Water Separation on the Switchable

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pH-Controllable On-Demand Oil/Water Separation on the Switchable Superhydrophobic/Superhydrophilic and Underwater Low-Adhesive Superoleophobic Copper Mesh Film Zhongjun Cheng,† Jingwen Wang,*,‡ Hua Lai,† Ying Du,† Rui Hou,† Chong Li,† Naiqing Zhang,† and Kening Sun*,† †

Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150090, People’s Republic of China ‡ Heilongjiang Science and Technology Information Research Institute, Harbin, Heilongjiang 150090, People’s Republic of China S Supporting Information *

ABSTRACT: Recently, materials with controlled oil/water separation ability became a new research focus. Herein, we report a novel copper mesh film, which is superhydrophobic and superhydrophilic for nonalkaline water and alkaline water, respectively. Meanwhile, the film shows superoleophobicity in alkaline water. Using the film as a separating membrane, the oil/water separating process can be triggered on-demand by changing the water pH, which shows a good controllability. Moreover, it is found that the nanostructure and the appropriate pore size of the substrate are important for realization of a good separation effect. This paper offers a new insight into the application of surfaces with switchable wettability, and the film reported here has such a special ability that allows it to be used in other applications, such as sewage purification, filtration, and microfluidic device.

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oped. Some examples include carbon nanotube film,17,18 multifunctional foam,21 and polydimethysiloxane (PDMS) coated nanowire membrane.33 Recently, inspired by the fish scale’s exceptional antipollution capacity in oil-polluted water,45 Xue et al. advanced a new hierarchical structured hydrogel coated mesh, which can also be used in the oil/water separation.46 Different from those separating films with superhydrophobicity/superoleophilicity, the new separating film is superhydrophilic and underwater superoleophobic, which is resistant to oils and can avoid the oil-polluted troubles. Although numerous separating membranes as mentioned above have been developed, in some special applications such as controlled filtration, removal of water from a microreactor system, and remote operation of oil/water separation units, these films would be unsuitable for the lack of good controllability. To overcome these difficulties, Kwon et al. first advanced a new electric field controlled method:47 the oil/ water separation process can be triggered on-demand through controlling the voltage between the liquid and the separating membrane. After that, Tian et al. reported a similar photocontrollable oil/water separation process on the nanorod-like ZnO membrane.48 However, all these methods need some severe conditions, such as high voltage electric field and special ultraviolet source. Meanwhile, for most common complex

INTRODUCTION In the past few years, with the increase of oil spill accidents and industrial oily wastewater, separation of oil/water mixtures has become a worldwide challenge and has aroused much attention.1 Among lots of separating technologies, membranebased technology is attractive for its energy-efficiency and applicability across a wide range of industrial process. Because wettability of the separating membrane is an important factor that influences the oil/water separation process, designing and fabricating membrane materials with special wettability would be a facile and effective way to solve the problem.2,3 Take inspiration from nature, such as a lotus leaf and the legs of water striders,4,5 people find that the hierarchical micro/ nanostructures can enhance the surface-wetting performances to realize the superhydrophobicity/superhydrophilicity and superoleophobicity/superoleophilicity.6−16 Porous materials with such wetting performances have been widely applied in separating oil and water mixtures,17−45 and it is found that the presence of hierarchical structures can enhance both the separating efficiency and the stability of the separating device. For example, Feng et al. reported a hierarchical structured polytetrafluoroethylene coating mesh.20 The film can be easily wetted by oils while showing antiwetting performance for water because of its superoleophilicity and superhydrophobicity; thus, oil can permeate the film while water cannot. After that, through designing special hierarchical structures on the substrates, many other separating membrane materials with such superhydrophobicity/superoleophilicity have been devel© XXXX American Chemical Society

Received: September 14, 2014 Revised: January 1, 2015

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Figure 1. (a, b) Low- and high-magnified SEM images of the original copper mesh substrates, respectively; (c, d) low- and high-magnified SEM images of the film obtained after immersion in the NaOH and (NH4)2S2O8 solution for 3 min, respectively.

mixtures such as oil-in-water emulsions, the roles of these films are also limited. In this regard, design and fabrication of a new separating film that can realize the controllable oil/water separation more conveniently and efficiently would be necessary and highly desired. In this paper, we report a new oil/water separating membrane, on which the separation process can be triggered on-demand by changing the water pH. The film was prepared through assembling the responsive thiol molecules on the Cu(OH)2 nanorod structured copper mesh substrate. When the water pH is varied from acid to alkaline, the film can transit from superhydrophobicity to superhydrophilicity and meanwhile shows superoleophobicity in basic water. Detailed research indicates that the nanostructures and appropriate pore size on the substrate are crucial for controllable separation. Because the obtained film is so smart, we believe it could be useful in some other fields, for instance, microfluidic devices and wastewater treatment.

with a gold layer (on a sputter-coater Leica EM, SCD500) and were modified with a thiol solution containing HS(CH2)9CH3 and HS(CH2)10COOH for approximately 12 h.51 The whole thiol concentration was 1 mmol L−1 in ethanol, and the ratio of two thiol molecules was varied. At last, these substrates were rinsed with abundant ethanol and were desiccated with N2. A smooth surface was treated with an identical process. Preparation of Oil/Water Mixture. The immiscible oil/ water mixtures were obtained by mixing oil (about 10 mL) and water (about 10 mL) in a beaker and simply shaking. The emulsions were obtained by ultrasonically treating the mixture (Vwater/Voil = 9:1) for about half an hour, and finally, the milky and white emulsion could be obtained. The obtained emulsions were stable for at least half an hour without special protection. Instrumentation and Characterization. The wetting performances on the film were investigated on a contact angle meter (JC 2000D5). For water contact angle measurement, a water droplet was put on the film. For underwater oil contact measurement, the substrates were first fixed in a quartzose container that was transparent and full of water. For 1,2-dichloroethane, the oil droplet was directly put on the film. For oils with lower density than water, such as petroleum ether, the oil droplet was released under the film through an inverted needle. The average values were achieved by examining five points on the identical film. The rolling angles were investigated by tilting the film with a dropler (4 μL) that came in contact with the film until the droplet started to slide. The morphology on the substrates was obtained on a scanning electron microscope (HITACHI, SU8000). Water with different pH values was obtained by adding HCl or NaOH. The water pH was measured on a pH meter (PB-10, sartorius). Photographs in Figure 4 and Figure 5 were obtained on a camera (Canon HF M41). The oil concentration was analyzed using an Infrared Spectrometer Oil Content Analyzer (CY2000, China).

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EXPERIMENTS Materials. Oil red (sudan III), (NH4)2S2O8, petroleum ether, NaOH, methylene blue, HCl, 1,2-dichloroethane, ethanol, and hexane were obtained from Tianjing Fine Chemical Co., China. Copper mesh substrates, 99.9%, were obtained from Shanghai Chemical Reagent Co., China. Diesel oil and gasoline were supplied from Sinopec group. Polydimethylsiloxane (silicon oil), HS(CH 2) 9 CH3 , HS(CH2)10COOH, (Aldrich, Germany), and deionized water were obtained from Milli-Q system, >1.82 MΩ cm. Sample Preparation. The nanostructures on the substrates were prepared through a similar method as already reported.49,50 In brief, the copper mesh substrates were first cleaned in an ultrasonic instrument with acetone and ultrapure water for about 15 min. Then, the clean copper mesh substrates were immerged into a water solution of (NH4)2S2O8 (0.1 M) and NaOH (2.5 M). After a certain time, the substrates were cleaned with pure water and further were desiccated with N2. After the growth of nanostructures, the substrates were covered B

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RESULTS AND DISCUSSION Figure 1a shows the scanning electron microscopy (SEM) image of the copper mesh substrate. One can observe that copper wires with average diameter of about 30 μm (Figure 1b) are weaved together; the average size of the formed square pores is about 48 μm. As reported, nanostructures on the separating substrates can enhance the wetting performance and improve the separating efficiency.20,21 Here, after a simple solution (containing both NaOH and (NH4)2S2O8) immersion process, Cu(OH)2 nanorods would be formed (Figure S1 of the Supporting Information (SI)). As shown in Figure 1c and d, after growth of Cu(OH)2 nanorods, the average diameter of copper wires increases to about 46 μm, and meanwhile, the average pore size decreases to about 32 μm. On the amplified image, it can be seen that the diameter of Cu(OH)2 nanorods ranges from about 80 to 150 nm, and the length of the nanorods approximates to 8 μm (Figures S2−S4 of the SI). After production of Cu(OH)2 nanostructures, the substrates were further modified with mixed thiol molecules HS(CH2)10COOH and HS(CH2)9CH3 to obtain the pHresponsivity.51 We find that the film prepared with XCOOH = 0.6 (XCOOH is the mole fraction of HS(CH2)10COOH in the thiol solution) can transit between superhydrophobicity and superhydrophilicity (Figures S5−S7 of the SI). Figure 2a

high as 160° (Figure 2c, Figures S13−S14 of the SI). Meanwhile, the film is low adhesive to the oil droplet, and the droplet can roll away the film with a sliding angle no more than 5° (Figure 2d), indicating that the film has a good oil resistant ability in alkaline water. Noticeably, such superoleophobicity can remain even after 2 days immersion in the water, demonstrating that the film has a good underwater superoleophobicity in water. In addition to the 1,2-dichloroethane, some other oils, such as diesel oil, silicon oil, hexane, and so forth, were also used as the test droplet. As shown in Figure 3, the underwater superoleophobicity and low adhesion

Figure 3. Statistics of the contact and sliding angles of various oils’ contact with the film in water with pH 12.

can also been observed for these oils, which means that the obtained film has a good universality regardless of the oil type (Figure S15 of the SI). The original smooth copper mesh substrates were also modified with the same mixed thiol molecules, and the wettability was also investigated. Although the pH-responsivity can be observed, the change is limited and the surface cannot reach the superhydrophobicity/superhydrophilicity for different water droplets (Figure S7 of the SI), further confirming that the nanostructures can effectively enhance the surface-wetting performances. As previously reported,20,51 nanostructures on the reticulated substrates are helpful to obtain the amplified wetting performances and to enhance the stability of the separating device. Thus, in the following research, only the Cu(OH)2 nanorod structured substrates will be considered. As described above, the obtained nanostructured film can transit from superhydrophobicity to superhydrophilicity as the water pH is increased, and meanwhile, the film also shows superoleophobicity in alkaline water. On the basis of these results, the on-demand separation of oil/water mixture can be realized on the film through changing the water pH. To illustrate the pH-induced separation process, a device using the obtained film is designed. As shown in Figure 4, the as-prepared film was clamped between two glass tubes, and the pHcontrollable separation process can be described as follows: first, put some pure water into the upper tube and ensure that the whole copper mesh film is under the water level and then add the mixture of water and oil. Take the separation of petroleum ether and water as an example (the red and blue color of oil and water are due to the presence of oil red and methylene blue). Because the film is superhydrophobic for nonalkaline water, the film can withstand the oil/water mixture for the strong negative capillary pressure (Figure 4a). When the

Figure 2. Images of a water droplet (4 μL) with pH (a) 7 and (b) 12 on the prepared film in air. (c) A 1,2-dichloroethane droplet (4 μL) sitting on the film in alkaline water (pH = 12). (d) A 1,2dichloroethane droplet (4 μL) rolling away from the film in alkaline water (pH = 12) with a low rolling angle.

displays a 4 μL neutral water droplet on the film, and the water contact angle (WCA) is about 152°, demonstrating a good superhydrophobicity (Figure S8 of the SI). Increasing water pH to the alkaline condition, the film would show superhydrophilicity with a WCA of about 6° (Figure 2b). Moreover, as more water is added, the water can pass through the film (Figures S9−S11 of the SI). In addition to the water wettability, the oil wettability on the film was also investigated. In air, the obtained film shows superoleophilicity, and the oil (1,2-dichloroethane, 4 μL) contact angle (OCA) is approximate to zero (Figure S12a of the SI). When the film is placed in water, the phenomena would be different. In water with pH 7, the film retains the superoleophilicity (Figure S12b of the SI), whereas in water with pH 12, it becomes superoleophobic, and the OCA is as C

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Figure 6. Statistics of separation efficiency of the as-prepared film for different oils.

Figure 4. pH-controllable oil/water separation device. (a) The pHresponsive film was clamped between two glass tubes. The film is superhydrophobic for nonalkaline water, and the mixture can be supported on the film. (b) By increasing the water pH to basic condition and by the film becoming superhydrophilic, the water would pass through the film and flow into the beaker, while the oil would be retained for the underwater superoleophobicity of the film.

performances. Figure 7 displays the WCAs and OCAs as a function of the pore size of the copper mesh substrates (all the

water pH is increased to the alkaline condition by adding some alkaline water, water permeates the film for the surface superhydrophilicity evoked capillary effect as well as its own gravity. Noticeably, oil cannot pass the film because of the negative capillary effect resulting from the underwater superoleophobicity (Figure 4b). Thus, the mixture of oil and water can be separated on-demand by altering the aqueous pH. In addition to immiscible oil/water mixture, oil-in-water emulsion can also be separated controllably using the same device. For emulsion, the copper mesh film with longer nanorods needs to be used (Figure 5a, from the SEM image, almost no obvious

Figure 7. Water and oil contact angles as a function of the substrate pore size.

films were obtained with the same immersion time and were modified with the solution with XCOOH = 0.6). It can be seen that all the films prepared with pore sizes from 35 to 160 μm have a good hydrophilicity and permeability for alkaline water. Meanwhile, in alkaline water (pH = 12), most of these films show superoleophobicity except those with the pore sizes larger than 140 μm. Thus, the substrates with pore size less than 140 μm are suitable to obtain the underwater superoleophobicity. Noticeably, for neutral water, only substrates with a pore size of about 48 μm is suitable to reach the superhydrophobicity. The film switching from superhydrophobicity to superhydrophilicity and showing underwater superoleophobicity are beneficial to realize good controllability, and one can conclude that the copper mesh with pore size of about 48 μm is the optimal substrate. From the above, it can be seen that the special pH-responsive superhydrophobicity/superhydrophilicity switch and underwater superoleophobicity are pivotal for the controllable oil/ water separation, and these abilities can be explained as follows. The pH-responsive superhydrophobicity/superhydrophilicity can be attributed to the following two factors: one is the variation of surface carboxylic acid groups between protonation and deprotonation as a function of aqueous pH,51,52 and the

Figure 5. (a) SEM image of copper mesh film obtained after 30 min immersion in NaOH and (NH4)2S2O8 solution. Photographs of hexane-in-water emulsion (b) before and (c) after separation.

pores on the membrane can be observed). Figure 5b and c is the photographs of the hexane-in-water emulsion before and after separation, respectively. From the two images, it can be seen that after separation, the milky white emulsion (the droplet sizes of emulsion are ranging from 5 to 40 μm observed by an optical microscopy) becomes clear, indicating a good separating effect. The separation efficiency was further exampled accurately using an infrared spectrometer oil content analyzer. As shown in Figure 6, for both immiscible oil/water mixture and oil-in-water emulsion (Figure S16 of the SI), after separation, the contents of oils are extremely low, indicating that high-separation efficiency can be realized on our films. In this work, we find that the pore size is also important because different pore sizes can result in different wetting D

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Langmuir other is the nanostructures on the substrates which can enhance the wetting performances.6,7 In this work, before modification with mixed thiol molecules, a gold layer has been covered on the Cu(OH)2 nanorods, which can react with the −SH groups, and the −COOH groups can be used as the functional groups.51,52 For nonalkaline water, the protonated carboxylic acid groups that show relative hydrophobicity present on the film. Together with the hydrophobic HS(CH2)9CH3 and the amplified effect of surface nanostructures, the superhydrophobicity and high contact angle can be observed on the obtained film, which can further be proved by the following equation:53

cos θr = f1 cos θ − f2

(1)

Here, θr and θ are the WCAs for a water droplet on the rough and flat copper substrates, respectively. f1 and f 2 are the fraction of solid and air under the water droplet, respectively (i.e., f1 + f 2 = 1). In this work, θr and θ are 152° (Figure 2a) and 93° (Figure S17 of the SI), respectively. According to eq 1, f 2 = 0.876, indicating that the air fraction is enough high to induce the superhydrophobicity. Thus, for nonalkaline water, the WCAs are higher than 150°. As the water pH is changed to the alkaline condition, the deprotonated carboxylic acid groups with better hydrophilicity are formed on the film (Figure S18 of the SI). Meanwhile, the hydrophilicity can be intensified by the nanostructures according to the Wenzel equation.54 As shown in Figure 1, the presence of Cu(OH)2 nanorods can effectively increase the surface roughness. Thus, water can enter into the Cu(OH)2 nanostructures for the three-dimensional capillary effect, and a low WCA can be seen on the film. After cleaning by pure water and further desiccating under N2, the deprotonated carboxylic acid groups returned to their initial state, and the film became superhydrophobic again. Therefore, reversible transition between superhydrophobic/superhydrophilic can be achieved by changing the aqueous pH, and the separating device shown in Figure 4 can be used repeatedly. The underwater superoleophobicity of the film in alkaline water can be ascribed to the cooperation between the hydrophilic surface composition and the nanostructures on the substrate.45 As discussed above, the film is superhydrophilic in basic water. After immersion into the alkaline water, the gaps between the Cu(OH)2 nanorods are occupied by the water. When the oil droplet contacts the film, a composite interface can be formed, and the superoleophobicity can be illustrated by the following modified Cassie equation:45 ′ = f cos θow + f − 1 cos θow

Figure 8. Schematic illustration of the liquid-wetting modes. (a) For nonalkaline water, the film shows hydrophobicity that can sustain the water because Δp > 0. (b) For alkaline water, the film displays hydrophilicity, and it cannot support any pressure because Δp < 0, so water can pass through the film. (c) After water permeation, some water can be trapped among the interspaces between the Cu(OH)2 nanorods, and thus, the film shows oleophobicity, and oil can be supported because Δp > 0.

where γ is the surface tension, l is the pore’s perimeter, R is the meniscus’s radius, A is the pore’s area, and θA is the advancing contact angle on the film. From eq 3, it can be seen that when θA > 90°, the film can withstand the pressure to some extent since the Δp > 0. In this work, for nonalkaline water, the film displays superhydrophobicity, and the θA is apparently larger than 90°; therefore, the water cannot pass the film spontaneously (Figure 8a). The theoretical maximum pressure that the film can support is about 5.39 ×103 pa. As to alkaline water, the film displays superhydrophilicity, and θA is nearly 0°. According to the above equation, Δp < 0, which means that the water can spontaneously pass through the film because the film cannot support any pressure (Figure 8b). After the permeation process, the interspaces between the Cu(OH)2 nanorods would be occupied by water for the superhydrophilicity of the film. The trapping of water can enhance the oil-repellent force, which can lead to the superoleophobicity and high OCA on the film (Figure 2c, the OCA is larger than 90°). Thus, the oil cannot pass through the film as the Δp> 0 (Figure 8c). From the above, it is clear that the film has a good pH-induced permeability for water, and after that, the oil can be retained on the film for the underwater superoleophobicity. Therefore, an

(2)

Here, θow ′ and θow are the OCAs of an oil droplet contact with the nanostructured copper mesh and flat copper substrates, respectively. f represents the area fraction of copper mesh substrate contacts with oil. In this work, θ′ow = 160° (Figure 2c), θow = 130° (Figure S17 of the SI), and f = 0.169, which means that about 83% contact area is the oil/water contact interface. Therefore, the superoleophobic and easily rolling performances can be observed. To make clear the pH-induced oil/water separation process, the water and oil wetting processes are modeled in Figure 8. Generally, an intrusion pressure Δp has to be overcome before the liquid can wet the pore bottom because the advancing contact angle θA has to be exceeded, which can be described as55,56 Δp =

2γ = −lγ(cos θA )/A R

(3) E

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oil/water mixture can be well separated with the film through altering the aqueous pH. The reason for only the substrate with pore size of 48 μm (Figure 7) shows that the superhydrophobicity can also be explained by the above equations. On substrates with pore size less than 48 μm, the liquid/solid contact fraction ( f1 in eq 1) is increased,57 which would result in the decrease of the WCA (according to eq 1). For those with sizes larger than 48 μm, increase of the pore size would decrease the static pressure (according to eq 3), and the hydrophobic force offered by the surface nanostructures would be decreased. As a result, the water can wet more substrates and the f1 in eq 1 would be increased. Thus, as the pore size is increased, lower hydrophobicity would be observed.

CONCLUSION In summary, we report a new pH-controllable oil/water separating membrane. The film was prepared by simply assembling responsive thiol molecules on the Cu(OH)2 nanorod structured copper mesh. For nonalkaline water, the obtained film displays superhydrophobicity, whereas for alkaline water, it becomes superhydrophilic. Meanwhile, the film also shows superoleophobicity in alkaline water. Combining the above properties, both the immiscible oil/water mixture and oil-in-water emulsions can be separated on-demand through changing the water pH. This paper reports a new strategy to realize the controllable separation of oil-and-water mixture, and the as-prepared membrane is so smart that it endows itself with a lot of potential applications, for instance, microfluidic devices, controllable filtration, and wastewater treatment. ASSOCIATED CONTENT

S Supporting Information *

XRD of the film; magnified and cross-sectional SEM image of Cu(OH)2nanorods; dependence of water contact angle on the immersion time; XPS results of the surfaces; dependence of the contact angles on the XCOOH; shapes of a water droplet sliding on the film; process of water permeating the film; reversible transition between the two extremes states; dependence of water and oil contact angle on the water pH. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (+86) 045186412153. Fax: (+86) 045186412153. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC Grant NO. 21304025), the Research Fund for the Doctoral Program of Higher Education of China (20112302120062), the assisted project by Heilong Jiang Postdoctoral Funds for scientific research initiation (LBHQ13063), and China Postdoctoral Science Foundation (2011M500650). F

DOI: 10.1021/la503676a Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/la503676a Langmuir XXXX, XXX, XXX−XXX