Fabrication of SuperhydrophobicâSuperoleophilic Fabrics by an

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Fabrication of Superhydrophobic-Superoleophilic Fabrics by Etching and Dip-Coating Two-Step Method for Oil-Water Separation Caili Zhang, Pei Li, and Bing Cao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00206 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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Industrial & Engineering Chemistry Research

Fabrication of Superhydrophobic-Superoleophilic Fabrics by Etching and Dip-Coating Two-Step Method for Oil-Water Separation Caili Zhang, Pei Li * and Bing Cao* College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China.

ACS Paragon Plus Environment

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ABSTRACT: A dual-scale roughness structure superhydrophobic-superoleophilic fabric was fabricated by first etching the microscale fibers with alkali, and then dip-coating in a mixed solution of polymer of intrisic microporisty (PIM-1) and fluorinated alkyl silane (PTES). Scanning electron microscopy analysis showed that the etching process created nanoscale pits on the fiber surface, and subsequently formed hierarchical structures on the fabric surface. Coating of PIM-1/PTES on the etched fibers significantly lowered the surface energy of the fibers, thus making the fabric surface possessed superhydrophobicity with a water contact angle of 158° and superoleophilicity with an oil contact angle of 0°. The obtained superwettable fabric was mounted in a leak-proof manner on the open end glass bottle, like an oil skimmer container. Such a new surface-tension-driven, gravity-assisted, one-step, oil-water separation device was used to separate oil-water mixture with separation efficiency as high as 99.96% after 30 recycles.

KEYWORDS: PIM-1, superhydrophobic-superoleophilic, oil-water separation, fabric

1. INTRODUCTION The frequent occurrences of oil spills and chemical leakages have posed a threat to biological and human safety, which urged us to find the simple and efficient solutions that can remove oil from water.1-3 In case of a light oil spill accident, the oil will stay on the top of water. Hence, the oil can be removed by simply burnt, oil skimmers or adsorption. However, in situ combustion is only partially successful, creates toxic fumes, and gradually losing efficiency when the floating oil layer thins.4,5 For adsorption method, oil-absorbent materials are commonly used. The most prevalent commercial oil-absorption materials are polypropylene-based felts. These materials have several drawbacks including inadequate absorption capacity, low


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selectivity, and poor recyclability. Other oil-absorbing materials, for example, zeolites, activated carbon, organoclays, straw, and wool fibers also have similar limitations.6-8 The third most common method for dealing with such situations is skimming. However, most oil skimmers are inefficient, required in large numbers, and leave much of the recovered oil mixed with water.9,10 Therefore, oil skimmer will be more competitive to other technologies if its oil-water selectivity can be greatly improved. Recently, many research works have been done to fabricate membranes with superhydrophobic and superoleophilic surfaces in order to achieve high oil-water separation efficiency since membranes with this kind of surface properties could repel water but absorb oil.11-13 However, currently methodologies to separate oil-water mixture using the above mentioned materials consist of two steps. First, collect the oil-water mixture in bulk, and second, pour it onto the surface of the separation material to make the oil-water separation happen. This approach can be cumbersome and energy-intensive for large-area oil spills. Therefore, it is important to develop a direct method that does not require a pouring step. So, if we stick the special wettability materials to the top of the oil skimmer container, the aims of high oil-water separation efficiency and in situ collection of oil will be simultaneously reached. Usually, mesh and fabric are selected as substrate filtration materials due to their microscale porous structure, mechanical and chemical stability, as well as its easy availability and low cost.14-20 Comparing mesh with fabric, the latter is more flexible and easier to be attached to the skimmer container. So we select fabric as the base material to fabricate superhydrophobic and superoleophilic surface. Since the as-received fabric does not have the ability to selectively permeate water over oil or wise versa, the surface of the fabric has to be modified to control its wettability so that increase its selectivity. The established theoretical and experimental results


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manifest that the superhydrophobic-superoleophilic surfaces can be obtained by either enhancing the surface roughness or tuning the surface energy21-24. According to Wenzel’s and Cassie’s theories, introducing an appropriate microstructure shall improve the hydrophobicity of the surface because of the introduction of air pockets beneath the water droplet, whereas an oleophilic surface may become more oleophilic owing to the capillary effect.25, 26 Therefore, in this study we report a simple way to fabricate superhydrophobicsuperoleophilic fabric by developing a dual-scale roughness structure and then dip-coating in a mixed solution of polymer of intrisic microporisty (PIM-1), a hydrophobic polymer27, and fluorinated alkyl silane (PTES) with low surface energy. The obtained superwettable fabric is mounted in a leak-proof manner on the open end glass bottle, like an oil skimmer container. Such a new surface-tension-driven, gravity-assisted, one-step, oil-water separation device is used to oil-water mixture separation. 2. EXPERIMENTAL SECTION 2.1 Materials 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethyl-1,1’-spirobisindane

(TTSBI,

97%)

was

purchased from Alfa Aesar. 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN, 99%), potassium carbonate (K2CO3, >99.5%) and anhydrous N,N’-dimethylformamide (DMF) were purchased from Sigma Aldrich. Hydrochloric acid (HCl, 37.5%), sodium hydroxide, hexadecane and anhydrous ethanol were obtained from Tianjin Fu Chen Chemical Reagents Factory. PET fabrics bought

from

local

department

store.

Tetrachloromethane,

and

1H,1H,2H,2H-

perfluorooctyltriethoxysilane (C14H19F13O3Si) (PTES) were purchased from J&K co. ltd. TTSBI was purified by re-crystallization in methanol, while TFTPN was sublimated under vacuum. Other chemicals were used as received. PIM-1 was synthesized via a nucleophilic substitution


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reaction between TTSBI and TFTPN using K2CO3 as the catalyst in DMF solution. The detailed synthesis procedure was reported in our previous work.27-29 The PIM-1(Mw=144,378 g/mol) was obtained by carrying out the reaction for 3 days. 2.2 Preparation of Sodium Hydroxide Etched Fabrics The as-received fabrics were cleaned with deionized water at 75 °C for 30 min to remove the impurities and dried at 100 °C. The cleaned fabrics were immersed into 300 g/L sodium hydroxide solution which put into a polyethylene zip-lock bags and then heated at 100 °C for 60 min. Finally, the fabrics were rinsed by abundant water until the pH of the wash water reached 7 and then dried at 100 °C in an oven. In the end, the etched fabrics were obtained and denoted as E-fabric. 2.3 Fabrication of PIM-1/PTES Dip-coating Fabrics. The etched fabrics were immersed into 1 wt% PIM-1/PTES solution for 10 seconds. The weight ratio of PIM-1/PTES is 90/10 (w/w). The obtained dip-coating fabrics were dried at ambient temperature. Finally, the sample was denoted as C-fabric. 2.4 Oil-Water Separation Experiments. To investigate the separation efficiency (the purity of oil in the filtrate after permeating through the membrane) of the C-fabric to immiscible oil-water mixtures, a novel approach for oil-water separation like reported in a reference30 was used. This method can easily collect floating oil. Specifically, the separation device made up of a superhydrophobic C-fabric mounted in a leak-proof manner on the open end of a 20 mL glass bottle. During the separation process, the bottle with a tilt angle was partly immersed into a floating oil-water mixture (hexadecanewater). Part of the C-fabric was submerged and remained in contact with both the floating oil and


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the water underneath. Because of the affinity of the oil toward the C-fabric, the floating hexadecane film wetted and spread over the C-fabric; the oil phase passed through the partly submerged C-fabric and flowed down along the inclined inner glass wall of the bottle, and collected by gravity at the bottom, while the water phase was not transmitted through the Cfabric and remained outside the collection bottle. The obtained filtrate was collected for purity tests. The water contents in the collected filtrates were measured using a Karl Fischer moisture titrator (SFY-3000). 2.5 Characterization A field emission scanning electron microscopy (FE-SEM) (Hitachi S-4700) equipped with an energy dispersive X-ray spectroscopy (EDS) was utilized to investigate the morphologies and elements of the coated fabric. Samples were sputter-coated with gold prior to examination. The surface wettabilities of the pristine and dip-coating fabrics were evaluated by measuring the contact angles of water using an OCA15EC (DataPhysics Instruments GmbH, Filderstadt, Germany) apparatus. The contact angle value was calculated by averaging over three contact angle values at different sites. In each measurement, an approximately 5 µL droplet was dispensed onto the fabric. The pore size distributions of the pristine and etched fabrics were measured using a mercury intrusion porosimetry (PoreMaster-60 GT) bought from Quantachrome Instruments (Boynton Beach, USA), and calculated from the mercury intrusion data using the Washburn equation. 3. RESULTS AND DISCUSSION Based on the Cassie’s theory, the surface geometric structure plays an important role to form a superhydrophobic surface. For instance, a hierarchical surface structure can introduce air pockets trapped between the droplet and the solid surface, minimizing the contact area.31,32 On


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one hand, the inherent arrangement of fabric fibres acts as an ideal platform for hierarchical micro-nano structure. On the other hand, to develop a binary micro-nano structure on the fiber surface, an etching and coating procedure is carried out on the PET fabrics, as schematically shown in Figure 1a. SEM was used to determine the surface morphologies of the as-received, etched and coated fabrics. It is found that the as-received fabric has a woven structure, as shown in Figure 1b, consisting of twisted yarns with microscale fibers of a diameter about 17.5 µm which shows a relatively smooth surface and evident grooves and striations along the fibres. In order to enhance the roughness of the fabric surfaces, PET fabrics were etched by sodium hydroxide. Figure 1c shows that pits are formed on the fiber surface after chemical etching by alkaline hydrolysis and the diameter of the fibers decreases to about 8 µm. The enhanced surface roughness of the fibres is good to obtain superhydrophobicity due to the air trapped among the nanoscale structures on the fibres formed by the growth of the coating. Also importantly, as shown in Figure 1d, coating of PIM-1/PTES does not cause great changes in the roughening morphology of the etched fibers.

Figure 1. (a) Schematic illustration of the fabrication of superhydrophobic fabrics through etching and dip-coating process. SEM images of (b) as-received fabrics, (c) etched fabrics (Efabric), and (d) etched fabrics after dip-coating with PIM-1/PTES (C-fabric).


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The etching process not only gives rise to the fabric surface roughness changed, but also changes the pore structure of the fabric. As shown in Figure 2a and 2b, due to the decrease in the fiber diameter after etching, the fabric woven structure becomes looser than as-received fabric. The fabric pore size distribution can be measured by a mercury intrusion method. According to Figure 2c and 2d, the pore diameters of the as-received fabric distribute in two parts. The large pores between 60-95 µm are the pores formed by woven, and the relatively small pores between 5-20 µm are the pores between fibers. After being etched by alkaline solution, the pore distribution is greatly changed. The sizes of pores formed by woven are dramatically increased, and it is too large to be detected by mercury intrusion method. The sizes of pores between fibers are increased from 5-20 µm to 10-35 µm. The fabric structure plays an important role in gravitydriven oil-water separation process, and the impact of pore size on separation efficiency will be discussed in the part of oil-water separation.

Figure 2. SEM images of (a) as-received fabric, (b) E-fabric. (c) and (d) are the corresponding pore size distribution obtained from the mercury intrusion method. Figure 3a and 3b are the magnified images of etched and coated fabrics, which demonstrate more clearly that the hierarchically dual-scale structured surface is formed. Figure 3c exhibits the


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energy dispersive X-ray spectra (EDS) analysis of the as-received and coated fabrics, respectively. Only peaks of C and O are found on the as-received fabric (Figure 3c), which manifests no other impurities exist on the fabric surface (The signal of Pt element comes from the gold spray). Whereas, besides C and O signals, F and Si signals which belong to PTES can be observed on the C-fabric (Figure 3d). Because of the limited amount of N atom in PIM-1 polymer backbone, the characteristic peak of N is not detected. The corresponding EDS elemental composition of etched fabrics and coated fabrics are shown in Table 1. Table 1. Elemental composition of etched fabrics and coated fabrics. Fabric H-fabric C-fabric

C 69.82 70.32

Composition (At. %) O F 30.18 / 16.63 10.79

Si / 2.26

Figure 3. SEM images of (a) E-fabric, (b) C-fabric. (c) and (d) are the corresponding EDS spectra. Figure 4a shows fabric appearance before and after dip-coating. Since the color of PIM-1 is yellow, after coating the color of the fabric changes from white to yellow. The wettabilities of the fabrics are evaluated. As shown in Figure 4b, the water jet from a pipet can bounce off the C-


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fabric surface without leaving a trace, indicating the weak interaction between water and the Cfabric surface. The etched fabric demonstrates superhydrophilic and water droplets are quickly absorbed and penetrate into its surface (Figure 4c). However, the C-fabric displays both superhydrophobic and superoleophilic behavior, so the oil droplet is quickly absorbed and disappears, while the water droplets stay on the C-fabric forming a highly spherical bead (Figure 4e). The C-fabric shows superhydrophobic and superoleophilic properties simultaneously with a high water CA of 158±1° (Figure 4d). Figure 4f shows that when C-fabric is immersed into water by an external force, air bubbles are trapped around the surface, forming a silver, mirror-like surface, demonstrating the high resistance of C-fabric to water permeation. Figure 4g shows the photographs of a water droplet (5 µL) touching and leaving the C-fabric surface. The water droplet is forced to sufficiently contact the fabric surface with an obvious deformation, and it is then lifted up. The corresponding photographs of the water droplet show almost no deformation when leaving the fabric surface, thus confirming the extremely low water adhesion for C-fabric.

Figure 4. (a) The appearance of the etched fabric (E-fabric) and after dip-coating PIM-1/PTES fabric (C-fabric). (b) A jet of water bouncing off the surface. (c) Water droplets quickly absorbed and disappeared on the E-fabric surface. (d) The water contact angle of C-fabric. (e) Water and oil droplets on the C-fabric. (f) The C-fabric immersed in water by an external force. (g) The photographs of a water droplet touching and leaving the C-fabric surface.


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The above results indicate that the C-fabric possesses both superhydrophobicity and superoleophilicity. Hence, it is an ideal material for separating oil-water mixtures. To prove this, several drops of hexadecane (lighter than water, dyed with oil red O) are dropped on the water surface to form a thin oil layer, then a piece of C-fabric is brought into contact with the oil layer. Consistent with our expectation, the hexadecane is fully absorbed within a few seconds, leaving a transparent region on the water surface (Figure 5a). Furthermore, the absorption of tetrachloromethane (heavier than water, dyed with oil red O) by C-fabric is also carried out to assessment the separation performance. When a piece of C-fabric is inserted into water to approach tetrachloromethane, the tetrachloromethane droplet is immediately sucked up by the superoleophilic fabric underwater (Figure 5b). All of the tetrachloromethane droplets can be removed by taking out the absorbed fabric from water. Nevertheless, due to the inherent poor uptake capacity of fabric, the application of this superhydrophobic-superoleophilic fabric for the absorption of oil from water may be restricted.

Figure 5. Photographs of the processes of using the C-fabrics to remove (a) hexadecane and (b) tetrachloromethane from water. In reality, to deal with an oil spill accidents, absorption method is not enough as it may require a lot of absorbent. Hence, it is necessary to utilize some oil/water separation devices. Like reported in the reference 30, a relative new, simple, surface tension-driven, gravity-assisted filtering device to collect oil floating on water is used. The collection of floating oil using this


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method can occur in active flow conditions, like a drift bottle, a fabric-capped bottle is anchored while floating in an ocean current. As shown in Figure 6a-c, an inclined bottle is immersed into water and there is no water penetrated into the bottle for the superhydrophobic nature of the Cfabric. (See Supporting Information video，video S1) In contrast, as shown in Figure 6d-f, when a bottle is anchored into the oil-water mixture (the collecting bottle size was chosen large enough to hold the entire oil-phase contained in the original mixture), part of the C-fabric is submerged and remains in contact with both the floating oil and the water underneath. (The whole separation process was recorded in a video and provided in the Supporting Information, video S2) Because of the affinity of the oil toward the C-fabric, the floating hexadecane film wets and spreads over the C-fabric; the oil phase passes through the partly submerged C-fabric and flows down along the inclined inner glass wall of the bottle, and is collected by gravity at the bottom, while the water phase does not transmit through the C-fabric and remains outside the collection bottle. This collection process is very quick and it only takes 30s for almost all oil collection. The separation efficiency (the purity of the collected oil filtrate) is 99.99 %. The mixtures of n-hexane-water, cyclohexane-water, petroleum ether-water, vegetable oil-water, soybean oil-water, and vacuum pump oil-water have also been successfully separated with high efficiencies using the same method as shown in Figure 6. No visible oil exists in the water phase after separation, and the separation efficiencies for all tested oil-water systems are higher than 99.94% (Figure 7a). This separation process is effective not only for a continuous film-type spill, but also with discrete floating droplets or isolated oil volumes. The high separation efficiency and separation rate is the results of both the superoleophilicity of the fabric surface (caused by lower surface tension and high surface roughness) and the appropriate porous structure of the fabric that lead to the fast permeate of the


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oil phase. At the initial stage when the C-fabric-capped bottle is anchored into the oil-water mixture, the oleophilic interaction and van der Waals forces play an important role between the experimental oils and fabric’s surfaces. Chemical compatibility between oil and C-fabric leads to minimum surface tension, which provides minimum energy barrier for oil to penetrate to the fibrous porous structures and then pass through the fabric. In contrast, we also use PIM-1/PTES coated as-received, no etching treated fabrics to separate the same oil-water mixture. At the same condition, the oil passing rate is lower than etched treatment fabric. (See Supporting Information video, video S3) The main reason attributes for the large pore size of the etched fabric. The bigger the pore size is, the easier for oil to pass through. This phenomenon is more obvious when separating highly viscos oil/water mixture.

Figure 6. Surface-tension-driven, gravity-assisted oil-water separation and collection with superhydrophobic fabric-capped device. (a-c) Water cannot penetrate into container. (d-f) Temporal stages of hexadecane oil collection side views. The oil-water separation performance of the superhydrophobic C-fabric after abundant of separation times under the same experimental condition is tested to evaluate it reusability. As presented in Figure 8, after 30 recycles, separation efficiency of superhydrophobic C-fabric


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ranges from 99.99% to 99.96% and water contact angle varies from 158° to 153°, indicating that the superhydrophobic C-fabric still has high separation efficiency. In addition, the C-fabric exhibits robust superhydrophobicity towards water with a broad range of pH (Figure 7b). In the above experiment, superhydrophobic C-fabric displays wonderful water-oil separation efficiency and reusability.

Figure 7. (a) The separation efficiency of different kinds of oil-water mixture. (b) The relationship between pH and the WCAs of the C-fabric.

Figure 8. The variations of (a) The water contact angle and (b) the separation efficiency with the increases in the recycle numbers of the C-fabric in an oil-water separation process. 4. CONCLUSIONS


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In summary, we have demonstrated an approach to prepare superhydrophobic and superoleophilic fabrics for oil-water separation. The obtained superwettable fabric was utilized to assemble an oil skimmer container. Such a new surface-tension-driven, gravity-assisted, onestep, oil-water separation device was used to separate oil-water mixture with stable separation efficiency as high as 99.96% after 30 recycles. ASSOCIATED CONTENT Supporting Information. Videos of oil-water mixture separation process. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (P.L.); [email protected] (B.C.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank the National Natural Science Foundation of China (51403012), the Major Project of Science and Technology Research from the Chinese Ministry of Education


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(308003), the State Key Laboratory of Organic-Inorganic Composites of BUCT (22010006013) and the Fundamental Research Funds for the Central Universities (buctrc201415).


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