Preparation of Superwetting Porous Materials for Ultrafast Separation

Article pubs.acs.org/Langmuir

Preparation of Superwetting Porous Materials for Ultrafast Separation of Water-in-Oil Emulsions Chih-Feng Wang* and Liang-Ting Chen Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan ABSTRACT: Functional materials with a superwetting surface property have been extensively explored to achieve emulsion separation. In this paper, we report a simple and inexpensive method for fabricating superhydrophobic/superoleophilic porous materials from polymeric sponges. These microstructured porous materials, which do not contain any fluorinated compounds, maintain their superhydrophobicity and superoleophilicity after long-term organic solvent immersion and display environmental stability. These superhydrophobic porous materials can effectively separate a wide range of water-in-oil emulsions including surfactant-free and surfactant-stabilized water-in-oil emulsions with high efficiency (>99.98%) and high flux (up to 155 000 L m−2 h−1 bar−1). Meanwhile, these materials exhibited excellent pH resistance and antifouling properties. The high performance of our superhydrophobic porous materials and their efficient, low-energy, costeffective preparation suggest that they have a great potential for practical applications.

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INTRODUCTION Nowadays, tremendous threats both on the environment and on human health are caused by large quantities of oily wastewater expelled from our daily life and industrial processes. With the expansion of oil production and transportation, there is an increasing potential for oil spills from industrial accidents or the sinking of oil tankers or ships.1,2 The International Tanker Owners Pollution Federation has reported that approximately 5.72 million tons of oil was lost as a result of over 1800 large oil tanker accidents from 1970 to 2015. In addition, oily wastewaters containing emulsified oil/water mixtures can cause severe environmental and ecological problems and can also threaten human life. Accordingly, there remains a need to develop new materials for the separation of immiscible oil/water mixtures and oil/water emulsions. Recently, spongelike porous materials possessing both superhydrophobic and superoleophilic properties have become attractive because of their capacity to efficiently separate oil/water mixtures.3−20 Wang et al. described microfibrillated cellulose fiber sponges possessing superhydrophobicity and superoleophilicity for the efficient separation of oil from water.3 Gao and co-workers prepared superhydrophobic and superoleophilic carbon soot sponges that absorb a broad range of organic solvents efficiently and with high selectivity.4 Wang et al. employed a thermally induced phase-separation technique to prepare superhydrophobic polypropylene sponges, which were applicable for water/oil separation.5 Although most of these materials can facilitate immiscible oil/water separation, they are not effective in removing water droplets of small diameters from water-in-oil emulsions, especially for surfactantstabilized emulsions having droplet sizes of less than 20 μm. Therefore, the need remains to develop efficient, cost-effective, © 2017 American Chemical Society

and mass-producible materials for the separation of surfactantfree and surfactant-stabilized emulsions. In this paper, we present a facile two-step method for the fabrication of superhydrophobic and superoleophilic porous materials. First, a melamine sponge was compressed into a compact form. Afterward, we anchored hydrophobic polymer coatings onto the frames of the sponges to change their wettability from hydrophilic to superhydrophobic. The asprepared superhydrophobic porous materials possessed excellent repellency not only for pure water but also for corrosive aqueous liquids. Interestingly, we found that the superhydrophobic porous materials could separate both surfactantfree and surfactant-stabilized water-in-oil emulsions with fluxes of up to 155 000 L m−2 h−1 bar−12 to 3 orders of magnitude higher than that of commercial filtration membranesand with high separation efficiencies (>99.98 wt % in terms of oil purity in the filtrate). The excellent performance of our superhydrophobic porous materials in water-in-oil emulsion separation and their simple preparation through an industrially feasible process suggest that they have great potential applicability in both academic and industrial settings.

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EXPERIMENTAL SECTION

Materials. Commercial melamine sponges were purchased from BASF. Span 80 was obtained from Acros. SE 1700, a hydrophobic adhesive, was supplied by Dow Corning. Superhydrophobic Porous Material. The superhydrophobic porous material was prepared using a two-step method. First, a Received: December 4, 2016 Revised: December 30, 2016 Published: February 1, 2017 1969

DOI: 10.1021/acs.langmuir.6b04344 Langmuir 2017, 33, 1969−1973

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Langmuir melamine sponge was compressed into a compact form having a volume approximately one-tenth of that of the pristine sample. The compressed melamine sponge (thickness: 48 mm) was immersed into a hydrophobic polymer solution (containing 2.50 g of SE 1700, 0.25 g of curing agent, and 50 mL of EtOAc) and then cured in an oven (80 °C, 16 h). Water-in-Oil Emulsions. Surfactant-free water-in-oil emulsions were prepared by mixing water with an oil (n-hexane, n-hexadecane, noctane, isooctane, or toluene; 1:9 v/v) and then sonicating for 1 h to produce a white solution. To prepare surfactant-stabilized water-in-oil emulsions, Span 80 (0.08 g) was dissolved in oil (n-hexane, nhexadecane, n-octane, or isooctane; 200 mL), water (2.0 mL) was added, and then the mixture was stirred for 3 h. For surfactantstabilized water-in-toluene emulsions, Span 80 (1.30 g) was added into toluene (200 mL), and then water (2.0 mL) also added. The mixture was stirred for 3 h. Instruments and Characterization. The microstructure of the superhydrophobic melamine sponge was characterized using a Hitachi S-4700 scanning electron microscope (acceleration voltage: 15.0 kV). The static contact angles and sliding angles of droplets (5 μL) were measured using an FDSA MagicDroplet-100 contact angle goniometer; each reported contact angle represents the average of six measurements. The water contents in the original emulsions and the corresponding collected filtrates were determined using an MKC-500 Coulometric Karl Fischer moisture titrator. Optical microscopy images were recorded using an Olympus BX51M instrument after placing a drop of an emulsion solution onto a biological counting board.

sponge before and after modification, respectively. Compared with that of the pristine sponge (Figure 1b), the surface morphology of the modified sponge changed such that the diameters of the pores decreased and the skeleton of the sponge was packed more compactly. Besides, the smooth skeleton of the original sponge was covered with the hydrophobic polymer after the modification. The highermagnification image (Figure 1c) of the hydrophobic polymer coating reveals hierarchical structures that existed in the form of craterlike nanostructures. Such a morphology raised the surface roughness dramatically and provided a composite interface in which air became trapped within the grooves beneath the liquid, thereby inducing superhydrophobicity. Typically, superhydrophobicity is lost under harsh environments, such as those containing corrosive acids, bases, and organic solvents. We found that our as-prepared superhydrophobic porous material possessed excellent repellency for corrosive aqueous liquids, including acidic (1.0 M HCl) and basic (1.0 M NaOH) liquids (Figure 1a). Droplets of these solutions stood spherically on the superhydrophobic sponge (contact angles of acidic, alkaline, and salt solutions were all greater than 150°) and rolled off readily. Moreover, the superhydrophobic porous material was very stable against long-term (150 h) immersion in organic solvents. Samples were removed every 6 h and dried before performing water contact angle measurements and sliding angle tests. Figure 2 reveals the negligible effect of long-term immersion in an organic solvent on the superhydrophobicity of the porous material.

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RESULTS AND DISCUSSION Generally, the wettability of a solid surface is controlled by its topographical microstructure and surface chemical composition. Combining hydrophobic polymer coatings with the rough surfaces of compressed melamine sponges provided us with the desired superhydrophobic porous materials. The as-prepared superhydrophobic porous materials shown in Figure 1a

Figure 2. Variations in water contact angle on the superhydrophobic porous material with immersion time in isooctane at ambient temperature.

Wastewater containing emulsified oil/water mixtures is a major environmental issue affecting a range of industries. Direct discharge of such wastewater is extremely harmful not only to the environment but also to human health. Therefore, there remains a need to develop efficient, low-energy, and costeffective processes to meet the stringent standards of emulsion separation. Interestingly, we found that our superhydrophobic porous material can be used for successful separation of emulsified oil/water. To test the separation ability, we passed a series of water-in-oil emulsions, including surfactant-free and surfactant-stabilized emulsions, through the superhydrophobic porous material under a suction vacuum pressure of 10 kPa. The oils were immediately absorbed and permeated through the sponge, causing the emulsion droplets to demulsify and leaving behind the water, similar to previous reports.21 All of the emulsified oil/water mixtures were well-separated and

Figure 1. (a) Photograph of water and oil droplets on the superhydrophobic porous material. (b,c) SEM images of (b) the pristine sponge and (c) the superhydrophobic porous material.

possessed a high water contact angle (160°) and superoleophilicity (contact angles of n-hexane, n-octane, nhexadecane, isooctane, and toluene: all close to 0°). On this superhydrophobic surface, water droplets possessed nearspherical shapes and rolled off easily. Figure 1b,c present the top-view scanning electron microscopy (SEM) images of the 1970

DOI: 10.1021/acs.langmuir.6b04344 Langmuir 2017, 33, 1969−1973

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Langmuir

Figure 3. Photographs of (a) surfactant-free and (b) surfactant-stabilized water-in-isooctane emulsions before and after separation using a superhydrophobic porous material.

collected through a single step. Figure 3a presents the results of separating a surfactant-free water-in-isooctane emulsion through the superhydrophobic porous material. The collected filtrate was transparent, whereas the original feed emulsion was milky white. To confirm the effective separation, we used optical microscopy to observe the droplets in the feed and in the collected filtrate. We did not observe any droplets in the collected filtrate over the whole view, implying that the water had been removed from the surfactant-free water-in-isooctane emulsion. The fluxes of the n-hexane, n-hexadecane, n-octane, isooctane, and toluene emulsions permeating through the superhydrophobic porous material were surprisingly high: 156 700, 27 630, 137 800, 138 100, and 136 600 L m−2 h−1 bar−1, respectively (Figure 4a). The superhydrophobic porous material also displayed high efficiency when separating surfactantstabilized water-in-oil emulsions. Figure 3b reveals that the components of a surfactant-stabilized water-in-oil emulsion could be separated well and collected in a single step. Similar to the results obtained when separating surfactant-free water-in-oil emulsions, we observed no oil droplets in the image of the filtrate, confirming that the superhydrophobic porous material was effective for the separation of the surfactant-stabilized water-in-oil emulsion. The fluxes of all surfactant-stabilized water-in-oil emulsions decreased slightly (155 400, 131 200, 130 900, and 124 400 L m−2 h−1 bar−1 for n-hexane, n-octane, isooctane, and toluene surfactant-stabilized emulsions, respectively), except for that of the surfactant-stabilized water-in-nhexadecane emulsion (9715 L m−2 h−1 bar−1) (Figure 3a). The fluxes of all emulsions were extremely high compared with those of commercial filtration membranes. As summarized in Figure 4b, oil purities of all separated emulsions were greater than 99.98%, with some even reaching up to 99.99 wt %, indicating extremely high separation efficiency. The purities of the original oils used in the experiments were also tested (Table 1); the purities of the filtrates nearly matched those of the pure reagents. Recently, some pioneer researchers have successfully developed superwetting materials for fast separation of water-

Figure 4. (a) Fluxes and (b) oil purities of filtrates after separating various surfactant-free and surfactant-stabilized water-in-oil emulsions.

in-oil emulsions (Table 2).21−33 Shi and co-workers developed ultrathin free-standing single-walled carbon nanotube (SWCNT) network films displaying hydrophobicity and superoleophilicity for ultrafast separation of emulsified oil/ water mixtures.21 Zhang et al. used an inert solvent-induced phase-inversion process to prepare superhydrophobic and superoleophilic poly(vinylidene fluoride) (PVDF) membranes 1971

DOI: 10.1021/acs.langmuir.6b04344 Langmuir 2017, 33, 1969−1973

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Langmuir Table 1. Purities of the Original Oils Used in the Experiments oils

oil purity (wt %)

n-hexane n-hexadecane n-octane isooctane toluene

99.99 99.99 99.99 99.99 99.98

that were applicable in the separations of both a surfactant-free emulsion and a surfactant-stabilized water-in-oil emulsion.22 Recently, Gu and co-workers, who synthesized superhydrophobic polymer/CNT hybrid membranes for a highly effective separation of various surfactant-stabilized water-in-oil emulsions.23 Chu et al. reported that silicone nanofilament-coated porous glass materials can effectively separate surfactantstabilized water-in-oil emulsions with high flux and high separation efficiency.30 Compared with many other kinds of special wettable materials, our superhydrophobic porous material possesses extremely high fluxes and displays outstanding separation efficiencies during the separations of waterin-oil emulsions (Table 2). The method we described herein possesses the advantages of being simple and inexpensive while being able to utilize non-fluorine-containing compounds. Oil fouling during oil/water separation processes is a common and tough issue for many filtration materials. An ideal material for oil/water separation should have good antifouling properties. We tested the antifouling performance of our superhydrophobic porous material by performing a cyclic experiment for the treatment of the surfactant-stabilized waterin-isooctane emulsion. For each cycle, 200 mL of the surfactant-stabilized emulsion was permeated through the sponge and then the sponge was simply washed with acetone. The variation in the flux during this process was also tested. Figure 5 reveals that the flux did not decrease obviously upon increasing the emulsion volume that permeated through the sponge within one cycle and that it returned completely to the starting permeation flux after cleaning. The oil purity in every cycle remained greater than 99.97 wt %; thus, the separation efficiency was not sacrificed during these cycles. These results reveal the excellent antifouling properties of the superhydrophobic porous material during long-term use in the treatment of water-in-oil emulsions.

Figure 5. Real-time monitoring of the separation flux and oil purity in the filtrate during the cycles of a surfactant-stabilized water-inisooctane emulsion separation test using a superhydrophobic porous material.

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CONCLUSIONS Separating water from emulsified oil/water mixtures has become a worldwide subject that is tough and challenging. The mixing of water and oil is a common problem in the petroleum industry. Herein, we have developed a facile, inexpensive method for the fabrication of microstructured porous materials exhibiting superhydrophobicity and superoleophilicity; these materials have practical use in the ultrafast separation of surfactant-free and surfactant-stabilized water-inoil emulsions. The superhydrophobic porous materials can separate surfactant-free and surfactant-stabilized water-in-oil emulsions with ultrahigh permeation fluxes of up to 156 700 and 155 400 L m−2 h−1 bar−1, respectively. The oil purities of all separated emulsions were greater than 99.98%, indicating extremely high separation efficiency. Meanwhile, this superhydrophobic porous material exhibited excellent antifouling properties. We believe that our special wettable materials are promising candidates for practical use in the treatment of wastewater produced industrially and in daily life, providing high-quality water as a result.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 886-7-6577711-3129. Fax: 886-7-6578444. ORCID

Chih-Feng Wang: 0000-0002-0758-7153 Notes

The authors declare no competing financial interest.

Table 2. Comparison of Various Special Wettable Materials Used for Surfactant-Stabilized Water-in-Oil Emulsion Separation materials

flux of surfactant-stabilized water-in-oil emulsions (L m−2 h−1 bar−1)

SWCNT network films PVDF membrane polystyrene/CNT hybrid membranes SiO2-nanoparticle-coated membranes Janus polymer/CNT hybrid membranes fluorinated-silica-nanoparticle-coated papers polydivinylbenzene (PDVB)-coated PVDF membrane SWCNT-based bilayer membrane perfluorodecyltriethoxysilane/CNT hybrid membrane silicone-nanofilament-coated porous glass substrates hydrophobic-polymer-coated melamine sponge

up to up to up to none up to up to up to up to up to up to up to 1972

17 000 1000 7500 9000 16 810 1500 54 000 42 500 120 000 155 000

oil purity (wt %)

ref

>99.95 >99.95 >99.94 >99.96 >99.98 >99.9 >99.98 >99.95 >99.98 >99.98 >99.98

21 22 23 24 25 26 27 28 29 30 this work

DOI: 10.1021/acs.langmuir.6b04344 Langmuir 2017, 33, 1969−1973

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ACKNOWLEDGMENTS This study was supported financially by the Ministry of Science and Technology, Taiwan, Republic of China, under contract no. MOST 104-2221-E-214-048-MY2.

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DOI: 10.1021/acs.langmuir.6b04344 Langmuir 2017, 33, 1969−1973