as-prepared fabric on the water side was obviously wrapped by a coating layer. It exhibited the typical microscopic “mountain-and-valley” morphology originated from the as-deposited PDA film [32], which is similar to that of PDA-treated fabric in the previous stage (more contrast data
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see Fig. A4). Unlike the fabric surface on the water side, the surface on another side orientated to the oil phase was relatively smooth. It was ascribed to the successful grafting of C18-NH2 molecules, which filled the “valley” to some extent, outside the predeposited PDA film. Meanwhile, the difference in microscopic morphology between opposite sides also confirmed water both from the top layer and the fabric channels can be used as a protective media to prevent hydrophobic reactions occurring on the water side in spite of the on-going hydrophobic modification on the oil side. The asymmetric surface chemistry and microstructure on the fabric generate the cross-sectional wetting chemical potentials, which could power the unidirectional liquid transportation.
Fig. 3. SEM images of surfaces on (a, d) the pristine fabric and both sides of the asprepared Janus fabric membrane, corresponding (b, e) water side, and (c, f) oil side
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located on fabric when preparing it. The images on the right side are corresponding magnified versions.
3.2. Water unidirectional transportation behaviours Water-infused fabric settled at the oil/water interface implies a large proportion of the fabric (settling in the upper water) will be protected and sealed by water. The amphiphilic C18-NH2 located at the oil/water interface ensured the oil side of fabric to realize the regional hydrophobic transformation while the water side maintaining hydrophilic. Thus, the as-prepared fabric would possess relatively a thin hydrophobic layer and a thick hydrophilic layer, demonstrating the feature of water unidirectional JMs [12, 31]. Water transportation behaviors across the fabric were observed at the oil/water interface. As shown in Fig. 4a, b, the fabric was placed at the oil/water interface steadily. The asprepared Janus fabric exhibited an obvious unidirectional transportation behavior to water droplets on the membrane surface with opposite placement ways. When a water droplet (blue color) was dropped on the hydrophobic side (Video 1), it exhibited and maintained a spherical state in the drop position and then penetrated the membrane, presenting a jellyfish shape. On the contrary, a water droplet was blocked when placing fabric upside down (Video 2). The different behaviors also appeared when supplying water droplets under the membrane (Fig. 5). In detail, water droplet penetrated fabric when the hydrophobic side is upward (Video 3) (the arrow and the inset magnified photo all points to the penetrated dyed water). The droplet was blocked when putting the fabric upside down (Video 4). All the aforementioned phenomena about water transportation behaviors exhibited the unidirectional transportation features of Janus fabric towards water. The
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underlying mechanism could be interpreted on the basis of the radius of curvature of membrane surface-contacted droplets and the hydrophilic/hydrophobic synergism. The wettability of the membrane surface (the water droplet contacting) determines the initial state of a water droplet on the membrane surface; spherical or spreading state. The spherical state of a water droplet on the hydrophobic side corresponds to a large curvature to generate a big Laplace pressure; spreading state on another side (hydrophilic side) corresponds to a negligible one. A large Laplace pressure derived from the spherical water droplet on the hydrophobic side and the synergistic hydrophilic/hydrophobic interactions from integrated asymmetric wettability powers the vertical penetration of water across the membrane (Fig. 4c). On the contrary, the small curvature of the spreading water state on the hydrophilic side cannot offer enough pressure to pass through the cross-section (Fig. 4d). The underlying driving force brought by the asymmetric wettability of the as-prepared JMs enables the great potential towards water collection.
Fig. 4. Water unidirectional transportation behaviors. The schematic and corresponding snapshots of water transportation across the as-prepared Janus fabric membrane at the oil/water interface; a water droplet (dyed by methylene blue) (a) penetrated the fabric when the hydrophobic side was upward and (b) was blocked when placing the fabric 15
upside down. The oil is petroleum ether (dyed by oil red O). The schematic illustration of difference in driving force (Laplace pressure, ΔP) generated by (c, d) Janus fabric membrane with different configuration modes on the oil/water interface (corresponds to the regions in the dashed box in Fig a, b).
Fig. 5. The schematic and snapshots of water droplets (supplied from below) on surfaces of Janus fabric at the oil/water interface. (a) A water droplet penetrated the fabric when the hydrophilic side was upward (the arrow points to the penetrated water, the inset is a locally magnified photo) and (b) was blocked when placing the fabric upside down. The oil is 1,2-dichloroethane (dyed by Oil red).
3.3. Water collection using laboratory-made collectors configured by Janus fabric membrane Water collection assays were unfolded using the as-prepared Janus fabric in virtue of its water-unidirectional transportation feature. As the demonstration, two collection devices were used to collect water. Therein, one collector was fabricated via capping a tube with Janus fabric and sealing it with an iron hoop. The fabric cap was fixed with the hydrophobic layer outside (Fig. 6a). The collector was transferred to a cup containing oil phase with some scattered water droplets on the bottom (Fig. 6b, Video 5). When the 16
fabric cap approached water droplets (dyed blue), it quickly sucked the water droplets around. All water droplets were sucked in along with moving the tube around, indicating as-prepared Janus fabric can realize water droplet collection below oil. It was attributed to the asymmetric wettability of fabric cap that only enables water to pass through it but blocks the oil phase from entering into the channels in the fabric. In detail, the external hydrophobic layer of the as-prepared Janus fabric enables the contacting water droplets to generate large Laplace pressure and to penetrate through the fabric via the synergistic incentive from the generated Laplace pressure and capillary action derived from the internal hydrophilic side. Meanwhile, the oil well wetted the external hydrophobic layer and the inner thicker hydrophilic layer effectively blocked the penetration of oil phase. The asymmetrical wettability of Janus fabric membrane endowed it with the selective transport capacity and helped it to realize the water collection under the oil.
Fig. 6. (a) The schematic and (b) snapshots of water collection (dyed by methylene blue) under the oil using a laboratory-made Janus membrane collector.
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In practice, sometimes we want to directly collect the water-soluble pharmaceutical ingredients from water-in-oil emulsions during the pharmaceutical production. Thus, researching how to efficiently collect water phase from water-in-oil emulsion is, in a sense, energy-saving and meaningful. In previous reports, the oil-unidirectional JM have successfully realized oil collection (the phase with a relatively low volume fraction in the emulsion) from oil-in-water emulsion, which is derived from the unilateral transport of oil phase [30,32,36]. Thus, we wonder whether we can extend this feature to the waterunidirectional transportation system. Theoretically, the water-unidirectional JM could also realize the separation of water phase from the water-in-oil emulsion in virtue of its “water-diode” character. Therefore, as proof of concept, the water-in-oil emulsion was prepared as the feed to conduct water collection using the Janus fabric. As shown in Fig. 7a, the Janus fabric-sandwiched device is used to conduct the water collection from water-in-oil emulsion. The emulsion feed exhibited an obviously pink milky trait, the filtrate is colorless and transparent (Fig. 7b). We further verified the filtrate is water when distinguishing it by oil-soluble dye Oil red. The permeation flux reached ca. 20.8 L m-2 h-1 with an oil rejection of 99.6 % during the gravity-driven emulsion separation. Furthermore, the optical microscopic views were determined (Fig. 7c,d); the feed exhibited a water droplet-scattered bi-phase trait, the filtrate was only one phase. Therefore, the water collection was realized from the water-in-oil emulsion feed using the as-prepared Janus fabric. Conventionally, the super-hydrophobic porous membranes are also applied to separate water-in-oil emulsions [43-54]. But the underlying mechanism is totally different; the super-hydrophobic membrane allows oil phase to pass through and defends water droplets outside owing to its super-hydrophobic and underwater super-
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oleophilic feature, whereas, the as-prepared JMs validly recover the water (the phase with a relatively low volume fraction in the emulsion) and defend oil effectively owing to its "water-diode" transport feature (Fig. A5).
Fig. 7. The schematic of particular water collection from water-in-oil emulsion using the as-prepared Janus fabric-sandwiched device and (b) photos of emulsion feed and filtrate, the optical images of (c) feed and (d) filtrate.
4. CONCLUSIONS A liquid/liquid interface-confined surface engineering strategy was developed to fabricate JM with the aid of mussel-inspired chemistry. The as-prepared fabric exhibited asymmetric chemistry, wettability, and surface morphology, indicating the successful fabrication of JM by our new strategy. Water-infused fabric membrane settled at the oil/water interface implies that the majority of fabric membrane on the top layer was protected and sealed by water. The immiscible oil below, containing amphiphilic C18-NH2,
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guarantees the realization of regional hydrophobicity transformation on the oil side of fabric membrane while the hydrophilicity of the water side maintaining unaffected. The as-prepared Janus fabric membrane exhibited the water unidirectional transportation feature across the membrane at the oil/water interface when supplying water droplets above or under the as-prepared JMs. Furthermore, two kinds of JM-assembled devices were successfully demonstrated for water collection both under oil and from the water-inoil emulsion, respectively. The interesting results indicate the enormous potential of liquid/liquid interface-confined surface engineering strategy for JM fabrication and liquid manipulation towards diverse advanced applications.
Acknowledgements The authors thank the financial support from National Natural Science Foundation of China (21878062, 21676063) and Open Project of State Key Laboratory of Urban Water Resource and Environment (Harbin Institute Technology) (No. HC201706).
Appendix A. Supplementary material The digital photos of the PDA-treated cotton fabric positioned at the immiscible oil/water interface, schematic of possible distribution feature of C18-NH2 (ODA) molecule in the oil/water mixture, SEM images of PDA-treated fabric surface and the water side of the as-prepared fabric.
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List of videos
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Video 1. A water droplet (blue color) dropped on hydrophobic side of as-prepared JM (the hydrophobic layer toward oil) at oil/water interface. Video 2. A water droplet (blue color) dropped on hydrophilic side of as-prepared JM (the hydrophobic layer toward water) at oil/water interface. Video 3. A water droplet (blue color, supplied from below) dropped on hydrophobic side of as-prepared JM (the hydrophobic layer toward oil) at oil/water interface. Video 4. A water droplet (blue color, supplied from below) dropped on hydrophilic side of as-prepared JM (the hydrophobic layer toward water) at oil/water interface. Video 5. Water collection (dyed by methylene blue) under the oil using a laboratorymade Janus collector.
HIGHLIGHTS
Interface-confined surface engineering realized asymmetric surface modification.
Asymmetric surface modification can construct brand-new Janus membrane (JM).
The as-prepared JM possesses water-unidirectional transportation ability.
The JM devices successfully collect water under oil or from water-in-oil emulsion.
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