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石彦龙, 汪志丹, 方芸, 吕涛, 冯晓娟, 冯雷, 杨武. 超疏水超亲油铜网的制备及其在油水分离中的应用[J]. 应用化学, 34(4): 472-480
SHI Yanlong, WANG Zhidan, FANG Yun, et al. Fabrication of Superhydrophobic-Superoleophilic Copper Mesh and Its Application in Oil-Water Separation[J]. Chinese Journal of Applied Chemistry, 34(4): 472-480  
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超疏水超亲油铜网的制备及其在油水分离中的应用
石彦龙a,b,*, 汪志丹a, 方芸a, 吕涛a, 冯晓娟a, 冯雷a, 杨武b
a河西学院化学化工学院,甘肃省河西走廊特色资源利用省级重点实验室 甘肃 张掖 734000
b西北师范大学化学化工学院,生态环境相关高分子材料教育部重点实验室 兰州 730070
通讯联系人:石彦龙,讲师; Tel/Fax:0936-8282066; E-mail:yanlongshi726@126.com; 研究方向:功能材料
修回日期:2016-07-28 接受日期:2016-09-29
基金:国家自然科学基金(20873101)、甘肃省高等学校科研项目(2016B-091)、甘肃省河西走廊特色资源利用省级重点实验室面上项目(XZ1603)、2015年国家级大学生创新创业训练计划项目(201510740004)、2016年甘肃省省级大学生创新创业训练计划项目(201610740004)和河西学院第六批大学生科技创新项目(001,117)资助;
摘要

以铜网为基底,通过浸涂法在其表面制得超疏水超亲油有机-无机复合薄膜,水滴、油滴在其表面的接触角分别为152°和10°。 线性低密度聚乙烯-SiO2纳米球构成的复合阶层结构及低表面能线性低密度聚乙烯涂层的协同作用使铜网产生独特的润湿性。 该铜网具有很好的自清洁性和抗腐蚀性,可用于油水混合物的有效分离。 与传统方法相比,该方法制备超疏水-超亲油薄膜方法简单、成本低、无氟,有望在实践中得到应用。

关键词: 超疏水; 超亲油; 线性低密度聚乙烯; SiO2; 油水分离
中图分类号:O647.5 文献标志码:A 文章编号:1000-0518(2017)04-0472-09
Fabrication of Superhydrophobic-Superoleophilic Copper Mesh and Its Application in Oil-Water Separation
SHI Yanlonga,b, WANG Zhidana, FANG Yuna, LYU Taoa, FENG Xiaojuana, FENG Leia, YANG Wub
aKey Laboratory of Hexi Corridor Resources Utilization of Gansu Universities,College ofChemistry and Chemical Engineering,Hexi University,Zhangye,Gansu 734000,China
bKey Laboratory of Eco-Environmental Related Polymer Materials of Ministry of Education,College of Chemistry and Chemical Engineering,Northwest Normal University,Lanzhou 730070,China
Corresponding author:SHI Yanlong, lecturer; Tel/Fax:0936-8282066; E-mail:yanlongshi726@126.com; Research Interests:functional materials
Revised:2016-07-28 Accepted:2016-09-29
Fund:Supported by the National Natural Science Foundation of China(No.20873101), the Scientific Research Foundation of the Higher Education Institutions of Gansu Province(No.2016B-091), the General Program of the Key Laboratory of Hexi Corridor Resources Utilization of Gansu Universities(No.XZ1603), 2015 National Training Program of Innovation and Entrepreneurship for Undergraduates(No.201510740004), 2016 Provincial Training Program of Innovation and Entrepreneurship for Undergraduates(No.201610740004), the Sixth Scientific Innovation for College Students of Hexi University(No.001, No.117);
Abstract

An organic-inorganic composite film with superhydrophobicity and superoleophobicity was constructed on copper mesh by facile dip-coating. The water contact angle on the film is 152° while the oil contact angle is less than 10°. It is confirmed that the cooperative effect of the hierarchical structures composed by linear low density polyethylene(LLDPE) and SiO2 nanoballs, and the nature of the LLPDE contribute to the wettability. The film with unique wettability exhibits self-cleaning and good anti-corrosion properties, and can be employed to effectively separate oil from water. Compared with traditional techniques for constructing superhydrophobic-superoleophilic film, the method adopted here is simple, low cost, fluorine-free, and may be applied in practice.

Keyword: superhydrophobicity; superoleophobicity; linear low density polyethylene; SiO2; oil-water separation

Wettability is defined as the process of replacing gas-solid contacts by liquid-solid, and which is often characterized by contact angle(CA). For a solid surface, on which the contact angle for water or oil is larger than 150° or less than 5°, it is called superhydrophobic/superoleophobic[1] or superhydrophilic/superoleophilic[2]. The typical example of superhydrophobicity in nature is the surface of lotus leaf, which is attributed to the synergistic effects of surface hydrophobic epicuticular wax and surface multiscale structures with randomly distribution micro-papillae covered by branch-like nanostructures[3,4]. Inspired by the superhydrophobicity effect of lotus leaf, constructions of superhydrophobic materials have aroused worldwide attention due to their great potential in fundamental research and practical applications, such as anti-corrosion, self-cleaning, anti-sticking, anti-biofouling, water proofing and oil-water separation, microfludic control, liquid transportation and drug delivery[5,6,7,8,9,10,11,12,13,14,15]. Traditionally, artificial surfaces with superhydrophobicity are often fabricated by first creating structures with micro/nanometer dual scale and subsequently modifying the rough surface with low surface energy materials[3]. Recently, with the increasing water pollution accidents caused by spilled oil(such as the 1967 Torrey Canyon oil spill and the latest 2011 Bohai Bay oil spill), which have disastrous impact on ecosystem and seriously threaten people's health, how to develop advanced techniques for collecting and removing spilled oil from water is highly desirable. Thanks to the advancing of artificial superhydrophobic material, a good and feasible solution to the headache problem may be obtained. Superhydrophobic materials with superoleophilicity, which are also referred to as “oil-removing” materials, can be employed to separate oil-water mixture because of their capacities of selectively absorbing oil and completely repelling water[16].

In the published literatures, metal meshes with pore-structured semipermeable barriers, flexible and inexpensive textiles, commercially available 3D(three dimensional) porous materials and gels with sponge-like porous structures are often employed as ideal substrates to construct superhydrophobic-superoleophilic materials for oil-water separation[17]. In 2004, Jiang's research group[18] first pioneered research in the application of superhydrophobic-supreoleophilic material in separating oil-water mixtures. Briefly, a stainless mesh with superhydrophobicity and superoloephilicity was obtained by dip-coating and subsequent drying, the contact angle of water on the coated stainless mesh is greater than 150°, whereas that of diesel oil is around 0°. With the unique wettability, water droplets could not penetrate through the coating while oil could easily pass through the film, consequently, with the aid of the mesh film, mixtures of oil and water could be effectively separated. Moreover, Wang et al[19] fabricated a superhydrophobic and superoleophilic film on copper mesh by combining electrochemical deposition and subsequent modification of the long-chain fatty acid monolayer, which could be employed for the effective separation of oil from water. Guo et al[20] also constructed a Cu(OH)2 film on copper mesh by chemical vapor corrosion, after being modified by 1-decanethiol, the film exhibited superhydrophobicity and superoleophilicity and could be applied to separate oil from water. Furthermore, copper mesh was also employed to construct a miniature oil containment boat by electrodepositing Cu2O film on its surface, and the boat can be used for separating oils from water due to its properties of superhydrophobicity and superoleophilicity[21].

As described above, copper mesh is an ideal substrate to construct superhydrophobic-superoleophilic materials for separating oil from water. However, the construction process described above are still a little complicated, expensive and not suitable for large scale production in practice. Therefore, it is highly desirable to explore a more facile and low-cost method to fabricate superhydrophobic-superoleophilic film for separation of oil-water mixtures. Herein, we report a facile procedure of constructing linear low-density polyethylene(LLDPE)-SiO2 composite film with superhydrophobicity and superoleophilicity on commercially available copper mesh by one-step dip-coating without further modification. The water CA on this film is found to be larger than 150°, and the diesel oil CA is less than 5°. To the best of our knowledge, no such procedures have been reported to date. The film with unique wettability shows self-cleaning, anticorrosion properties and can be employed to separate oil-water mixtures with high efficiency. From the point of practical applications, this kind of film may be more practical and is expected to be applied in the construction of chemical engineering materials and microfluidic devices. Compared with the traditional techniques for constructing superhydrophobic materials, the method presented here is facile, low-cost, fluorine-free, and versatile for large area fabrication of superhydrophobic materials.

1 Experimental
1.1 Materials and reagents

Copper mesh, which is knitted by wires with a diameter of 135 μm and the length and width of pore size of the copper mesh is about 375 μm and 285 μm, respectively, was obtained from Shanghai Jinshan Chemical factory, China. LLDPE was provided by Lanzhou Petrochemical Company, China. And xylene(analytical grade, ≥99.0%) was purchased from Beijing Chemical Works, China. Tetraethylorthosilicate(TEOS, analytical grade, ≥28.4, counted as SiO2) was obtained from Sinopharm Chemical Reagent Co., Ltd. Ammonium hydroxide(NH3·H2O, analytical grade, 25%~28%, counted as NH3) was obtained from Sichuan Xilong Chemical Co., Ltd, China; Methyl trimethoxysilane(MTMS, analytical grade, 98%) was purchased from Aladdin Industrial Corporation, Shanghai, China. The absolute ethanol(EtOH, analytical grade, ≥99.7%) was provided by Tianjin Fuyu Fine Chemical Co., Ltd.

1.2 Pretreatment of copper mesh

Copper mesh was first ultrasonically rinsed in 0.1 mol/L HCl, ethanol and distilled water for 10 min each, and then dried in an oven at 75 ℃ for 30 min before dipping coating.

1.3 Fabrication of SiO2 sol-gel

SiO2 gel was prepared by the base-catalyzed hydrolysis of tetraethylorthosilicate[22]. Firstly, ammonium hydroxide, distilled water and absolute ethanol were mixed and stirred for 30 min at room temperature. Then, a mixed solution of TEOS and absolute ethanol was added dropwisely to the above solution and kept stirring at 60 ℃ for 90 min. Subsequently, another mixed solution of MTMS and absolute ethanol was also added into the solution and kept stirring for 24 h at 60 ℃. In the final solution, the molar ratio of MTMS/TEOS/NH3·H2O/H2O/EtOH is 0.24:0.24:1.04:4.00:13.92.

1.4 Fabrication of composite film of LLDPE-SiO2by one step of dipping-coating

In this step, 1.5 g LLDPE was firstly dissolved in 60 mL xylene and kept stirring for 30 min at 120 ℃, and then, 30 mL SiO2 sol was added dropwisely into the LLDPE solution and stirred for another 30 min. Afterward, the cleaned copper mesh was vertically dipped into the solution and drawn out with a speed of 12 cm/min. To ensure adequate coating of mixed LLDPE-SiO2 on copper mesh, the dip-coating procedures were performed twice. Subsequently, the copper mesh coated by LLDPE-SiO2 was kept in a fridge at 8 ℃ for 24 h, and then, the copper mesh began to exhibit unique properties of superhydrophobicity and superolephilicity. The schematic process of fabricating superhydrophobic/superoleophilic copper surfaces is shown inFig.1.

Fig.1 Schematic illustration of the fabrication of superhydrophobic-superoleophilic copper mesh

1.5 Characterizations

Surface microstructures were observed by a Quanta 450 scanning electron microscope(SEM, FEI, USA). Water contact angles(CA) and sliding angles(SA) were measured by a SL200KS(Kino, USA) contact angle measurement system at room temperature, which is equipped with a video camera and a tilting stage. Water or diesel oil droplets(about 8.0 μL) were dropped carefully onto the coated copper mesh and the average contact angle value was obtained by measuring five different positions of the same sample.

Electrochemical corrosion behavior of samples in 3.5%(mass fraction) NaCl solution was performed by using a CHI 660E electrochemical workstation(Shanghai CH Instruments, China) at room temperature. A saturated calomel electrode(SCE) was used as a reference electrode, the copper mesh and a platinum stick were employed as the working and counter electrodes, respectively. The corrosion rate of samples was investigated by polarization curves, which were obtained with a scanning rate of 5 mV/s at ambient temperature.

2 Results and discussion

SiO2 gel was formed due to the following steps of hydrolysis and subsequent condensation polymerization. With the addition of LLDPE, particles of SiO2 were embedded in LLDPE, and then attached on copper mesh by dip-coating.

Si(OEt)4+4H2O=Si(OH)4+4EtOH

Si(OH)4= nSiO2+2 nH2O

It is well known that the wettability of a solid surface is governed by its surface morphology and chemistry.Fig.2 A~2 B are low-magnification SEM images showing that the copper wires are fully coated by the film of mixed SiO2-LLDPE, but the pores between wires are still kept well and the pore sizes are about 200~300 μm.Fig.2 C is a higher-magnification view showing that the microballs of SiO2 were imbedded in LLDPE, and there are many cavities with irregular geometry, and the diameters of cavities vary from 1 μm to 20 μm. Moreover, A high-magnification image reveals that microballs of SiO2 imbedded in LLDPE with higher density are piled up and the diameter is approximately 400~600 nm, as shown inFig.2 D.

Fig.2 SEM micrographs of a copper mesh coated by SiO2-LLDPE at different magnifications. Inset in D is a photograph of a 8 μL water droplet on the dip-coated copper mesh
A, B.corresponds to low magnification; C, D.corresponds to high magnification

As depicted above, the hierarchical structures with micro(copper wires and LLPDE) and nanometer(SiO2spheres) scale are favorable for the formation of superhydrophobicity. As for the formation mechanism of hierarchical structures, according to the literatures[23,24], the lower solvent evaporation temperatures increase the nucleation rate, pore formation and crystallization time, which is beneficial for a higher overall crystallinity. Consequently, the inhomogeneity and size of the pores increase with the lower solvent evaporation temperature. In addition to hierarchical structures, a low surface energy is also required for obtaining superhydrophobicity, which is provided by the long chains of LLDPE[25], and the XPS spectra were employed to reveal the composition of film. The XPS full survey spectra of the film on copper mesh prepared by dip-coating are given inFig.3. It is shown that the XPS signals of C, O, Si and Cu are observed on superhydrophobic surface. The XPS spectra of typical elements of Cu2 p3/2, Cu2 p1/2, O1 s and Si2 p, which are centered at 936, 956, 530 and 101 eV, respectively, demonstrate that the film coated on copper mesh is composed of LLDPE and SiO2.

Fig.3 XPS full spectra of the as-prepared film on copper mesh by dip-coating

As described above, with the synergistic effect of micro/nano-sized structures and low surface energy LLDPE, the dip-coated copper mesh exhibits properties of superhydrophobicity and superoleophilicidy. On the as-prepared copper mesh, the water droplet could retain spherical shape. With a slight tilt and the water droplet began to slide off, the static water contact angle and sliding angle were estimated to be about 152° and 8°, respectively. However, when a droplet of diesel oil was placed on the dip-coated copper mesh, it collapsed quickly and penetrated through the copper mesh, and the oil contact angle was roughly estimated to be about 0°. As proposed by Cassie and Baxter[26,27], the superhydrophobicity, characterized by the high contact angle and small sliding angle of water droplets on dip-coated copper mesh, has been attributed to a layer of air pockets formed between water droplets and copper mesh surface, which completely prevent the penetration of water droplets on dip-coated copper mesh. According to the Cassie equation:

cos θ*= f1cos θY-f2

where f1 and f2 are the area fractions occupied by solid and air on the contacting interfaces of water droplet and as-prepared copper mesh( f1 +f2=1), θ* is apparent CA of water dropet on superhydrophobic copper mesh coated by SiO2-LLDPE, and θY is the intrinsic CA of water droplet on flat copper substrate coated by SiO2-LLDPE, which is estimated to be about 100°. With the given value of θ*=152°and θY=100°, and then, f1 and f2 can be calculated to be about 14% and 86%. The results reveal that on the composite contacting state, water droplets are suspended on superhydrophobic copper mesh due to the existed air layer, consequently, water droplets begin to slide off with a small tilt.

In addition, in order to confirm the existence of “air pocket” on dip-coated copper mesh, the as-prepared copper mesh was immersed in water and a silver mirror-like appearance was observed on the surface compared with the pristine cooper mesh, as shown inFig.4 A and 4 B. This mirror-like interface is formed due to the trapped air layer between water and the superhydrophobic copper mesh, which further confirms that the contacts of water droplets and SiO2-LLDPE coated copper mesh is a composite state of Cassie-Baxter.

Fig.4 Photographs of pristine( A, C, E) and superhydrophobic( B, D, F) copper meshes immersed in water( A, B), covered by graphite dusts( C, D) after being washed by water droplets( E, F)

It has long been known that lotus leaves exhibit excellent water-repellence and self-cleaning performance. The as-prepared copper mesh with superhydrophobicity could also offer such a self-cleaning capability. First, dusts of graphite was put on both superhydropobic and pristine copper mesh, as shown inFig.4 C and 4 D, after that, water droplets with a volume of 8.0 μL were carefully dripped onto the copper mesh surface covered by graphite dusts, and it was found that the dusts on superhydrophobic copper mesh could be washed away easily as water droplets rolled off the surface while most dusts on pristine copper mesh still adhered to the surface, as shown inFig.4 E and 4 F. The distinct difference of adhesion between superhydrophobic and pristine copper mesh reveals that the self-cleaning characteristic could be improved by the formation of superhydrophobicity, and the adhesion work of water droplets on the superhydrophobic copper mesh could be roughly estimated by employing the following Young-Dupre equation[28,29].

Wad= fs· γL(cos θY+1)

where γL is the surface tension of water, γL=7.2×10-2 N/m, and the CA in the Young model θY=100°, which was obtained by measuring the CA of a flat copper coated by SiO2-LLDPE, fs is the area fraction of a water droplet in contact with SiO2-LLDPE surface, which is equals to the value of f1( f1=14%) in Cassie Baxter equation. With the known values, the adhesion work of Wad can be estimated as about 8.3×10-3 N/m. The small adhesion work is responsible for the high CA, low sliding angle and the excellent self-cleaning performance.

2.1 Oil-water separation

In the past decades, porous materials with superhydrophobicity and superoleophilicity have aroused broad interest, which are believed to be promising for highly efficient oil/water separation because of their selective filtration and absorption. Herein, the as-prepared copper mesh with superhydrophobicity and superoleophilicity could also be used to separate water-oil mixture. In this system, when mixtures of oil and water were poured on top of the copper mesh, diesel oil firstly falls off, spreads out, penetrates through the copper mesh and falls into the beaker, while water droplets roll over the copper mesh surface and subsequently fall into the petri dish, as shown inFig.5 A~5 C. With the process, the initial 50 mL oil-water mixtures(the volume of water and oil is 25.00 mL, respectively), were effectively separated, and the volumes of collected water and oil were 24.5 mL, 24.0 mL, respectively, as seen inFig.5 D. With the measured values, the corresponding separation efficiency of water and oil were calculated to be about 98% and 96%, respectively. As a material for potential application in practice, the recyclability and the durability of the obtained superhydrophobic-superoleophilic copper mesh should be evaluated. After the experiment of oil-water separation, the contaminated copper mesh was rinsed thoroughly by alcohol to remove the absorbed oil. After that, the rinsed copper mesh was dried in an oven at 60 ℃ for 30 min, and the initial superhydrophobicity-superoleophilicity was easily recovered. As for the oil-water separation, the as-prepared copper mesh could be repeatedly used for at least 10 times, and the separation efficiencies were not obviously decreased. In addition to diesel oil, the as-prepared copper mesh can also be employed to separate dodecane and hexdecane from water with high efficiency.

Fig.5 Photographs of the procedures of oil-water separation with the as-prepared copper mesh
A.oil-water mixtures; B, C.oil and water after separation; D.collected water and oil
The inset in B is an image of water droplet on superhydrophobic copper mesh, water and diesel oil were respectively stained with methylene blue and oil red dye for clear observation

2.2 Anticorrosions

The anticorrosive performance of copper with superhydrophobicity was evaluated by the potentiodynamic polarization curves. As shown inFig.6, extrapolation of cathodic and anodic polarizations curves to their intersection point provides both the corrosion potential and the corrosion current. Before surface treatment, the corrosion current and current potential of naked copper mesh in 3.5% NaCl aqueous solution is 1.3×10-4 A and -0.25 V, respectively. However, the dipping of SiO2-LLDPE on copper mesh made the corrosion potential shift positively to -0.21 V, and the corrosion current( Icorr) was significantly decreased from 1.3×10-4 to 2.09×10-5 A. The positive shift of corrosion potential and the drastic decrease of corrosion current reveal that the existence of superhydrophobic film on copper mesh could significantly improve the anticorrosion performance of copper mesh. Compared with the naked copper mesh, three distinct regions appeared in the anodic polarization region of dip-coated copper mesh[30]:(Ⅰ)Copper was oxidized to Cu+ in the potential extended from the Tafel region to the maximum current density value. (Ⅱ)Cu+combined with Cl-, and the anodic current declined rapidly until the minimum value reached. (Ⅲ)CuCl combined with chloride ions to form CuC l2-, and the diffusion velocity of CuC l2-is greater than Cl-, so the current density increases again due to the reaction of CuCl+Cl-→CuC l2-.

Fig.6 Polarization curves of the pristine and superhydrophobic copper mesh

As described in literature[31], the anticorrosive performance of a coating is mainly dependent on the following three aspects: water sorption of the coating, transport of water in the coating and accessibility of water to the coating/substrate interface. Therefore, it is reasonable to believe that the coating of SiO2-LLDPE dipped on copper mesh could sufficiently prevent the water adsorbing onto the copper mesh substrate, and eventually makes the copper exhibit superior corrosion resistance in wet environment.

In addition, the large amount of air trapped on superhydrophobic copper mesh immersed in water could behave as a dielectric for a pure parallel plate capacitor, which can effectively inhibit the electron transfer between the electrolyte of NaCl and the copper substrate, and finally improve the corrosion resistance[32,33].

3 Conclusions

In conclusion, we have shown the preparation of a superhydrophobic-superoleophilic copper mesh by a facile dip-coating method. On the copper mesh dip-coated by linear low density polyethylene(LLDPE), the water contact angle is larger than 150° and the oil contact angle is less than 10°. Moreover, the copper mesh treated by dip-coating exhibits remarkable loading capacity, significant anticorrosion performance and superhydrophobic cleaning. With the special performance of superhydrophobicity and superoleophilicity, the copper mesh could be employed to separate oil-water mixture with high efficiency and superior recyclability, which may be expected to be applied in practice.

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