A novel solution-controlled hydrogel coated mesh for oil/water separation based on monolayer electrostatic self-assembly

Abstract

Solution-controlled hydrogel coated materials for oil/water separation have been successfully fabricated. The as-prepared mesh can switch between superhydrophilicity/underwater superoleophobicity and superhydrophobicity/superoleophilicity through self-assembly and dis-assembly pathways. The PDMAEMA hydrogel coated mesh is firstly prepared via photo induced free radical polymerization. A facile monolayer electrostatic self-assembly method was involved by immersing the hydrogel coated mesh in an ethanol solution of stearic acid for a few minutes. The stearic acid modified mesh converts to superhydrophobicity/superoleophilicity. More importantly, the mesh can return to the original state swiftly after the dis-assembly of the monolayer by immersion in a NaOH aqueous solution. The unique superiority of this material is that the transition of wettability can be achieved in situ, so that oil and water can pass through the mesh in sequence through the control of the NaOH solution. Therefore, the as-prepared meshes have great potential in practical applications dealing with both water-rich and oil-rich oil/water mixture separation with high separation efficiency (>99.3%). The synthetic method is simple, time saving and cost saving.

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1 Introduction

Oil/water separation has always been an important field which is worth exploring. Frequent oil spill accidents and large quantities of oily wastewater discharged from all kinds of industries and our daily life have caused serious environmental pollution.^1–5 In recent years, increasing attention has been paid to the study of oil/water separation materials with special wettability. Wettability is an intrinsic property of a solid surface that is governed by both the inherent chemical composition and the surface geometry.^6–11 Compared with conventional oil/water separation methods utilizing ultrasonic, gravity, adsorption and so on, materials with special wettability are much more stable and effective.¹²

Based on the differences of wettability, three types of separation materials can be summarized: “oil-removing” materials with superhydrophobicity/superoleophilicity,^1,13–17 “water-removing” materials with superhydrophilicity/underwater superoleophobicity and smart separation materials.^18–21 Among these materials, the last type has triggered more attention since it can realize the controllable changes in wettability via altering environment stimuli such as pH,^22–24 light,^25,26 temperature,^27–29 electricity,³⁰ solvent,³¹ etc. For example, a pH-responsive material can be prepared through modifying a self-assembled monolayer of HS(CH₂)₁₁CH₃, HS(CH₂)₁₁NH₂ and HS(CH₂)₁₀COOH on a gold electro-deposited silica wafer.³² By fabricating PNIPAM hydrogel on a silica wafer, a thermo-responsive material can be obtained.²⁸ However, these stimuli-responsive materials have their limitations when used in oil/water separation: the raw materials are too expensive (such as gold), the wettability transition of these smart surfaces can only be changed under certain environment conditions, which usually cannot be realized easily in practical applications. Besides, responsive processes of these materials often require multi-step preparations and sometimes a long period of time.³³ Therefore, it is necessary to come up with new facile and time saving methods to achieve the transition of wettability.

In our work, a solution-controlled hydrogel coated mesh for oil/water separation has been successfully fabricated. Poly (dimethyl amino) ethyl methacrylate (PDMAEMA) hydrogel was chosen as the coating because of its easy way to synthesize, three-dimensional network and remarkable special wetting performance in water.³⁴ The function of PDMAEMA hydrogel is its excellent inherent superhydrophilic behavior when pH is less than 13 at ambient temperature. PDMAEMA hydrogel is a pH responsive material which has been discussed in our previous work. The hydrogel exhibits hydrophilicity/underwater oleophobicity when pH is less than 13 and hydrophobicity/oleophilicity when pH is larger than 13. But the pH of oil cannot be controlled easily and may affect the purity of oil. Therefore, we want to make up this shortage and achieve the transition of wettability through facile self-assembly and dis-assembly methods. Since the PDMAEMA hydrogel has plenty of tertiary amine groups which conjugated with carboxyl groups in stearic acid through electrostatic attraction, with the modification of stearic acid on PDMAEMA hydrogel, the mesh can be used for both oil-rich and water-rich oil/water separation by control.³⁵ As shown in Scheme 1, firstly, the PDMAEMA hydrogel coated stainless steel mesh was obtained through photo induced free radical polymerization. This hydrogel-coated mesh possesses superhydrophilicity and underwater superoleophobicity in an oil/water/solid three phase system. Then, the wettability can be transformed via a monolayer electrostatic self-assembly pathway. When the hydrogel-coated mesh was immersed in an ethanol solution of stearic acid for a few minutes, the stearic acid monolayer could be constructed on the surface of hydrogel driven by the electrostatic attraction between the positively charged tertiary amine groups in PDMAEMA and the negatively charged carboxyl groups in stearic acid. Thus the stearic acid modified mesh displayed superhydrophobicity and superoleophilicity in a liquid/air/solid three phase system. More importantly, it could return to the original state swiftly after the dis-assembly of the monolayer through being immersed in a NaOH aqueous solution.

Scheme 1 Schematic description of the as-prepared mesh and the solution-controlled wettability transition: (a) the explanation of wettability conversion at molecule level, stearic acid molecules can be modified on PDMAEMA molecules and removed using NaOH solution. (b) The preparation processes of the mesh. (c) Wettability transition of the mesh, PDMAEMA hydrogel-coated mesh displays superhydrophilicity and underwater superoleophobicity while stearic acid modified mesh owns superhydrophobicity and superoleophilicity.

The unique superiority of this material is that the transition of wettability can be achieved in situ, so that oil and water can pass through the mesh in sequence through the control of NaOH solution. Other advantages such as simple synthetic methods, time saving, cost saving and high separation efficiency (>99.3%) are obvious. Besides, the method of electrostatic self-assembly can decorate the monolayer on surface easily.³⁶ Moreover, since the material is governed by solution, the change of wettability is not influenced by external stimuli (electricity, light, temperature and so on), which means this mesh can display different wettability in the same condition. In practical applications, the hydrogel coated mesh can be used in the separation of water-rich oil/water mixtures while the stearic acid modified mesh can be used in oil-rich oil/water mixtures separation.

2 Experimental section

2.1 Materials and instrumentation

Dimethyl amino ethyl methacrylate, stearic acid, N,N′-methylene bisacrylamide and 2,2′-diethoxyacetophenone were purchased from J&K Co. Ltd., Beijing, China. Other agents were all purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, P. R. China. All of the reagents were used directly. The surface morphology of the hydrogel-coated mesh and stearic acid modified mesh was obtained via an environmental scanning electron microscope Quanta200. Contact angles (CAs) were measured with an OCA15 machine (Data-Physics, Germany) at ambient temperature. The contact angle of each sample was acquired by testing five different positions and taking an average. The X-ray photoelectron spectra (XPS) were carried out by a Thermo escalab 250Xi spectrometer using an Al Kα X-ray source (1486.6 eV). As for separation efficiency, the water content in oil was measured by using Karl Fischer titrator (Cou-Lo Aquamax KF Moisture Meter, UK) while the oil content in water was obtained by the infrared spectrometer oil content analyzer (CY2000, China).

2.2 Electrodeposition of copper on stainless steel mesh

Cu electrodeposition was performed on each clean stainless steel mesh with pore size about 50 μm (300 mesh size) twice in an aqueous solution of 0.5 M CuSO₄ at a 1.5 V constant potential for 400 s. The whole process is carried out at room temperature. Stainless steel mesh was chosen as the cathode and the working electrode was copper mesh. The material was rinsed and dried carefully after electrodeposition.

2.3 Fabrication of solution-controlled mesh

The PDMAEMA hydrogel-coated meshes were obtained through photo initiated free radical polymerization. The Pregel solution consisted of dimethyl amino ethyl methacrylate (DMAEMA), N,N′-methylene bisacrylamide (BIS), 2,2′-diethoxyacetophenone (DEOP) and PAM (number average molecular weight M_n = 3 000 000) as precursor, crosslinker, initiator and adhesive agent, respectively (50 : 1.5 : 1 : 0.5 by weight). After adding the above agents into distilled water and stirring for 12 h to form a homogeneous solution, the stainless steel mesh was immersed in the solution completely and drawn out carefully with the sticky pregel liquid adhering to the wires. The superhydrophilic/underwater superoleophobic PDMAEMA coated mesh was fabricated by placing the sample under UV (365 nm) light for 90–120 min. Modification of stearic acid was performed via immerging the hydrogel-coated mesh into an ethanol solution of 0.1 M stearic acid for 15 min at ambient temperature, thus the wettability was switched to superhydrophobicity and superoleophilicity. After dipping the mesh into an aqueous solution of 1 M NaOH for 5 min, the wettability regained to the original state.

2.4 Oil/water separation experiments and the test of separation efficiency

The as-prepared mesh was fixes between two fixtures which were attached with glass tubes. The experiments were carried out by pouring oil/water mixtures (50 v/v%) onto the mesh. For the PDMAEMA hydrogel-coated mesh, the fixtures were placed vertically. The characterization of separation efficiency was performed through calculating the rejection coefficient (R(%)) by testing the oil content in water before and after separation:

In eqn (1), C_o and C_p are the oil content in water before and after separation, respectively. While for the stearic acid modified mesh, the fixtures were tilted with an angle of 20° and separation efficiency was determined directly by testing the water content in oil using Karl Fischer titrator.

3 Results and discussion

3.1 Surface morphology

The scanning electron microscopy (SEM) images of PDMAEMA hydrogel coated mesh and stearic acid modified mesh are shown in Fig. 1. We can see the porous stainless steel mesh (300 mesh size) after electron-deposition of copper in Fig. 1a, which demonstrates that the Cu particles have been deposited on mesh surface uniformly. These particles contribute to the roughness of the surface and the polymerization of pregel solution. Fig. 1b is the image of hydrogel-coated mesh. The stainless steel mesh is successfully covered with PDMAEMA on the wires. Although some of the mesh pores still exist hydrogel, the following experiments prove that there is no influence on the oil/water separation since the hydrogel is hydrophilic itself. According to the Wenzel equation:³⁷

cos θ_r = rcos θ (2)

here, r is the roughness factor of the surface (r ≥ 1), θ_r is the contact angle of rough surface. From the equation, we can deduce that the wettability can be magnified by increasing the roughness factor. Fig. 1d is the high-magnification image of a single wire of hydrogel coated mesh, random papillae with hierarchical micro-structures can be observed clearly. Thus these hierarchical structures can significantly enhance the wettability, changing it from hydrophilicity to superhydrophilicity. Fig. 1c is the image of stearic acid modified mesh. The color change of the surface demonstrates that a monolayer of stearic acid has been grafted to the surface of hydrogel. The morphology is similar to Fig. 1b so the roughness factor does not change, long alkane chains of the stearic acid lead to the transition of the wettability from superhydrophilicity/underwater superoleophobicity to superhydrophobicity/superoleophilicity.

Fig. 1 SEM images of the as-prepared mesh: (a) large-area view of the stainless steel mesh after the electro-deposition of Cu. (b) The PDMAEMA hydrogel-coated mesh with microstructures. (c) The mesh after stearic acid modification. (d) The high-magnification image of the hydrogel coated mesh from which the random papillae with hierarchical micro-structures can be observed.

Moreover, to prove that the electro-deposition process of copper is necessary, the PDMAEMA hydrogel coated mesh without electrodeposition was prepared as a control. From the SEM images (see details in ESI, Fig. S1†), the amount of PDMAEMA hydrogel covered on the smooth mesh was much less than in the mesh after electrodeposition. The wettability of the hydrogel coated mesh is not uniform, varied from superhydrophilicity to hydrophilicity. Besides, the transition of wettability could not be realized through the testing of water contact angles (ESI, Fig. S2 and Table S1†). The results indicate that the electrodeposition of copper can contribute to the process of polymerization and improve the adhesion between the mesh and the hydrogel.

3.2 XPS analysis of the chemical composition

The chemical composition of the material at different states measured by XPS has further proved that the monolayer of stearic acid can be successfully modified on the hydrogel surface and removed by a NaOH solution. The basic elements surveyed by scanning binding energy from 0 to 1200 eV are carbon (C), oxygen (O) and nitrogen (N). As shown in Fig. 2, three peaks at 284.9, 399.5 and 551.8 eV are labelled and represent C 1s, N 1s and O 1s, respectively. Table 1 is the elements concentration of C, N and O. As seen from the spectrum and table, after the hydrogel-coated mesh is modified with stearic acid, the contents of C increase from 64.74% to 90.23%. This is attributed to the alkane chains and carboxyl groups of stearic acid molecules. Since the modification process is driven by the electrostatic attraction between the positively charged tertiary amine group in PDMAEMA and the negatively charged carboxyl group in stearic acid (Scheme 1), the long alkane chains are on the top of the surface while the carboxyl groups and tertiary amine groups are at the bottom. Thus the contents of O and N decrease sharply, there is even no element N can be observed after modification. Compared to PDMAEMA coated mesh, there is no visible change of the composition of C, N and O after soaking the stearic acid modified material into an aqueous solution of 1 M NaOH, which illustrates that the mesh can return to the original state.

Fig. 2 XPS analysis of the hydrogel-coated mesh, stearic acid modified mesh and the mesh after immersing in NaOH solution: (a) survey scans the spectral region from 0 to 1200 eV. (b–d) High-resolution XPS C 1s, O 1s and N 1s narrow scans as a function of electron binding energy.

Table 1 Element concentration of the hydrogel coated mesh, the stearic acid modified mesh and the mesh after immersion in NaOH solution based on XPS analysis

	Hydrogel coated mesh	Stearic acid modified mesh	After immersion in NaOH solution
C 1s	64.74	90.23	63.37
N 1s	3.14	0	2.46
O 1s	32.12	9.77	34.17

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3.3 Mesh wettability performance

The surface wettability of the material was characterized using the water contact angle measurement. Fig. 3 shows the water contact angles (WCA) and underwater oil contact angles (OCA) of the PDMAEMA hydrogel coated mesh. A drop of water (2 μL) could penetrate the surface rapidly so the WCA was almost 0°, while an oil droplet (2 μL) displayed spherical shape underwater and the OCA reached to 155.9 ± 2.4°. More importantly, oil contact angles of n-hexane, petroleum ether, gasoline, diesel and dichloroethane were also measured in Fig. 3c, all of the OCAs were larger than 150°. The results demonstrate that this hydrogel coated mesh possesses superhydrophilicity and underwater superoleophobicity suitable to various oils. When the material was dipped in an ethanol solution of stearic acid for 15 min, long hydrophobic alkane chains were modified on the surface of hydrogel. Therefore, the wettability of the material converted to superhydrophobicity with a WCA of 155.9 ± 0.9° and superoleophilicity with an OCA of nearly 0° (Fig. 4a and b).

Fig. 3 The PDMAEMA hydrogel-coated mesh shows special wettability: (a) a water droplet (2 μL) penetrates the mesh quickly. (b) Photograph of a gasoline oil droplet (2 μL) on the mesh underwater with a contact angle of 155.9 ± 2.4°. (c) Underwater oil contact angles of the hydrogel coated mesh for various oil samples, all the OCAs are above 150°.

Fig. 4 (a and b) Special wettability of the stearic acid modified mesh with a water contact angel of 155.9 ± 0.9° and an oil contact angle of 0° (c and d) The wettability of the mesh after immersing in NaOH solution with a water contact angle of nearly 0° and a underwater oil contact angle of 149.4 ± 0.5°.

Besides, this stearic acid modified mesh had a low adhesion for water so the water droplet stayed on the probe, even could not stick to the surface. After the mesh was immersed in an aqueous solution of NaOH for 5 min, the WCAs and underwater OCAs were shown in Fig. 4c and d. Obviously, the monolayer of stearic acid has been removed successfully and the wettability has returned to superhydrophilicity/underwater superoleophobicity.

To prove that the material has low adhesion force, experiments were designed (see details in the ESI, Fig. S3†). For the PDMAEMA hydrogel coated mesh, the material was placed underwater, oil was dropped on the surface, forming the words THU. The oil droplets exhibited ball-shape on the surface and could roll off easily. The phenomenon illustrated that the hydrogel coated mesh possessed superoleophobicity and had low adhesion for oil. As for the stearic acid modified mesh, it was placed in air and water was dropped on the surface. Similarly, the water droplets were spherical and could roll off rapidly, which proved the superhydrophobicity and low adhesion of the material.

3.4 Oil/water separation

Oil/water separation capacities were characterized by a series of proof-of-concept studies. To simulate the separation processes in real applications, gasoline was chosen as the oil sample. The oil/water separation experiments of the PDMAEMA coated mesh were conducted (see details in ESI, Fig. S4 and Video. S1†). The hydrogel coated mesh was fixed between two fixtures which were attached with glass tubes. The fixtures were placed vertically.

When oil/water mixtures (50 v/v%) were poured directly onto the material, water penetrated through the mesh quickly while oil was blocked above the surface due to the superhydrophilicity and underwater superoleophobicity of the hydrogel. Besides, the oil/water separation experiment of the stainless steel mesh was also carried out as a control (see details in ESI, Fig. S5†), both water and oil could pass through the mesh fleetly, indicating the stainless steel mesh did not own the capacity of oil/water separation. The tests demonstrate the function of PDMAEMA hydrogel. After the separation, the mesh was rinsed by distilled water carefully and immersed into an ethanol solution of stearic acid. Since the wettability has changed to superhydrophobicity and superoleophilicity, the separation device was placed with a tilt angle of 20° to ensure that the lighter gasoline could contact with the surface before water. As shown in Fig. 5a and b, the stearic acid modified mesh could separate oil/water mixtures successfully, gasoline passed through the surface freely and water was kept above the mesh. Then a small amount of NaOH solution was added to the water in situ and the water mixtures penetrated the mesh instantly (ESI, Video. S2†), indicating the removal of the stearic acid (Fig. 5c and d). To insure that the monolayer was removed thoroughly, the mesh was immersed in NaOH solution for 5 min. As illustrated in Fig. 6e and f, the material has returned to the original superhydrophilic and underwater superoleophobic state in which water penetrated the mesh while oil was kept in the upper glass tube (see details in the ESI, Video. S3†). Moreover, recycling experiments were conducted by testing WCAs (ESI, Fig. S6†), the wettability of the as-prepared mesh could be transited for several times, between superhydrophilicity of WCA nearly 0° and highly hydrophobicity of WCAs larger than 140°. Compared with the mesh without electron deposition, this mesh is much better in reversibility. In our future work, other attempts such as introducing thiol groups will be used to strengthen the affinity between the hydrogel and the mesh for better recycling tests.

Fig. 5 Oil/water separation experiments of the stearic acid modified mesh and the mesh after immersion in NaOH solution: (a) the stearic acid modified mesh was fixed between two glass tubes with a tilt angel of 20°. (b) After pouring the gasoline/water mixtures, gasoline passed through the mesh quickly while water was blocked above the surface. (c and d) When a small amount of NaOH was added into water, the mixtures penetrated the mesh in a few seconds, indicating the removal of the monolayer of stearic acid. (e and f) After dipping into NaOH solution, the mesh returned to the original state, water can pass through the mesh freely while gasoline stayed on the mesh.

Fig. 6 Separation efficiency of the as prepared meshes for n-hexane and gasoline. Both of the hydrogel coated mesh and the stearic acid modified mesh have a high separation efficiency.

Oil/water separation efficiency was also tested to characterize the performance of the material. Gasoline and n-hexane were chosen as the oil samples. For the PDMAEMA coated mesh, the characterization of separation efficiency was performed via testing the oil content in water before and after separation. While for the stearic acid modified mesh, the separation efficiency was determined directly by testing the water content in oil. As demonstrated in Fig. 6, no matter the hydrogel coated mesh or the stearic acid modified mesh, the separation efficiencies were all above 99.3%, indicating the excellent separation capacity of the material.

4 Conclusions

In summary, a solution-controlled hydrogel coated mesh for oil/water separation based on monolayer electrostatic self-assembly has been successfully fabricated. The PDMAEMA hydrogel coated mesh displays superhydrophilicity/underwater superoleophobicity suitable for various oils. After modification of a monolayer stearic acid on the hydrogel, the wettability converts to superhydrophobicity/superoleophilicity so the mesh can selectively separate oil from oil/water mixtures. More importantly, the mesh can return to the original state after the dis-assembly of stearic acid molecules by the immersion in aqueous solution of NaOH. XPS analysis has confirmed the results. Because the wettability of the as prepared material can be converted via the control of NaOH solution in situ, the oil and water can pass through the mesh in sequence. The synthetic method is simple, time saving, cost saving. Therefore, the as-prepared meshes have great potential in practical applications dealing with both water-rich and oil-rich oil/water mixtures separation with high separation efficiency (>99.3%).

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation (51173099, 21134004), the National High Technology Research and Development Program of China (2012AA030306), and the National Research Fund for Fundamental Key Projects (2011CB925700).

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Footnotes

† Electronic supplementary information (ESI) available: Low adhesion force experiments, WCA images, photos and videos of oil/water separation experiments. See DOI: 10.1039/c4ra09140j

‡ Weifeng Zhang and Yingze Cao contribute equally to this work.

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