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The novel one-step, in-situ thermal polymerization fabrication of robust superhydrophobic mesh for efficient oil/water separation Bin Jiang, Hongjie Zhang, Luhong Zhang, Yongli Sun, Lidong Xu, Zhaoning Sun, Wenhao Gu, Zhenxing Chen, and Huawei Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03063 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Industrial & Engineering Chemistry Research

The novel one-step, in-situ thermal polymerization fabrication of robust superhydrophobic mesh for efficient oil/water separation Bin Jiang, Hongjie Zhang, Luhong Zhang, Yongli Sun, Lidong Xu, Zhaoning Sun, Wenhao Gu, Zhenxing Chen, Huawei Yang*

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China.

Corresponding author at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People's Republic of China. Tel./fax: +86 2227400199. E-mail address: [email protected]

ACS Paragon Plus Environment


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Abstract In this work, a brand new one-step in-situ thermal polymerization (ISTP) preparation of highly stable polymer coated superhydrophobic materials has been reported. Basing on the thermal initiation and nonvolatility of ionic liquid (IL) precursor, robust polymeric layer could be in-situ generated and coated to meshes under air atmosphere, while the anchored nanoparticles could provide hierarchical micro/nanostructure. An “oxidative crosslinking” effect was found and the possible mechanism was proposed. As expected, the obtained mesh exhibited superhydrophobicity with water CA of 158° and superoleophilicity with oil CA of 0°. Besides, the mesh showed selfcleaning effect with a low sliding angle. As for application evaluation, the mesh could act a filter for the highly efficient separation of a series of oil–water mixtures. More importantly, the mesh exhibited excellent stability and durability towards ultrasonic, abrasion treatment, long-term storage, and even under strongly acidic, alkaline and saline environment conditions. In summary, this work provided a novel, facile and scalable method in the fabrication of superhydrophobic surface.


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1. Introduction With the growth of human society and economy, frequent oil spill incidents and industrial oily wastewater have seriously caused ecological environment destruction. Therefore, separation of oil/water mixtures has become an increasingly important subject worldwide. Various traditional industrialized methods, including physical,1,

2

chemical,3

and biological approaches,4 have been used to separate oil/water mixtures, while most of these methods are expensive, energy-consuming, inefficient or easy to cause secondary pollution, etc. Therefore, the development of efficient and inexpensive methods for separating oil/water mixtures is desirable.5-9 Recently, superhydrophobic surfaces with a water contact angle (CA) of higher than 150° have aroused broad attention for both scientific research and practical applications.6, 10-18

In nature, superhydrophobic and superoleophilic surfaces can be constructed through

the strategy of increasing surface roughness with hierarchical micro/nanostructures and chemically modifying them with low surface energy substances.19-26 Among the superhydrophobic porous materials, the stainless steel mesh (SSM) exhibits great ruggedness, porosity and durability, etc.27 Thus, it is regarded as good candidate for treating industrial oily wastewater continuously. Recently, Jiang et al. firstly fabricated superhydrophobic/ superoleophilic mesh films, showing high oil/water separation efficiency and selectivity.28 Following that, various approaches have currently been proposed, including sol-gel method,29 hydrothermal method,30 crystallization control,31 chemical vapor deposition,32 phase inversion,33 electro-deposition method,34,

35

self-

assembly,36 electro-spinning technique,37 spray coating method,38 etc. However, most of these methods have limitations in large-scale fabrication. The toxic reagents with volatility are harmful to the environment and human health. Specific equipment and complicated fabrication processes also hinder their large scale applications. Also, a time-consuming and low atomic utilization process increased the cost. Moreover, lack of durability, poor toughness and chemically instability toward complex oily wastewater environments also hamper their range of application. In our previous work,15 we reported a covalent layer-bylayer grafting (LBLG) functionalized superhydrophobic SSM fabricated by the method of



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covalent self-assembly within multilayers. The as-prepared mesh exhibited excellent stability under various complex conditions except for strong acidic/alkaline environments due to the hydrolysis reaction of amide groups. According to the above analyses, it is still challenging to fabricate superhydrophobic and superoleophilic surfaces with excellent durability, chemical stability and recyclability by using a one-step, low-cost and environmentally friendly manufacturing method.39 In the recent years, ionic liquids (ILs), salts with unique physical properties, such as a negligible vapor pressure, low viscosity and high thermal stability have been wildly researched.40-42 ILs possess various advantages including tunable solubility, designable structure and chemical and thermal stability which make them excellent candidates for functional material. To date, most researchers are focused on the catalytic and biotechnological application of ILs,40,

43, 44

but pay little attention to their potential

applications in superwettability material for oil/water separation.45 For the ILs can be highly functionalized, based on the rational design, the physical and chemical properties of ILs can be expected in principle. In this work, a novel one-step, in-situ thermal polymerization (ISTP) method was proposed to fabricate superhydrophobic materials. To the best of our knowledge, the ISTP method has never been reported in the field of superhydrophobicity research. More specifically, N,N-dimethyl-dodecyl-(4-vinylbenzyl) ammonium chloride (DDVAC) was synthesized and adopted as the IL precursor. Basing on its thermal initiation property, two kinds of polymer DDVAC-N and DDVAC-O were fabricated by ISTP method under nitrogen and air atmosphere, respectively. Comparison experiments and characterizations verified that the DDVAC-N was a polymer but easy to be dissolved, while DDVAC-O was a highly cross-linked polymer with excellent stability. The possible formation mechanism of DDVAC-O was systematically analyzed. Then, a kind of superhydrophobic SSM was fabricated depending on the DDVAC-O in-situ formed and coated to the mesh (Scheme 1), while its hierarchical micro/nanostructure surface was achieved by wrapping SiO2 nanoparticles in. Besides, more detailed assessments about its morphology characteristics and surface wettability performances were carried out. The as-prepared SSM was then


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applied in oil/water separations. The repeatability, stability and durability of the SSM in various complex corrosion conditions was also investigated.

Scheme 1 Fabrication of superhydrophobic mesh through ISTP approach. 2. Experiments 2.1. Materials and chemicals The 304 SSM (with mesh number of 200 corresponding to pore size of 74 µm) was purchased from the local market. 1-(chloromethyl)-4-ethenyl-benzen (95%), N,N-dimethyl dodecylamine (98%), SiO2 nanoparticles (50 nm, analytical grade) were obtained from Shanghai Macklin Biochemical Co., Ltd., China. Acetone, methanol, ethanol, n-octane and dichloromethane (analytical grade) were obtained from Tianjin Jiangtian Chemical Technology Co., Ltd., China. All chemical reagents were used without further purification. 2.2. Preparation of IL precursor The precursor DDVAC was synthesized by 1-(chloromethyl)-4-ethenyl-benzen and N,N-dimethyl dodecylamine. DDVAC was prepared according to earlier report with some modification.46,

47

N,N-dimethyl dodecylamine (0.1 mol) was dispersed in 100 mL

methanol into a flask with magnetic stirring at -20 °C. 1-(chloromethyl)-4-ethenyl-benzen (0.1 mol) which was dissolved in 100 mL methanol was slowly added into the flask by a dropping funnel. The mixture was stirred at -20 °C for 2 h after dropping and further stirred at 30 °C for 24 h. After that, the solvent methanol was removed through rotary evaporation at 30 °C. The obtained solid product was extracted with hot acetone twice, then filtered and dried. 1H NMR spectrum of DDVAC is shown in Fig. S1.



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2.3. Preparation of superhydrophobic mesh The as-prepared DDVAC (100 mM) and SiO2 nanoparticles (0.5 wt%) were dispersed in CH2Cl2, then the DDVAC & SiO2 solution was prepared. The SSM with a size of 5×5 cm2 used as substrate was ultrasonically cleaned in ethanol, dichloromethane, deionized water in sequence and dried. The cleaned mesh was immersed in DDVAC & SiO2 solution for 5 s, then taken out, and put on a shelf until no liquid dripping down. After most of the solvent was volatilized (2 min), the treated mesh was put in N2 or air environment at 150 °C for 6 h to fabricate the SSM-N or SSM-O, respectively. 2.4. Preparation of polymer by ISTP In order to investigate the reaction mechanism, DDVAC was also used to prepare the polymers directly. For short, the synthesized DDVAC (100 mM) was dissolved in CH2Cl2, and the DDVAC solution was put in air environment at 150 °C for 6 h subsequently. The prepared polymer was named by DDVAC-O. Another polymer DDVAC-N was prepared by changing the air environment into nitrogen. 2.5. Oil/water separation In the oil/water separation experiment, the as-prepared mesh SSM-O was fixed between two stainless steel flanges attached with glass tubes. The device was placed tilted so that oil/water mixtures could flow down by gravity force, 50 mL of oil/water mixture (1:1, v/v) was poured into the upper glass of device subsequently. The infrared spectrometer oil content analyzer (CY2000, China) was used to measure the content of oil. The separation efficiency was calculated by the oil rejection coefficient (R (%)):

R (%) = (1 – CP/C0) × 100 where C0 and CP are the oil concentrations of the original oil/water mixture and the collected water in the upper glass tube after separation. Separation efficiencies were obtained from the average values of five measurements. 2.6. Characterization 1

H NMR spectra was recorded on a VARIAN INOVA 500 MHz spectrometer. The

Fourier transform infrared spectroscopy (FT-IR) of samples was recorded on a Bio-Rad FTS 6000 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) data were


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obtained with an electron spectrometer (ULVACPHI, PHI 5000 VersaProbe). Solid-state 13

C NMR was recorded on Infinityplus 300 MHz spectrometer. The CAs of water and oil

were measured on an optical contact angle & interface tension meter (SL200KS, KINO), water CAs were measured with 3 µl droplets by five measurements performed at different positions on the same sample. The morphologies of the mesh surface were observed by field emission scanning electron microscopy (SEM, Hitachi S-4800). Before scanning, the samples were cleaned to remove physical adsorbing substance. 3. Results and discussion 3.1. Comparison and possible formation mechanism of DDVAC-N and DDVAC-O Several mechanisms for the ISTP concerning styrene were proposed before, but there is no consensus as to the correct mechanism.48 According to the Mayo and Flory mechanism,49,

50

in a certain reaction environment, styrene compounds could generate

monoradical initiators or diradical initiators to initiate polymerization. Kelli et al. considered that styrene could abstract a hydrogen atom from the diradical to generate monoradical initiators, thus starting the chain polymerization process.48 Thus, the styrene compounds DDVAC has the potential of thermal initiation function as well. In this work, attempting of ISTP of DDVAC was firstly carried out under N2 atmosphere. Then, the structural differences of DDVAC and DDVAC-N were assessed by 1H NMR spectrum (Fig. S1, Fig. S2). The results verified that DDVAC could be polymerized through ISTP process. Besides, a kind of polymer DDVAC-O was fabricated under aerobic environment to compare the different characteristics of each other. Thus, comparison experiments were carried out to investigate the superhydrophobic performances and solvent stabilities of the as-prepared SSM-O and SSM-N. As shown in Fig. 1a-b, the SSM-O and SSM-N both exhibited hierarchical micro/nanostructure surfaces. Moreover, the inset of Fig. 1a and Fig. 1b illustrate that the two meshes were superhydrophobic with a water CA of 158° ± 1.3° and 157° ± 0.9°, which is attributed to the low energy surface energy substance and SiO2 nanoparticles anchored in the mesh surface. Then, the two kinds of meshes were treated by an ultrasonic cleaner with a power of 1 kW in dichloromethane for 24 h. The treated SSMO maintained a high water CA of 155° ± 0.7° (inset of Fig. 1c), and an SEM image (Fig.



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1c) demonstrates that the surface morphology had no obvious change. However, the treated SSM-N shows an SEM image similar to that of the original SSM (Fig. 1d), and the water CA reduced to 120° ± 1.2° (inset of Fig. 1d). The SSM-O was further tested by a series of solvents under ultrasonic condition, and all the results proved that the SSM-O possessed excellent solvent stability, while the SSM-N was the opposite. It is worth mentioning that a comparison experiment was carried out without SiO2 nanoparticles in the precursor solution. The water CA of the prepared mesh was only 134° ± 0.8°. Therefore, the SiO2 particles could embed in the mesh surface and form a hierarchical micro/nanostructure, which provides an essential factor of superhydrophobic property.

Figure 1 SEM characterizations and solvent stability tests of the as-prepared SSM-O and SSM-N. (a) Surface morphology SSM-O, the inset illustrates the SSM-O was superhydrophobic with a water CA of 158°. (b) Surface morphology SSM-N, the inset illustrates the SSM-N was superhydrophobic with a water CA of 157°. (c) Surface morphology of the SSM-O has no change after use. The inset illustrates the SSM-O maintained superhydrophobicity after 24 h ultrasonic treatment. (d) The SSM-N exhibits a smooth and clear surface after 24 h ultrasonic treatment, the superhydrophobicity of the mesh was destroyed. The inset illustrates the shape of a water droplet on the treated mesh.

As mentioned above, the IL precursor DDVAC on the mesh surface can be polymerized through ISTP. However, as is well-known, oxygen is a kind of polymerization inhibitor. Nevertheless, the generated polymer DDVAC-O possessed impeccable solvent stability,


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indicating that DDVAC-O was a highly cross-linked polymer and oxygen played an indispensable role in the crosslinking process. Thus, a series of characterizations were employed to analyze the composition differences between DDVAC-O and DDVAC-N. Firstly, the composition and chemical state of the elements of DDVAC-O and DDVACN were analyzed by XPS. Fig. 2a illustrates the typical XPS survey spectra by scanning bonding energy from 0 to 1200 eV, DDVAC-O was composed of C, O, N and Cl, while DDVAC-N was only consisted of C, N and Cl. As is illustrated in Table 1, a very high surface element C content of 82.75 atm.% owing to the long carbon chain derived from DDVAC. Interestingly, element O which is not existed in DDVAC, also accounts for 12.55 atm.% in the surface. These results ensure that oxygen participates in the process of DDVAC-O formation. As shown in Fig. 2b, the O 1s profile of DDVAC-O could be separated into two peaks, the predominant peak at 531.8 eV can be assigned to C-O bond, and peak at 533.4 eV is corresponding to C=O specie.51,

52

Thus, the obtained results

suggest that oxygen-containing groups were generated through the ISTP process. Then, as shown in Fig. 2c, the analyses of FTIR spectra were carried out to investigate the differences of DDVAC-O and DDVAC-N (The full spectra were shown in Fig. S3). In the FTIR spectrum of DDVAC-N, an absorption peak at 1219 cm-1 can be assigned to C-N stretching vibration,53 while typical peaks at about 1119 and 1383 cm-1 are attributed to the C-C stretching vibration of straight-chain and benzenoid rings, respectively,54,

55

which

suggests that DDVAC-N was constructed from repeating units of DDVAC. As for the spectrum of DDVAC-O, a series of additional peaks located at 1014, 1105, 1267, 1610 and 1718 cm-1 can be clearly observed, compared with DDVAC-N. The absorption peaks at 1014 and 1105 cm-1 are assigned to the C-O-C stretching vibration in aliphatic ether bonds, peaks at 1267 and 1610 cm-1 are attributed to the C-O-C stretching vibration and C=O stretching vibration in ester bonds, whereas typical peaks at about 1718 cm-1 is attributed to the C=O stretching vibration in carbonyl groups.56, 57 In addition, as shown in Fig. 2d, the Solid-state

13

C NMR was adopted to investigate the carbon element chemical state of

DDVAC-O and DDVAC-N, the results also proved the existence of ether, carbonyl and ester bonds in the DDVAC-O.



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Table 1 Element Contents of DDVAC-O and DDVAC-N Based on XPS Analysis elements

DDVAC-O

DDVAC-N

C 1s

82.75

92.43

N 1s

2.46

3.89

O 1s

12.55

0

Cl 2p

2.24

3.68

Figure 2 Characterizations of DDVAC-O and DDVAC-N. (a) Wide scan XPS spectrum of DDVAC-O and DDVAC-N, (b) O 1s peak fitting of DDVAC-O, (c) FTIR analyses of DDVAC-O and DDVAC-N (600-2000 cm-1). (d) Solid-state 13C NMR analysis of DDVAC-O and DDVACN.

In summary, the above-mentioned characterizations of XPS, FTIR and Solid-state

13

C

NMR bear testimony to the existence of abundant oxygen-containing functional groups in the DDVAC-O, indicating the formation mechanism of DDVAC-O was totally different from DDVAC-N, and oxygen directly participated in the formation of cross-linked structures. Therefore, according to Mayo and Flory theory,49,

50

the formation mechanism of

DDVAC-N was firstly proposed. As shown in Scheme 2, radical initiation proceeds by a Diels-Alder dimerization of styrene, then the styrene generates the monoradical initiators A• and HM• that initiate polymerization. Flory considered that styrene dimerizes to form a


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singlet 1,4-diradical (•M2•), then the diradical initiators was capable of starting the chain polymerization process. Based on the Flory process, DDVAC could be polymerized into a cross-linking polymer under N2 atmosphere. However, the chain propagation could only occur between vinyl groups, and furthermore the reaction system was lack of mobility, thus the polymerization degree of DDVAC-N was significantly limited. Thus, DDVAC-N was verified to be a soluble polymer.

Scheme 2 Mayo and Flory mechanisms.

In contrast, the formation process of DDVAC-O is quite different. Based on some previous reports, the possible formation mechanism is provided as follows (Scheme 3). The total hypothesis mechanism was based on radical reaction and separated into four parts: chain-initiation, chain-propagation, chian-termination and possible additional reaction. According to Mayo & Flory theory, styrene compounds could generate monoradical initiators or diradical initiators to initiate polymerization (equation (1)), which is the chain-initiation process. Chain-propagation involves the interaction of free radicals with molecules of the original compound (equation(2)). As shown in equation (3), alkyl radicals can be easily formed as peroxyl RO2• radicals under high temperature and aerobic environment. The generated peroxyl RO2• radicals could collide with alkyl radicals R•, and the radical annihilation occurs, which is the main reason of oxygen inhibition in a polymerization process. Nevertheless, the O-O bond of ROOR’ can be broken at high



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temperature, then alcoxyl radicals were formed (equation (4)).58, 59 In equation (5), peroxyl RO2• radicals interact with hydrocarbon molecules leads to the formation of hydroperoxides.60 For the majority of organic compounds the C-H bond dissociation energy is less than 90 kcal/mol , the reaction (5) is exothermic since rupture of the C-H bond is accompanied by the formation of the O-H bond of the hydroperoxide (90 kcal/mol).61 Thus, the reaction (5) is easy to occur, and radicals are able to transfer to nonvinyl groups. As a result, during the oxidation process, degenerate chain branching reactions occur with participation of intermediate products (ROOH) accumulating as well as new chains (R’•) formation.62 As shown in equation (6), the molecules of ROOH undergo rupture at the O-O bond comparatively readily with formation of free radicals, which in turn initiate new oxidation chains.63 Since considerable quantities of ROOH are accumulated, their decomposition and the formation of free radicals may proceed mainly by a bimolecular reaction between two ROOH molecules (equation (7)).64 It should be noted that alkyl, alcoxyl and peroxyl radicals (R•, RO• and RO2•) are highly reactive, and they are continously generated and rapidly consumed in this process, thus making their concentration rapidly reach a stationary value.65 Deducing from equation (2)-(7), it could be concluded that the occurrence of degenerate chain branching reaction (equation (5)) is the main reason for the formation of highly cross-linked polymer, and the interations between alkyl radicals (R•) and oxygen actually play the vital role. Finally, after the chaintermination process (equation (8)-(10)) and some possible additional reactions (equation (11)-(12)),59, 61, 63, 65, 66 the cross-linked polymer (DDVAC-O) was formed and carbonyl, ether and ester bond exist in its sturcture. According to the proposed mechanism, reaction temperature and reaction time may have an great impact on the ISTP process.49, 50 Thus, a series of SS Meshes concerning different reaction conditions were fabricated. As is shown in Table 2, a lower temperature (120 °C) could realize superhydrophobicity, which is attributed to the low energy surface energy substance on the mesh surface. However, the superhydrophobicity was easy to be destroyed after ultrasonication. The inadequate crosslinking degree of the polymer prepared at lower temperature might be the main reason. By contrast, the SSM-O prepared


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at a higher temperature (180 °C) could not realize the superhydrophobic property with the water CA of only 141° ± 1.2°, while the excellent stability was verified by ultrasonication treatment. The excessive oxidation of hydrocarbons at high temperature may be the reason for the loss of superhydrophobicity. On the other hand, the superhydrophobicity of SSM-O could be realized in a low preparation time (3 h) as well, while it was not stable enough. The reason was the same with the aforementioned low preparation temperature. Besides, the mesh prepared with a longer reaction time (12 h) was superhydrophobic and possessed extremely strong stability. However, the superhydrophobicity and stability can be achieved with a lower reaction time (6 h), thus leading energy-saving and time-saving. Therefore, the reaction temperature and reaction time of the ISTP process were determined to be 150 °C and 6 h.

Scheme 3 Possible formation mechanism of DDVAC-O.



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Table 2 Water CAs of SSM-O under different preparation conditions Entry

Temperature (°C)

Time (h)

CAa

CAb

1

120

6

157° ± 0.8°

139° ± 1.6°

2

150

6

158° ± 1.3°

155° ± 0.7°

3

180

6

141° ± 1.2°

140° ± 0.6°

4

150

3

157° ± 0.9°

147° ± 1.3°

5

150

12

155° ± 1.5°

153° ± 1.1°

CAa: measured water CAs before ultrasonication in dichloromethane; CAb: measured water CAs after ultrasonication in dichloromethane. 3.2. Detailed morphology characterization and wettability performance of the mesh The SEM images are used to represent surface morphologies of the SSM before and after the ISTP process. Fig. 3a illustrates that the original SSM substrate is knitted by stainless steel wires, which exhibits a smooth and clear surface. After the ISTP process, the low-magnification view of SSM-O exhibits a two dimensional surface with an average pore diameter of 74 µm (200 mesh size) without any blockage (Fig. 3d), indicating the asprepared mesh can provide channels for oil to easily flow though the mesh. Besides, a topview of the SSM-O illustrates that the original mesh has been completely covered by the polymer and the surface formed a hierarchical micro/nanostructure with numerous papillaes (Fig. 3b). Thus, the high-magnification shows the SiO2 nanoparticles anchored in the mesh could be clearly identified (Fig. 3c). Besides, as shown in Fig. S4, the FTIR spectrum of SSM-O could demonstrate the abundant existence of SiO2 nanoparticles on the mesh as well. The hierarchical micro/nanoscale roughness porosity of the coated mesh surface is essential for superhydrophobicity.


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Figure 3 SEM images of (a) morphology of the original SSM, (b) top-view of the SSM-O, (c) high-magnification view of the SSM-O anchored with SiO2 nanoparticles, (d) low-magnification view of the SSM-O, and the inset illustrates low-magnification view of the original SSM surface.

The surface wettability performance was investigated by an optical contact angle & interface tension meter. Fig. 4a illustrates that the water CA of the original mesh was 118° ± 0.6°, and the water droplets settled on the surface of the original mesh forming a hemisphere (inset of Fig. 4c). Then, ISTP method was adopted to achieve superhydrophobicity. The precursor DDVAC could in-site thermal polymerize on the outer surface of the mesh to reduce the surface free energy, and SiO2 nanoparticles anchored in the polymer could increase the roughness of the surface. Thus, the SSM-O exhibited special wettability that the water droplet stayed on the mesh surface keeping a highly spherical shape (Fig. 4c), and the water CA sharply increased to 158° ± 1.3° (Fig. 4b), indicating the excellent superhydrophobicity was achieved. Fig. 4d illustrates the wetting behavior of n-octane droplets on the mesh, and the oil CA is 0°. As a result, diesel could be absorbed within few seconds due to its superlipophilicity property (Fig. 5a). The superhydrophobicity and superoleophilicity of the SSM-O indicate its potential to be applied in oil/water separation. As shown in Fig. 4e-f, the sliding angle was then measured to be 3° ± 0.6° according to the previous method.15 Video 1 shows that the superhydrophobic mesh possessed an excellent rolling characteristics like the lotus leaf. With a low sliding angle and superhydrophobicity property, the as-prepared mesh could



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realize “lotus effect”, which means small droplets can easily roll off a surface and carry away the adherent dirt, thus realizing self-cleaning.67,

68

As shown in Fig. 5b, the high-

speed photograph of a 5-µl water droplet dropped on a slanted surface of the SSM-O, the water drop bounced from the superhydrophobic surface immediately. Video 2 shows that the water drop sprang from the mesh surface without any remain as well, indicating a strong repelling force of the surface to water. On the contrary, the same test was applied on the original SSM surface, the water drop spread and adhered to the surface (Fig. 5c). Thus, when a water droplet slid down the SSM-O surface, the droplet could carry off the dust (activated carbon particles) from the surface, thus indicating its self-cleaning effect (video 3).

Figure 4 Water contact angle of (a) the original mesh, (b) the SSM-O. (c) Optical images of water droplets (dyed with methylene blue for clear observation) sit on the surface of the superhydrophobic SSM-O, the inset shows the water droplets adhered to the surface of the original mesh. (d) Oil contact angle of the SSM-O. (e) Advancing water CA. (f) Receding water CA.


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Figure 5 High-speed photograph of (a) the SSM-O absorbing a diesel oil droplet, (b) a water droplet dropped on a slanted surface of the SSM-O surface, (c) a water droplet dropped on a slanted surface of the original mesh surface.

3.3. Oil/water separation research The cross-linking effect in DDVAC-O can be helpful for the enhancement of its stability and durability. Owing to the robust superhydrophobicity and superoleophilicity, the asprepared SSM-O has great potential to separate oil/water mixtures. Herein, a series of experiments were carried out to test the oil/water separation efficiency of SSM-O (Fig. 6 and Video 4) and a schematic description of the oil/water separation process is shown in Fig. 6a. Fig. 6b illustrates the detailed structure of the device. The mesh was fixed between two stainless steel fixtures. Both of the fixtures were attached with glass tubes (D = 25 mm), the lower tube was a straight tube (L = 10 mm) and the upper tube was a bending tube (L = 40 mm). In addition, the device was placed with a tilt angle of 15°, which could be convenient for feeding. As pouring the n-octane/water mixture into the upper tube, the n-octane (dyed with oil red) passed through the mesh and flowed into the beaker quickly with the driving force of gravity, while the water was repelled and kept in the upper glass tube (Fig. 6c-d and Video 4), indicating an excellent separation efficiency of the asprepared mesh.

Figure 6 Oil/water separation process of the as-obtained SSM-O: (a) schematic description, (b) before separation, the mesh was fixed between two stainless steel flanges.; (c) a mixture of n-



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octane (dyed with red oil) and water was poured into the upper glass tube and n-octane passed through the mesh quickly after separation, (d) the amplified picture shows that after separation, water remained in the upper glass and no oil could be found.

Then, mixtures of soybean oil/water, petroleum ether/water, gasoline/water, and diesel oil/water mixtures were also separated with high efficiency. No visible red oil was found in the water in the upper glass tube. As shown in Fig. 7a, the separation efficiencies of the SSM-O for a series of oil/water mixtures is all above 99.8% except for soybean oil (96.21% ± 0.31%) due to its complex composition and high viscosity. In addition, the asprepared mesh can be easily cleaned and stored for reuse. Thus, the mesh was cleaned in dichloromethane and dried after each repeated cycle, followed by another separation process. Fig. 7b shows that the mesh surface was still very rough and massive SiO2 nanoparticles were still anchored by the polymer on the surface when it was reused for 40 cycles, and the inset in Fig. 7b demonstrated that mesh surface still maintained superhydrophobicity and superoleophilicity. The SSM-O maintains high separation efficiency of 99.8% after 40 times reuse by taking the n-octane /water mixture as an example (Fig. 7c), showing the mesh had excellent reusability for oil/water separation.

Figure 7 (a) Oil/water separation efficiencies of the SSM-O for a series of oil/water mixtures. (b) Surface morphology has no change after 40 times reuse (n-octane/water mixture). The inset illustrates the shape of a water (left) and a n-octane (right) droplet on the mesh after 40 times


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reuse. (c) Separation efficiency remains high after 40 times reuse. (d) Digital photo of the water intrusion pressure of the SSM-O.

Water intrusion pressure and oil flux and were also introduced to further study the separation efficiency of the SSM-O. The intrusion pressure (P) was measured using the following equation:

P = ρghmax where ρ is the density of water, g is the acceleration of gravity, and hmax represents the maximum height of water that the as-prepared mesh can hold up. Intrusion pressure was measured for repeating five times. As shown in Fig. 7d, the water (dyed with methyl blue) in the glass tube could not pass through the as-prepared mesh, the maximum bearable height that the SSM-O could support was 30.11 ± 0.38 cm. Calculated by the maximum height of water, the average water intrusion pressure was determined to be 2.95 ± 0.04 kPa. And the intrusion pressure of n-octane was 0 kPa. Besides, the oil flux F was calculated using the equation:

F=V/(S×t) where V is the volume of oil that pass through the mesh, here, fixed to 0.1 L. S is the effective area of the mesh surface (19.6 cm2), and t is the required time (3.26 s) for the permeation of 0.1 L n-octane. Here, the oil flux was measured to be 15.65 ± 0.35 Lm-2s-1. From the results discussed above, water intrusion pressure and oil flux are relatively high compared to earlier reports, 13, 69, 70 which illustrates that the as-prepared mesh is capable of separating a large amount of oil/water mixtures. 3.4 Durability and stability of the mesh In practical industrial applications, superhydrophobic surfaces are always required to be stable and durable. The mechanical durability was tested via abrasion treatment. The abrasion test was carried out according to a modified procedure based on GB/T 29865–2013 by using the abrasion tester (Y571L(A), Lai Zhou, China). In the experiment, the untreated mesh was the abrasion partner. The sample was fixed onto the stainless steel column and moved



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repeatedly with a load pressure of 11.1 N (for one cycle). After certain cycles of abrasion, the water CA on the rubbed area of the sample was measured. As shown in Fig. 8a, water CA reduces monotonously with the abrasion cycles increasing, which is mainly attributed to the loss of surface roughness and the removal of DDVAC-O/SiO2 coatings at the surface. Importantly, the SSM-O possesses superhydrophobicity for over 8000 cycles of abrasion treatment. However, after 10000 cycles of abrasion, the mesh loss its superhydrophobicity and water CA decreased to 148.6° ± 0.8°. SEM characterization (inset of Fig. 8a) revealed that nearly no papillaes could be observed from the top surface of the mesh. Thus, the SSM-O proved to possess excellent mechanical durability. Besides, the asprepared mesh was evaluated in dichloromethane by ultrasonication as we mentioned above in 3.1 section, which had verified its excellent solvent stability. In addition, the anticorrosion property of a superhydrophobic mesh is also of vital importance for the practical oil/water separation, because acidic, alkaline and saline environments are common phenomena in wastewater treatments. Thus, the durability of the as-prepared mesh was evaluated in those harsh environments. The saline solutions with NaCl concentration of 010 wt% as well as acidic/alkaline solutions with pH value ranging from 0 to 14 were employed as the harsh environments. Firstly, the as-prepared meshes were immersed in those rigorous conditions for 24 h, respectively. Secondly, corrosive droplets with corresponding solutions were put on those treated mesh surface, and water CAs and oil/water separation efficiencies were then measured. Fig. 8b illustrates the corrosion resistance under different saline environments. According to the results, all the water CAs remained high (>154°), and separation efficiencies of n-octane/NaCl aqueous mixtures were all above 99.8%. Fig. 8c shows the corrosion resistance in acidic/alkaline solutions with pH range from 0 to 14. No obvious fluctuation in both water CAs and separation efficiencies was observed with the variation of the pH values. Besides, as shown in in Fig. S5, the stability of meshes can be capable of remaining longer by immersing in the mixed solution with 10 wt% NaCl and 1 mol/L HCl for 3 days. The results indicate that the SSMO had excellent stability in corrosive acidic, alkaline and saline environments, which is very important for separating oily wastewater in various complex conditions. Furthermore,


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the durability of the mesh was also evaluated by long-term storage treatment. As shown in Fig. 8d, water CAs and separation efficiencies at set intervals were measured by storing the mesh

for

three

months

under

ambient

atmosphere.

It

is

obvious

that

the

superhydrophobicity could not be destroyed with water CAs of more than 152° after the treatment, indicating that the long-term storage had negligible influence on the mesh. In short, the excellent stability under different corrosion environments make the as-prepared mesh a good candidate for large-scale oil/water separation.

Figure 8 (a) Effect of abrasion situation on the wettability of the superhydrophobic meshes, the inset shows the SEM images of the mesh after abrasion treatment. (b) Effect of saline corrosion situation on the wettability and separation efficiencies of the as-prepared superhydrophobic meshes. (c) Effect of acidic/alkaline corrosion situations on the wettability and separation efficiencies of the superhydrophobic meshes. (d) Effect of long-term storage time on the wettability and separation efficiencies of the as-prepared superhydrophobic meshes.

4. Conclusion In this paper, a one-step, ISTP method was first proposed to fabricate superhydrophobic materials. Comparison experiment results verified that the ISTP of DDVAC in air atmosphere could in-situ produce a robust polymeric layer. The “oxidative crosslinking” effect and possible reaction mechanism were proposed. The degenerate chain branching



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reaction as well as the interations between alkyl radicals (R•) and oxygen actually played the vital roles for the formation of the highly cross-linked polymer DDVAC-O. Futhermore, the reaction temperature and reaction time of the ISTP process were determined

to

be 150

°C

and

6 h.

Then,

the

as-prepared

mesh

exhibited

superhydrophobicity with water CA of 158° and self-cleaning effect. As for oil/water separation application, the SSM-O exhibited a high intrusion pressure (2.95 kPa), a high oil flux (15.65 Lm-2s-1) and a high separation efficiency after 40 times reuse cycle (99.8%). More importantly, the obtained mesh exhibited excellent durability and stability towards various complex conditions. In summary, this work provides a facile and scalable method to fabricate superhydrophobic surface, as well as the “oxidative crosslinking” mechanism for theoretical guidance. It is expected that the as-prepared mesh can be widely used for large-scale oil/water separation in a practical process. In addition, the precursor could coat onto other substrates with strong adhesion effect, which would greatly expand the value and scope for application. Supporting Information 1

H NMR spectrum of the DDVAC (Figure S1). 1H NMR spectrum of DDVAC-N

(Figure S2). FTIR analysis of the DDVAC-O and DDVAC-N (Figure S3). FTIR analysis of the SSM-O (Figure S4). Effect of mixed corrosion solution on the wettability and separation efficiencies of the as-prepared superhydrophobic meshes (Figure S5). Acknowledgements We are grateful for the financial support from National Key R&D Program of China (No. 2016YFC0400406). References (1) Zhang, L.; Xiao, H.; Zhang, H.; Xu, L.; Zhang, D., Optimal design of a novel oil–water separator for raw oil produced from ASP flooding. J. Petrol. Sci. Eng. 2007, 59, 213-218. (2) Santo, C. E.; Vilar, V. J. P.; Botelho, C. M. S.; Bhatnagar, A.; Kumar, E.; Boaventura, R. A. R., Optimization of coagulation-flocculation and flotation parameters for the treatment of a petroleum refinery effluent from a Portuguese plant. Chem. Eng. J. 2012, 183, 117-123. (3) Abdelwahab, O.; Amin, N. K.; El-Ashtoukhy, E. S. Z., Electrochemical removal of phenol from oil


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Novel One-Step, in Situ Thermal Polymerization ... - ACS Publications

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