Development of Omniphobic Desalination Membranes Using a

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Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold Jongho Lee, Chanhee Boo, Won-Hee Ryu, Andre D. Taylor, and Menachem Elimelech ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02419 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Applied Materials & Interfaces

Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold Jongho Lee,† Chanhee Boo,† Won-Hee Ryu,† André D. Taylor† and Menachem Elimelech *,†,‡ †Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286 ‡

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment

(NEWT), Yale University KEYWORDS. Omniphobicity, Electrospinning, Membrane distillation, Desalination, Saline wastewater

ABSTRACT. In this study, we present a facile and scalable approach to fabricate omniphobic nanofiber membranes by constructing multilevel re-entrant structures with low surface energy. We first prepared positively charged nanofiber mats by electrospinning a blend polymersurfactant solution of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and cationic surfactant (benzyltriethylammonium). Negatively charged silica nanoparticles (SiNPs) were grafted on the positively charged electrospun nanofibers via dip coating to achieve multi-level re-entrant structures. Grafted SiNPs were then coated with 1 ACS Paragon Plus Environment


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fluoroalkylsilane to lower the surface energy of the membrane. The fabricated membrane showed excellent omniphobicity as demonstrated by its wetting resistance to various low surface tension liquids, including ethanol with a surface tension of 22.1 mN/m. As a promising application, the prepared omniphobic membrane was tested in direct contact membrane distillation to extract water from highly saline feed solutions containing low surface tension substances, mimicking emerging industrial wastewaters (e.g., from shale gas production). While a control hydrophobic PVDF-HFP nanofiber membrane failed in the desalination/separation process due to low wetting resistance, our fabricated omniphobic membrane exhibited a stable desalination performance for eight-hour operation, successfully demonstrating clean water production from the low surface tension feedwater.

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1. INTRODUCTION Superhydrophobic surfaces have been of great interest in industrial applications, including non-wetting fabrics,1 self-cleaning optical devices,2 energy-efficient marine vehicles,3 and desalination membranes.4 Numerous approaches have been proposed to develop superhydrophobic surfaces, primarily employing low surface energy materials and high surface roughness.5-6 In spite of their excellent water repellency, most of the superhydrophobic surfaces are prone to wetting by organic liquids, including oils, alkanes, and alcohols. When superhydrophobic surfaces are exposed to environments with organic contaminants, their wetting resistance may be compromised, leading to loss of self-cleaning ability, severe fouling, or poor sealing performance by swelling.7 Therefore, omniphobic surfaces that are able to repel both water and organic liquids are highly desired for reliable anti-wetting performance. Like superhydrophobic surfaces, an omniphobic surface requires minimal surface energy. Extremely low surface energy materials, such as fluorinated polyhedral oligomeric silsesquioxanes (~10 mJ/m2),8 have been used for omniphobic surface modification. However, omniphobicity cannot be achieved by simply lowering the surface energy, because wetting by liquids with low surface tension (e.g., hexadecane with surface tension of 27.5 mN/m) is thermodynamically favorable even for such low energy surfaces. Specifically, omniphobic surfaces require a re-entrant structure to develop a local kinetic barrier for transition from the meta-stable Cassie-Baxter state to the fully wetting Wenzel state for low surface tension liquids.7-10 Notably, it has been shown that multilevel re-entrant structures are effective at increasing surface omniphobicity.11-13 Accordingly, omniphobic surfaces featuring multilevel re-entrant structures coated with ultra-low surface energy materials have been developed for oil-repelling textiles,14-15 antifouling solar cells,16 and wastewater desalination.17



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Membranes with surface omniphobicity would be of significant importance in desalination of challenging water sources, especially for high-salinity wastewaters from major industries such as textile, chemical production, and petroleum industries. Reverse osmosis (RO) is a state-of-the-art membrane-based desalination technology widely used for seawater desalination. However, RO is not an ideal option to treat high salinity wastewaters because, in many cases, the salinity or osmotic pressure of such wastewaters is far beyond the limit of RO operation (~70,000 ppm). For instance, the salinity of wastewater from shale gas production can reach up to ~ 360,000 ppm.18 Membrane distillation (MD) can be a promising alternative to desalinate hypersaline wastewaters.18-20 MD is a membrane-based thermal separation process utilizing low grade heat, where transport of water vapor is driven by a vapor pressure gradient across a hydrophobic microporous membrane.21 Membranes with anti-wetting property are critical for successful desalination in MD. However, conventional hydrophobic MD membranes fabricated from fluoropolymers, such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), cannot resist wetting by low surface tension organic contaminants commonly found in industrial wastewaters. Therefore, development of omniphobic membranes is essential to extend the applications of MD for desalination of challenging industrial wastewaters. Moreover, omniphobic MD membranes may find promising applications in emerging energy sectors such as biofuel production.22 Electrospinning is a promising and versatile technique to fabricate omniphobic membranes, because electrospun nanofibers with cylindrical shape feature a re-entrant structure7 and could be further engineered for additional levels of re-entrant structures.23 MD membranes require high porosity for high water flux performance, and pore size in the submicrometer range for high liquid entry pressure.24 Highly porous nanofiber mats with


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adjustable pore size can be readily prepared by electrospinning, using a wide range of polymers.25-26 In addition, the fibrous network offers multiple layers of re-entrant structures, which renders electrospun fiber mats a suitable platform to fabricate MD membrane with high wetting resistance. Previous studies have demonstrated successful fabrication of electrospun fiber mats with surface omniphobicity.7-8, 27-29 While the success of these studies is encouraging, a method that can realize mechanically and chemically robust omniphobic MD membranes with multilevel re-entrant structure by fully utilizing the versatility of electrospinning has not been available. In this study, we present a facile approach to fabricate a robust omniphobic membrane by constructing multilevel re-entrant structures with low surface energy. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is selected as a model membrane material owing to its superior mechanical and chemical properties. We first prepared an electrospun PVDF-HFP nanofiber mat to serve as a scaffold for fabricating the omniphobic membrane. Multi-level re-entrant structures were realized by grafting silica nanoparticles (SiNPs) on the nanofibers, which were then functionalized to attain low surface energy. The fabricated membrane exhibited excellent omniphobicity, repelling various low surface tension liquids and achieving high desalination performance of highly saline waters with low surface tension substances. The combined multilevel re-entrant structure with low surface energy enabled by the SiNPs allows for the use of a wide range of polymers as a membrane material with desired chemical and mechanical properties. 2. MATERIALS AND METHODS

2.1. Materials and Chemicals Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), benzyltriethylammonium chloride (BTEAC), sodium dodecyl sulfate (SDS), acetate buffer solution (pH 4.65), 5 ACS Paragon Plus Environment


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dimethylacetamide (DMAc), and Ludox SM colloidal silica (30 wt% suspension in water) were purchased from Sigma Aldrich. Acetone was purchased from J.T. Baker. (Heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (FDTS) was purchased from Gelest. Deionized (DI) water was obtained from a Milli-Q ultrapure water purification system (Millipore, Billerica, MA).

2.2. Preparation of PVDF-HFP Nanofibrous Membrane by Electrospinning PVDF-HFP (3.0 g) was dissolved in a mixture of 12 mL acetone and 8 mL DMAc. The blend of PVDF-HFP and BTEAC was prepared by adding 150 mg of BTEAC into the previously prepared PVDF-HFP solution, such that 10 wt% of BTEAC to PVDF-HFP in the blend was achieved. For complete mixing, the solutions were stirred on a hot plate (~110 °C) for 24 hours. Approximately 7 mL of solution of PVDF-HFP or PVDF-HFP with BTEAC were loaded into a Luer-Lok Tip syringe (Becton Dickinson and Company, NJ) with a metal 25gauge needle (Iwashita Engineering, Inc.). The syringe was placed horizontally in a syringe pump (KD Scientific, Holliston, MA), and a voltage supply (Gamma High Voltage Research Inc., Ormond Beach, FL) was connected to the needle tip via an alligator clip. The fibers were collected on an aluminum foil set on the rotating drum. The applied voltage, flow rate, and relative humidity employed for PVDF-HFP electrospinning were 17 kV, 0.7 mL/h, and 50%, respectively. Higher applied voltage (26 kV), lower flow rate (0.5 mL/h), and lower relative humidity (30%) were employed for the electrospinning the blend of PVDF-HFP and BTEAC. Identical temperature (~30 °C) and needle-to-collector distance (13 cm) were maintained during the electrospinning of the PVDF-HFP solution and the blend of PVDFHFP and BTEAC. Hereafter, the electrospun nanofiber mat obtained from the blend of PVDF-HFP and BTEAC is designated as PVDF-HFP/BTEAC.

2.3. Membrane Modification to Achieve Surface Omniphobicity 6 ACS Paragon Plus Environment

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Electrospun mats comprising an array of cylindrical nanofibers that feature re-entrant structure were utilized as a substrate. In order to achieve multilevel re-entrant structures, silica nanoparticles (SiNPs) were grafted on the nanofibers of PVDF-HFP/BTEAC via a simple

dip-coating

protocol

(Figure

1).

SiNPs

were

then

coated

with

perfluorodecyltrichlorosilane (FDTS) to lower the membrane surface energy. A 0.005 wt% silica nanoparticle suspension was prepared in an acetate buffer solution with an ionic strength of ~1 mM. The pH of the suspension was adjusted to 4 to promote electrostatic attraction between the negatively charged SiNPs and the positively charged nanofibers. To ensure complete wetting of the electrospun nanofibers by the SiNP suspension, the membrane was prewetted with several droplets of ethanol prior to the dip-coating. After 1 hour of dipcoating, the membrane was thoroughly rinsed by acetate buffer solution and DI water, and then heat-treated on a hot plate at 120 °C for 3 h. The SiNP-grafted membrane (SiNPsPVDF-HFP/BTEAC, hereafter) was placed in a petri dish with desiccants. After dry nitrogen gas was blown for 10 min, 100 µL of FDTS were added in the petri dish, which was left in a vacuum oven at 100 °C for 24 hours. Thermal annealing of the silanized membrane on the hot plate at 120 °C for 3 hours completed the modification process. Figure 1

2.4. Membrane Characterization The morphology of the membranes was characterized by field emission scanning electron microscopy (FESEM, Hitachi SU-70). The effective pore size and distribution were obtained from a custom-built capillary flow porometer using a wet/dry flow method30-31 (see Supporting Information S1 for details). Zeta potentials of membranes were estimated from streaming potentials measured in a rectangular channel comprising two pieces of the membranes as top and bottom surfaces of the channel (EKA, Brookhaven Instruments, 7 ACS Paragon Plus Environment


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Holtsville, NY) as described elsewhere.32 The streaming potentials were measured with 1 mM KCl and 0.1 mM KHCO3 as background electrolytes, and the pH was adjusted by adding HCl or KOH solution. Zeta potentials of SiNPs were calculated from electrophoretic mobility measurements at different pH and 1 mM NaCl (ZetaPals, Brookhaven Instruments, Holtsville, NY) by using the tabulated data of Ottewill and Shaw.33 The thermal properties and phase characteristics of the membrane samples were analyzed using thermogravimetric analysis (TGA, TA Instruments Q50, New Castle, DE) in air atmosphere. The weight change of the membrane samples was measured with increasing temperature up to 900 °C to identify the thermal decomposition behavior of polymers and specific weight portion of each component in the membrane samples. For visualization of wetting resistance of the membranes, food dyes were added to water, 3 mM sodium dodecylsulfate, and ethanol, while Oil Red O, an oil-soluble dye, was added to decane. Contact angles were measured by a goniometer (OneAttension, Biolin Scientific, NJ) with a digital camera recording the shape of each testing liquid droplet without a dye. Static contact angle was determined 10 seconds after dispensing 1.5 µL of the testing liquid on the membrane surface.

2.5. Membrane Distillation of Low Surface Tension Feed Solutions Comparative desalination performance experiments utilizing direct contact membrane distillation (MD) were conducted with the control PVDF-HFP and modified PVDF-HFP nanofiber membranes. The membranes were inserted into a custom-built transparent acrylic cell with channel dimensions of 77-mm long, 26-mm wide, and 3-mm deep. The effective membrane area exposed to feed and permeate (distillate) streams was 20.0 cm2. Hot feed and cold permeate streams were circulated using two variable gear pumps (Cole-Parmer, Vernon Hills, IL) and the temperatures were kept constant at 60 ºC (feed) and 20 ºC (permeate) using 8 ACS Paragon Plus Environment

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two recirculating water baths (Polystat Standard, Cole-Parmer, Vernon Hills, IL). The feed and permeate were circulated at cross-flow velocities of 12.8 and 8.5 cm/s, respectively, in a co-current mode. Spacers were inserted in both feed and permeate channels to support the membrane and maintain the membrane geometry in the cell. For the initial two hours of MD runs, 1 M NaCl solution at 60 °C without SDS was used as a feed. SDS was then introduced to the feed every two hours to progressively lower the surface tension of the feed solution. The SDS concentrations of the feed solution after sequential additions were 0.1, 0.2, and 0.3 mM, and the corresponding surface tensions of feed solution were ~41, ~33, and ~31 mN/m, respectively.34 Water vapor flux, Jw, across the membrane was measured by monitoring the increase in permeate mass using a digital balance (CUW 6200H, CAS). NaCl concentration in the permeate was measured using a calibrated conductivity meter (Oakton Instruments, Vernon Hills, IL), from which the salt rejection, R, was calculated. 3. RESULTS AND DISCUSSION

3.1. Membrane Morphology SEM images depicting the morphology of the pristine and modified PVDF-HFP electrospun nanofiber mats are shown in Figure 2. The average diameter of PVDF-HFP/BTEAC electrospun fibers (197.2 ± 44.0 nm) was noticeably smaller than that of PVDF-HFP without the cationic surfactant (718.2 ± 87.8 nm). This decrease in diameter of fibers electrospun by polymer solution with ionic surfactant is consistent with previous studies.35-37 The geometry of electrospun fibers is determined by characteristics of the polymer solution, such as polymer concentration, type of solvent, viscosity, surface tension, and conductivity, as well as electrospinning conditions, including electrical field strength and relative humidity.35 Since the mass fraction of ionic surfactants (i.e., BTEAC) is only 10 wt% of PVDF-HFP, the 9 ACS Paragon Plus Environment


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addition of BTEAC has a little effect on the viscosity and surface tension of the polymer solution, which are mainly determined by polymer (i.e., PVDF-HFP) concentration. However, the increased charge density attributed to the ionic surfactants in the polymer solution leads to a greater elongation of the fibers driven by the applied electric field, resulting in thinner nanofibers.35 Accordingly, the effective pore size of PVDF-HFP/BTEAC (i.e., 0.42 ± 0.09 µm) was significantly smaller than that of PVDF-HFP (i.e., 2.23 ± 0.26 µm; Supporting Information S1). Figure 2. To provide a platform for chemical modification to lower the surface energy and obtain multilevel re-entrant structures, SiNPs were grafted on the nanofiber by a simple dip-coating protocol. For the pristine PVDF-HFP nanofiber mat without BTEAC (cationic surfactant), no SiNPs were observed on the electrospun fibers after dip-coating (denoted as SiNPs-PVDFHFP in Figure 2b) due to the absence of surface charge on the electrospun fibers. On the other hand, nanofibers of the PVDF-HFP/BTEAC mat were entirely coated by SiNPs (SiNPsPVDF-HFP/BTEAC, hereafter), indicating a strong electrostatic attraction between the negatively charged SiNPs and positively charged nanofiber surface (Figure 2d, Figure S2 and S3). To verify the robust bonding between the SiNPs and the nanofibers, the PVDFHFP/BTEAC mat was immersed in a glass petri dish filled with an acetate buffer solution at pH 4 and sonicated for three minutes. No SiNPs loss was observed from the nanofibers after exposure to the strong physical stress, indicating that a stable and strong bond was formed by electrostatic attraction (Figure S2).

3.2. Membrane Characteristics The grafting of SiNPs on the nanofibers via electrostatic interaction can be explained by the surface zeta potentials of the nanofibers and the SiNPs. The zeta potentials of the electrospun 10 ACS Paragon Plus Environment

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nanofiber mats with and without BTEAC are presented in Figure 3a. The zeta potential of the pristine PVDF-HFP mat was negative for the entire pH range (pH 3 – 9) investigated, with an extrapolated isoelectric point of ~2.8. Because PVDF-HFP does not possess ionizable functional groups, the observed negative zeta potential may be attributed to adsorption of anions, such as OH- or Cl- from the background electrolyte, on the nanofiber surface, as proposed elsewhere.38-39 As expected, zeta potential of the PVDF-HFP nanofiber mats electrospun with cationic surfactant (i.e., PVDF-HFP/BTEAC) was noticeably less negative compared to that of the pristine PVDF-HFP mat, with an isoelectric point of 5.4. Although the cationic surfactants impart positive charges on the nanofibers, the negative zeta potentials above pH 5.4 are likely due to the adsorption of anions on the nanofibers, supported by the same trend of more negative zeta potential with increasing pH for the pristine PVDF-HFP membrane. Figure 3. Zeta potential of the SiNPs (Figure 3a) was negative for the entire pH range investigated due to deprotonation of silanol groups.40 Accordingly, to maximize SiNP surface coverage of the electrospun fibers, dip coating was performed at pH 4, where the zeta potential of the PVDF-HFP/BTEAC mat is positive while that of the SiNPs is negative. The elemental composition of the membranes was investigated by attenuated total reflectance Fourier transform infrared (ATR FTIR) spectroscopy. Figure 3b shows the 1600 – 600 cm-1 wavenumber region of the FTIR spectra, covering the characteristic absorbances of the main functional groups in the control and modified PVDF-HFP nanofiber mats. Several functional groups of PVDF-HFP were identified, including peaks at 1402, 1173, and 1073 cm-1, attributed to wagging of CH2, anti-symmetric stretching of CF2, and out-of-plane deformation of CF3, respectively.41 Peaks regarding crystallinity of the PVDF-HFP were also 11 ACS Paragon Plus Environment


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detected; the strong peaks at 875 and 840 cm-1 correspond to amorphous and beta phases of PVDF-HFP, respectively.41-42 For the PVDF-HFP/BTEAC membrane, two small but noticeable peaks at 757 and 705 cm-1 were observed; the peaks correspond to the stretching mode of aromatic C-H bonds in BTEAC, a monosubstituted benzene.43 After dip-coating of the SiNPs-PVDF-HFP/BTEAC (i.e., nanofibrous membrane with ionic surfactants) in SiNP suspension, two noticeable phenomena were observed. First, a small peak at 1110 cm-1 corresponding to silanol groups on the SiNPs was detected due to the dense SiNPs grafting on the nanofibers (Figure 2b). The low magnitude of the signal is attributed to the small mass portion of SiNPs compared to the substrate (PVDF-HFP), as quantified by thermogravimetric analysis (TGA), which will be discussed later. Second, the two peaks associated with BTEAC disappeared after the dip-coating process. This observation implies that the surfactants leached out from the nanofibers, since they are not covalently linked to the substrate material and are also soluble in aqueous media. The binding of SiNPs to the nanofiber surface was quite robust, and remained intact even after the membrane was subjected to physical stress by bath sonication (Figure S1). This finding suggests that the small amount of surfactants provides sufficient surface charges on the nanofibers to realize strong electrostatic attraction with SiNPs, thereby enabling irreversible SiNP binding on the membrane surface. Irreversible SiNP binding via electrostatic attraction was also observed in previous studies44-45 where nanoparticles with amine moieties (positively charged) were grafted onto a negatively charged polyamide membrane surface and remained on the surface even after harsh physical (i.e., bath sonication) and chemical (i.e., pH 2, pH 12, or 0.6M NaCl) treatment. The compositional change of the nanofibers during the omniphobic membrane fabrication was investigated by TGA. Figure 3c shows total mass fraction of the PVDF-HFP and PVDF12 ACS Paragon Plus Environment

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HFP/BTEAC nanofiber mats with increasing temperature. A thermal decomposition of PVDF-HFP was observed at ~ 480 °C, as reported in the literature.46 For the PVDFHFP/BTEAC mat, thermal decomposition of additional compound was observed at 210 – 250 °C, leading to ~ 10 % reduction of the total mass. This result is consistent with the relative mass fraction of BTEAC added to PVDF-HFP (10 wt%), implying that BTEAC decomposes at the specified temperature range. TGA also allows for estimation of the amount of the SiNPs grafted on the nanofibers from the remaining mass fraction above ~ 600 °C, since SiNPs do not thermally decompose in the investigated temperature range. For the pristine PVDF-HFP mat which was immersed in SiNP suspension (i.e., SiNPs-PVDF-HFP), the mass fraction profile was almost identical to that of the pristine PVDF-HFP (Figure 3d). The estimated mass fraction of adsorbed SiNPs relative to PVDF-HFP was negligibly small, (i.e., 0.055 wt%, Supporting Information S2), due to the absence of surface charge. TGA analysis of the SiNP grafted PVDF-HFP/BTEAC mat (i.e., SiNPs-PVDFHFP/BTEAC) revealed two distinct features. First, the mass fraction of BTEAC was only ~ 0.6 % of the total mass (Figure 3d and left inset), substantially lower than the original fraction (i.e., 10 wt% in the PVDF-HFP/BTEAC mat). Consistent with the previous FTIR analysis, this observation indicates that most of BTEAC leached out during the dip-coating process. Nevertheless, as we discussed earlier, the SiNP coating maintained a stable binding. Second, the mass fraction of SiNPs was estimated to be 2.01% relative to mass of PVDF-HFP (right inset in Figure 3d, Supporting Information S4), which is substantially higher than the SiNP mass fraction in the SiNPs-PVDF-HFP membrane, thereby proving the efficacy of nanoparticle grafting.

3.3. Wetting Resistance of Fabricated Membranes 13 ACS Paragon Plus Environment


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We investigated wetting resistance of the fabricated membranes by measuring the contact angle of liquids with different surface tensions (γ) (Figure 4). Due to the hydrophobic nature of PVDF-HFP and the porous structures of the nanofiber mat, the PVDF-HFP membrane exhibited a relatively high water contact angle of ~130 °. The PVDF-HFP membrane also retained a droplet of 3 mM sodium dodecyl sulfate (SDS) (γ = 57 mN/m). Although cylindrical fibers feature re-entrant structure 7-8, the pristine PVDF-HFP nanofiber mat failed to hold low surface tension liquids  mineral oil (γ ≈ 30 mN/m), decane (γ = 23.8 mN/m), and ethanol (γ = 22.1 mN/m)  likely due to a relatively high surface energy of PVDF-HFP (i.e., critical surface energy ~25 mN/m) 47. Figure 4. The FDTS coated SiNPs-PVDF-HFP/BTEAC membrane, on the other hand, exhibited superhydrophobicity, with a water contact angle of ~ 150°, and also retained all the testing liquids including mineral oil, decane, and ethanol (Figure 4). We attribute this excellent wetting resistance to two combined factors: (i) the multilevel re-entrant structure achieved by the SiNPs on the PVDF-HFP electrospun fiber mats and (ii) the low surface energy resulting from surface fluorination. The spherical SiNPs create the second level re-entrant structure on the cylindrical nanofibers, which serves as an additional barrier to surface wetting 15, 16, 17. In addition, the SiNPs provide a platform of dense hydroxyl groups for silanization with FDTS, which significantly lowers the surface energy (i.e., critical surface energy ~12 mN/m).48, thereby resulting in the observed surface omniphobicity.

3.4. Desalination of Low Surface Tension Saline Solutions We employed the omniphobic membrane for desalinating a highly saline, low surface tension aqueous solution, mimicking high-salinity wastewaters (e.g., oil- and gas-produced water).


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Membrane distillation (MD) is an emerging technology that can separate water from highly saline source waters via phase change utilizing a low-grade heat source.21 In direct contact MD, the most basic MD configuration, the saline feed water at moderately elevated temperature (40 – 70°C) and permeate water at low (ambient) temperature are in contact with each side of a hydrophobic microporous membrane. A vapor pressure gradient developed by the temperature difference drives water vapor from the feed to the permeate side of the membrane, while non-volatile solutes are rejected49 (Figure 5a). It is critical to prevent the membrane from wetting, which leads to leakage of solutes through the wetted pores and failure of the desalination process (Figure 5b). However, conventional hydrophobic microporous membranes, such as PVDF or PTFE, are prone to wetting upon exposure to source waters that contain organic contaminants such as oils or surfactants.17, 50 To demonstrate desalination of high salinity and low surface tension water mimicking shale gas wastewater by MD, we used a 1 M NaCl solution with varying concentrations of sodium dodecyl sulfate (SDS, an anionic surfactant) as a feed, and deionized (DI) water as a permeate. We note that surface tension of surfactant solutions is significantly reduced in the presence of electrolytes.34, 51-53 Although the mechanism has not been fully understood, it is generally accepted that for ionic surfactants, electrolytes screen the charged polar head groups, leading to a higher surfactant density at the liquid-gas interface.51 For example, although the surface tension of 0.1 mM SDS in DI water is ~ 67 mN/m,54 the surface tension is substantially reduced to ~ 41 mN/m in 1 M NaCl solution with the same SDS concentration.34 Since typical saline industrial wastewaters contain various organic contaminants, including surfactants, a highly wetting resistant MD membrane is essential to successfully desalinate source waters with low surface tension.



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The pristine PVDF-HFP membrane was first tested in a co-current mode in MD using 1 M NaCl at 60 °C and DI water at 20 °C as feed and permeate solutions, respectively. Figure 5c shows time traces of water flux through the membrane and salt rejection determined from measuring the permeate water electrical conductivity. The observed constant water flux with complete salt rejection indicates that the PVDF-HFP membrane allows only water vapor to pass through, while the non-volatile salt is rejected. However, when 0.1 mM SDS was added to the feed (1 M NaCl at 60 °C), which leads to a feed surface tension of ~ 41 mN/m, the water flux suddenly increased and concomitantly salt rejection decreased rapidly. This observation clearly indicates pore wetting of the pristine PVDF-HFP membrane. In such a case, the feed water with dissolved salt (NaCl) directly flows through the wetted membrane, driven by the slightly larger hydraulic pressure on the feed than that of at the permeate side (Figure 5b). On the other hand, the water flux and salt rejection of the omniphobic membrane were stable during the eight-hour MD experiment, regardless of SDS addition to the feed, even at a concentration of 0.3 mM, corresponding to a feed surface tension of ~ 31 mN/m. In addition, the omniphobic membrane showed a stable MD operation up to 20 h, even after being subjected to 3 min bath sonication (Supporting Information S5). This observation clearly demonstrates the robust wetting resistance of our fabricated omniphobic membrane and the potential for separation of water from source waters contaminated by low surface tension organic compounds. We also note that after the MD experiments, the SiNPs were found firmly attached to the nanofibers, confirming the robust irreversible coating of the nanofiber mats (Figure S1). Figure 5. 4. CONCLUSION 16 ACS Paragon Plus Environment

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We presented a facile approach to fabricate omniphobic membranes by constructing a hierarchical re-entrant structure followed by surface fluorination. The first level of structure was realized by the cylindrical morphology of electrospun PVDF-HFP nanofibers, with positive charges imparted by electrospinning the polymer solution with a cationic surfactant. The second level structure was attained by grafting spherical SiNPs onto the positively charged nanofibers via simple dip-coating. The SiNPs also provided a platform to lower the surface energy of the membrane by silanization using fluorinated alkylsilanes. The oncegrafted SiNPs formed robust binding with the nanofibers, not affected even under strong physical stress. The fabricated nanofibrous membrane exhibited excellent wetting resistance against liquids with a wide range of surface tensions, including aqueous (e.g., water and SDS solution) and organic solvents (e.g., mineral oil, decane, and ethanol). Finally, as a promising application of the novel fabrication approach, we demonstrated in MD experiments that the omniphobic membrane can desalinate a highly saline feed water with surface tension lowered by surfactants, mimicking shale gas wastewater, while the pristine PVDF-HFP membrane, which represents a conventional hydrophobic membrane, failed due to pore wetting. In our approach for omniphobic membrane fabrication, the surface omniphobicity is independent of the type of base polymers used and therefore allows for the use of a wide range of polymers that possess the desired physical and chemical properties. We expect the developed fabrication approach to broadly impact various industrial sectors, including water treatment, textile, transportation, and biomedical industries, which would benefit from highly wettingresistant surfaces against water and organic liquids. ASSOCIATED CONTENT



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Supporting Information. Measurement of Effective Pore Size Distribution. Binding robustness of SiNPs on the nanofibers. Effects of Dip-coating Conditions on SiNPs Grafting. Estimation of mass fraction of SiNPs grafted on the nanofibers based TGA data. Long-term Performance of Omniphobic Membranes for MD. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge the support received form the National Science Foundation through the Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500) and the Department of Defense through the Strategic Environmental Research and Development Program (SERDP, Project ER-2217). W.-H.R. acknowledges support from The NatureNet Program of the Nature Conservancy. Facilities used were supported by the Yale Institute of Nanoscale and Quantum Engineering (YINQE) and Chemical and Biophysical Instrument Center (CBIC).


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REFERENCES 1.

Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L. C.; Seeger, S., A Simple,

One-Step Approach to Durable and Robust Superhydrophobic Textiles. Adv Funct Mater

2008, 18 (22), 3662-3669. 2.

Zhu, J.; Hsu, C. M.; Yu, Z. F.; Fan, S. H.; Cui, Y., Nanodome Solar Cells with

Efficient Light Management and Self-Cleaning. Nano Lett 2010, 10 (6), 1979-1984. 3.

Nosonovsky, M.; Bhushan, B., Superhydrophobic Surfaces and Emerging

Applications: Non-adhesion, Energy, Green Engineering. Curr Opin Colloid In 2009, 14 (4), 270-280. 4.

Lawson, K. W.; Lloyd, D. R., Membrane Distillation. J Membrane Sci 1997, 124 (1),

1-25. 5.

Roach, P.; Shirtcliffe, N. J.; Newton, M. I., Progess in Superhydrophobic Surface

Development. Soft Matter 2008, 4 (2), 224-240. 6.

Li, X. M.; Reinhoudt, D.; Crego-Calama, M., What Do We Need for a

Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem Soc Rev 2007, 36 (8), 1350-1368. 7.

Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.;

McKinley, G. H.; Cohen, R. E., Designing Superoleophobic Surfaces. Science 2007, 318 (5856), 1618-1622. 8.

Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Robust

Omniphobic Surfaces. P Natl Acad Sci USA 2008, 105 (47), 18200-18205. 9.

Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H.,

Fabrics with Tunable Oleophobicity. Adv Mater 2009, 21 (21), 2190-+. 10.

Liu, K. S.; Tian, Y.; Jiang, L., Bio-inspired Superoleophobic and Smart Materials:

Design, Fabrication, and Application. Prog Mater Sci 2013, 58 (4), 503-564.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.

Page 20 of 30

Kota, A. K.; Mabry, J. M.; Tuteja, A., Superoleophobic Surfaces: Design Criteria and

Recent Studies. Surface Innovations 2013, 1 (2), 71-83. 12.

Pan, S. J.; Kota, A. K.; Mabry, J. M.; Tuteja, A., Superomniphobic Surfaces for

Effective Chemical Shielding. J Am Chem Soc 2013, 135 (2), 578-581. 13.

Fujii, T.; Aoki, Y.; Habazaki, H., Fabrication of Super-Oil-Repellent Dual Pillar

Surfaces with Optimized Pillar Intervals. Langmuir 2011, 27 (19), 11752-11756. 14.

Hoefnagels, H. F.; Wu, D.; de With, G.; Ming, W., Biomimetic Superhydrophobic

and Highly Oleophobic Cotton Textiles. Langmuir 2007, 23 (26), 13158-13163. 15.

Leng, B. X.; Shao, Z. Z.; de With, G.; Ming, W. H., Superoleophobic Cotton Textiles.

Langmuir 2009, 25 (4), 2456-2460. 16.

Fukada, K.; Nishizawa, S.; Shiratori, S., Antifouling Property of Highly Oleophobic

Substrates for Solar Cell Surfaces. J Appl Phys 2014, 115 (10). 17.

Lin, S. H.; Nejati, S.; Boo, C.; Hu, Y. X.; Osuji, C. O.; Ehmelech, M., Omniphobic

Membrane for Robust Membrane Distillation. Environ Sci Tech Let 2014, 1 (11), 443-447. 18.

Shaffer, D. L.; Chavez, L. H. A.; Ben-Sasson, M.; Castrillon, S. R. V.; Yip, N. Y.;

Elimelech, M., Desalination and Reuse of High-Salinity Shale Gas Produced Water: Drivers, Technologies, and Future Directions. Environ Sci Technol 2013, 47 (17), 9569-9583. 19.

Duong, H. C.; Chivas, A. R.; Nelemans, B.; Duke, M.; Gray, S.; Cath, T. Y.; Nghiem,

L. D., Treatment of RO Brine from CSG Produced Water by Spiral-wound Air Gap Membrane Distillation - A Pilot Study. Desalination 2015, 366, 121-129. 20.

Wang, P.; Chung, T. S., Recent Advances in Membrane Distillation Processes:

Membrane Development, Configuration Design and Application Exploring. J Membrane Sci

2015, 474, 39-56. 21.

Khayet, M., Membranes and Theoretical Modeling of Membrane Distillation: A

Review. Adv Colloid Interfac 2011, 164 (1-2), 56-88.


Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22.

Liu, D. E.; Cerretani, C.; Tellez, R.; Scheer, A. P.; Sciamanna, S.; Bryan, P. F.; Radke,

C. J.; Prausnitz, J. M., Analysis of Countercurrent Membrane Vapor Extraction of a Dilute Aqueous Biosolute. Aiche J 2015, 61 (9), 2795-2809. 23.

Liao, Y.; Wang, R.; Fane, A. G., Fabrication of Bioinspired Composite Nanofiber

Membranes with Robust Superhydrophobicity for Direct Contact Membrane Distillation.

Environ Sci Technol 2014, 48 (11), 6335-6341. 24.

Nejati, S.; Boo, C.; Osuji, C. O.; Elimelech, M., Engineering Flat Sheet Microporous

PVDF Films for Membrane Distillation. J Membrane Sci 2015, 492, 355-363. 25.

Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S., A Review on Polymer

Nanofibers by Electrospinning and Their Applications in Nanocomposites. Compos Sci

Technol 2003, 63 (15), 2223-2253. 26.

Tijing, L. D.; Choi, J. S.; Lee, S.; Kim, S. H.; Shon, H. K., Recent Progress of

Membrane Distillation Using Electrospun Nanofibrous Membrane. J Membrane Sci 2014,

453, 435-462. 27.

Guo, M.; Ding, B.; Li, X. H.; Wang, X. L.; Yu, J. Y.; Wang, M. R., Amphiphobic

Nanofibrous Silica Mats with Flexible and High-Heat-Resistant Properties. J Phys Chem C

2010, 114 (2), 916-921. 28.

Han, D. W.; Steckl, A. J., Superhydrophobic and Oleophobic Fibers by Coaxial

Electrospinning. Langmuir 2009, 25 (16), 9454-9462. 29.

Sharma, B.; Verma, R.; Baur, C.; Bykova, J.; Mabry, J. M.; Smith, D. W., Ultra Low

Dielectric, Self-cleansing and Highly Oleophobic POSS-PFCP Aryl Ether Polymer Composites. J Mater Chem C 2013, 1 (43), 7222-7227. 30.

Khayet, M.; Velazquez, A.; Mengual, J. I., Modelling Mass Transport through a

Porous Partition: Effect of Pore Size Distribution. J Non-Equil Thermody 2004, 29 (3), 279299.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

31.

Page 22 of 30

Li, D. P.; Frey, M. W.; Joo, Y. L., Characterization of Nanofibrous Membranes with

Capillary Flow Porometry. J Membrane Sci 2006, 286 (1-2), 104-114. 32.

Elimelech, M.; Chen, W. H.; Waypa, J. J., Measuring the Zeta (Electrokinetic)

Potential of Reverse-Osmosis Membranes by a Streaming Potential Analyzer. Desalination

1994, 95 (3), 269-286. 33.

Ottewill, R. H.; Shaw, J. N., Electrophoretic Studies on Polystyrene Latices. J

Electroanal Chem 1972, 37 (Jun), 133-142. 34.

Matijevic, E.; Pethica, B. A., The Properties of Ionized Monolayers .1. Sodium

Dodecyl Sulphate at the Air-Water Interface. T Faraday Soc 1958, 54 (9), 1382-1389. 35.

Zong, X. H.; Kim, K.; Fang, D. F.; Ran, S. F.; Hsiao, B. S.; Chu, B., Structure and

Process Relationship of Electrospun Bioabsorbable Nanofiber Membranes. Polymer 2002, 43 (16), 4403-4412. 36.

Qin, X. H.; Wang, S. Y., Interior Structure of Polyacrylonitrile(PAN) Nanofibers with

LiCl. Mater Lett 2008, 62 (8-9), 1325-1327. 37.

You, Y.; Lee, S. J.; Min, B. M.; Park, W. H., Effect of Solution Properties on

Nanofibrous Structure of Electrospun Poly(lactic-co-glycolic acid). J Appl Polym Sci 2006,

99 (3), 1214-1221. 38.

Childress, A. E.; Elimelech, M., Effect of Solution Chemistry on the Surface Charge

of Polymeric Reverse Osmosis and Nanofiltration Membranes. J Membrane Sci 1996, 119 (2), 253-268. 39.

Elimelech, M.; Omelia, C. R., Effect of Electrolyte Type on the Electrophoretic

Mobility of Polystyrene Latex Colloids. Colloid Surface 1990, 44, 165-178. 40.

Kirby, B. J.; Hasselbrink, E. F., Zeta Microfluidic Substrates: 1. Theory, Experimental

Techniques, and Effects on Separations. Electrophoresis 2004, 25 (2), 187-202.


Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41.

Sim, L. N.; Majid, S. R.; Arof, A. K., FTIR Studies of PEMA/PVdF-HFP Blend

Polymer Electrolyte System Incorporated with LiCF3SO3 Salt. Vib Spectrosc 2012, 58, 57-66. 42.

Ulaganathan, M.; Nithya, R.; Rajendran, S., Surface Analysis Studies on Polymer

Electrolyte Membranes Using Scanning Electron Microscope and Atomic Force Microscope. INTECH Open Access Publisher: 2012. 43.

Cherdoud-Chihani, A.; Mouzali, M.; Abadie, M. J. M., Study of Crosslinking Acid

Copolymer/DGEBA Systems by FTIR. J Appl Polym Sci 2003, 87 (13), 2033-2051. 44.

Tiraferri, A.; Kang, Y.; Giannelis, E. P.; Elimelech, M., Superhydrophilic Thin-Film

Composite Forward Osmosis Membranes for Organic Fouling Control: Fouling Behavior and Antifouling Mechanisms. Environ Sci Technol 2012, 46 (20), 11135-11144. 45.

Tiraferri, A.; Kang, Y.; Giannelis, E. P.; Elimelech, M., Highly Hydrophilic Thin-

Film Composite Forward Osmosis Membranes Functionalized with Surface-Tailored Nanoparticles. ACS Applied Materials & Interfaces 2012, 4 (9), 5044-5053. 46.

Cao, J. H.; Zhu, B. K.; Ji, G. L.; Xu, Y. Y., Preparation and Characterization of

PVDF-HFP Microporous Flat Membranes by Supercritical CO2 Induced Phase Separation. J

Membrane Sci 2005, 266 (1-2), 102-109. 47.

Angulakshmi, N.; Nahm, K. S.; Choi, J.-M.; Hwang, Y. J.; Stephan, A. M.,

Electrochemical Properties of Coconut Shell Flour-Incorporated Poly(vinylidenehexafluoropropylene)-Based Electrospun Membranes for Lithium Batteries.

Science of Advanced Materials 2013, 5 (6), 606-611. 48.

Brzoska, J. B.; Benazouz, I.; Rondelez, F., Silanization of Solid Substrates - a Step

toward Reproducibility. Langmuir 1994, 10 (11), 4367-4373. 49.

Khayet, M.; Matsuura, T.; Mengual, J. I.; Qtaishat, M., Design of Novel Direct

Contact Membrane Distillation Membranes. Desalination 2006, 192 (1-3), 105-111.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50.

Page 24 of 30

Kong, J.; Li, K., Oil Removal from Oil-in-water Emulsions Using PVDF Membranes.

Sep Purif Technol 1999, 16 (1), 83-93. 51.

Woolfrey, S. G.; Banzon, G. M.; Groves, M. J., The Effect of Sodium-Chloride on the

Dynamic Surface-Tension of Sodium Dodecyl-Sulfate Solutions. J Colloid Interf Sci 1986,

112 (2), 583-587. 52.

Corrin, M. L.; Harkins, W. D., The Effect of Salts on the Critical Concentration for

the Formation of Micelles in Colloidal Electrolytes. J Am Chem Soc 1947, 69 (3), 683-688. 53.

Becher, P., Nonionic Surface-Active Compounds .5. Effect of Electrolyte. J Coll Sci

Imp U Tok 1962, 17 (4), 325-&. 54.

Hernainz, F.; Caro, A., Variation of Surface Tension in Aqueous Solutions of Sodium

Dodecyl Sulfate in the Flotation Bath. Colloid Surface A 2002, 196 (1), 19-24.


Page 25 of 30

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Figure 1. Schematic diagram of omniphobic membrane fabrication. (i) Generation of a positively charged nanofibrous membrane by electrospinning a mixture of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and benzyltriethylammonium chloride (BTEAC) at a 10:1 mass ratio, dissolved in a dimethylacetamide (DMAC) and acetone mixture. (ii) Dip-coating of nanofibers with silica nanoparticles (SiNPs) via electrostatic attraction in sodium acetate buffer at pH 4 and an ionic strength of approximately 1 mM. (iii) Silanization of the overlaying SiNPs using perfluorodecyltrichlorosilane (FDTS) in a vacuum desiccator at 100 °C for 24 hours. (iv) Thermal annealing of the silanized nanofibers on hot plate at 120 °C for three hours.



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Figure 2. Effect of cationic surfactant (BTEAC) on grafting efficiency of SiNPs to PVDF-HP nanofibers. Scanning electron microscope (SEM) images of nanofibers of pristine PVDF-HFP membrane (a) before and (b) after immersion in SiNPs solution; SEM images of nanofibers in PVDF-HFP / BTEAC (c) before and (d) and after immersion in SiNPs solution. The insets in (a) and (b) show distributions of diameters of nanofibers, with average diameters (da) of 718.2 ± 87.8 nm and 197.2 ± 44.0 nm for PVDF-HFP and PVDF-HFP / BTEAC, respectively. The insets in (c) and (d) are high magnification images for a single nanofiber.


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Figure 3. (a) Zeta potentials of PVDF-HFP and PVDF-HFP / BTEAC membranes as well as silica nanoparticles (SiNPs). (b) ATR FT-IR spectra of (1) PVDF-HFP, (2) PVDF-HFP / BTEAC, (3) SiNPs-PVDF-HFP / BTEAC, and (4) silanized SiNPs-PVDF-HFP / BTEAC membranes. Thermogravimetric analysis (TGA) of PVDF-HFP and PVDF-HFP / BTEAC membranes: (c) before SiNPs grafting showing the thermal decomposition of BTEAC around 210 – 260 °C and (d) after SiNPs grafting showing the near-absence of mass fraction reduction around the same temperature range (i.e., 210 – 260 °C). Left inset in (d) is a magnified window near the BTEAC decomposition temperature. Right inset in (d) shows mass fractions of SiNPs grafted on the membranes relative to the mass of PVDF-HFP, estimated from residual mass after complete decomposition of PVDF-HFP (Supporting Information, S4).



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Figure 4. Comparison between the pristine PVDF-HFP and omniphobic membranes of (a) contact angles and (b) photographs of tested liquids with different surface tensions. In (b), dark areas for pristine PVDF-HFP membrane indicate that the low surface tension liquids (mineral oil, decane, and ethanol) wicked into the membrane, while the omniphobic membrane repelled all the liquids. Standard deviations of contact angles were obtained from at least three different measurements.


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Figure 5. Measurement of water transport and salt leakage through the fabricated desalination membranes in direct contact membrane distillation (MD) experiments. NaCl solution (1 M) at 60 °C with added SDS and DI water at 20 °C were used for feed and permeate streams, respectively. (a) Schematic illustrating water vapor transport in MD from the hot feed to the cold permeate side; the non-volatile salt ions are rejected by MD membrane. (b) Schematic of “failure” of the membrane distillation process due to membrane pore wetting by surfactants, where salt ions leak through the wetted pores. Time traces of water vapor flux across the membranes and salt rejection for (c) the pristine PVDF-HFP and (d) the omniphobic membrane, based on sequential doses of sodium dodecyl sulfate (SDS); the corresponding surface tensions of the feed streams are also indicated.


Na+ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

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100 nm

Vapor

Hot Feed

Water

Cold Permeate

SDSACS Paragon OilPlus Environment Decane Ethanol

Development of Omniphobic Desalination Membranes Using a

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