Highly Hydrophobic and Superoleophilic Nanofibrous Mats with

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Highly hydrophobic and superoleophilic nanofibrous mats with controllable pore sizes for efficient oil/water separation Botao Song, and Qing Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02500 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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Highly hydrophobic and superoleophilic nanofibrous mats with controllable pore sizes for efficient oil/water separation Botao Song*, Qing Xu Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710069, Shaanxi, People’s Republic of China. * Corresponding author. Tel.: +86 029 8153 5026. E-mail address: [email protected] (Botao Song).

Abstract: Both the wettability and pore size of the filtration materials are of great importance in oil/water separation. However, conventional strategies mainly focused on fabrication of filtration materials with special wettability, regardless of the pore size. Herein, we demonstrated the design and construction of special wettable nanofibrous mats with tunable pore sizes as filtration materials for selective and efficient separation of oil from oil/water mixtures. The nanofibrous mats with different pore sizes were prepared by the electrospinning approach with stainless steel wire mesh as collector, and the results indicated that the pore size of nanofibrous mat gradually increased with the decrease of mesh number. The wettability behavior results demonstrated that all the nanofibrous mats showed highly hydrophobic and superoleophilic properties. Owing to the special wettability and porous structure the nanofibrous mats were sequentially applied for oil/water separation, and the nanofibrous mats showed excellent ability to separate both layered oil/water mixture and water-in-oil emulsion, moreover, it was also found the oil flux could be highly improved by controlling the pore size of the nanofibrous mat and the oil flux of the nanofibrous mat with the largest pore size was about 10 times higher than the conventional nonwoven mat which had the smallest pore size. The developed nanofibrous mats with controllable pore sizes can therefore be practically used as highly efficient filtration materials in oily water management. Keywords: pore size, wettability, electrospinning, oil/water separation

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Introduction: The abundant discharge of oily wastewater and frequent oil spill accidents have already caused serious global environmental problems.1,2 The oil-polluted water usually contains toxic chemicals, which are catastrophic for ecosystem and even human health. Therefore, a wide variety of approaches have been developed to separate the oily water, such as skimming, centrifugation, in situ burning and filtration. Among them, filtration method has been proven to be an effective method for oil/water separation, due to its high separation efficiency, relative low operation costs and excellent recycle performance. Based on this, various filtration materials have been designed and the main strategy is to modify the commercially porous materials (e.g., cotton fabric, metal wire mesh, microfiltration membrane) to be superwettable (superhydrophobic-superoleophilic, or superhydrophilic-underwater superoleophobic), which results in the selective and efficient removing of only one phase (water or oil) from the oil/water mixtures.3-11 Besides the super wettability property, pore size of the filtration materials is also critical during the separation process.12-17 Generally, filtration material with large pore size always results in a high separation flux. For example, Shi and coworkers tune the pore size of the single-walled carbon nanotube (SWCNT) network films in the nanometer scale for separation of surfactant-free emulsions and surfactant-stabilized emulsions. Therefore, design of filtration materials with both opposite wetting behaviors to water and oil and controllable pore sizes is highly desirable, which can not only improve the separation efficiency but also greatly increase the separation flux. Recently, nanofibrous mats fabricated by the electrospinning approach have been frequently applied for separation of oil/water mixtures, which is due to the relative simplicity and versatility of this technique and the porous structure of the resultant nanofibrous mats.18-29 Moreover, it is also found that the electrospinning process can enhance hydrophobicity of the hydrophobic raw materials because of the improved surface roughness after electrospinning,22,30 which further enhances the affinity of the porous nanofibrous mats to oil and the repellency to water. It is known that the fabrication of the nanofibrous mats with different pore sizes is challenging by the


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conventional electrospinning

technique.

Generally,

during

the

conventional

electrospinning process the nanofibers will tightly and randomly deposit on the conductive collector (the iron plate, the rotated drum), and the pore of the mat is very small and is also not easy to be controlled.31-33 In this study, we modify the conventional electrospinning technique with the using of stainless steel wire mesh as collector, and it is found that the deposition of the nanofibers has the priority in the conductive region rather than the nonconductive region of the stainless steel wire mesh, and only a few nanofibers are deposited in the voids between the wire, which results in low fiber density and large pore size in the wire interspaces. Different from Shi’s study, our study aims to separate the layered oil/water mixtures and the surfactant-free emulsions. For separation of such two kinds of oil/water mixtures, it would be better to modulate the pore size of the filtration materials in the micrometer scale. For separation of surfactant-free emulsions, the droplets of the surfactant-free emulsions are generally in the micrometer scale, thus fabrication of filtration materials with the micro-sized pores is enough to efficiently retain the droplets from the emulsions, and there is no need to prepare filtration materials with nano-sized pores to separate such large droplets, which may decrease the liquid flux during the oil/water separation. In addition, during the separation of the layered oil/water mixtures, the filtration materials with the micro-sized pores are also desirable for achieving high liquid flux. In this study, an environment-friendly polymer polyvinyl butyral (PVB) is chosen to fabricate nanofibrous mats by electrospinning due to its stability and hydrophobicity,34 and the stainless steel wire meshes with different mesh numbers are used as collectors. Firstly, the morphology and pore size of the resultant nanofibrous mats are systematically investigated. Then, the wettability of the nanofibrous mats is further studied. Finally, the nanofibrous mats with different pore sizes are applied for separation both the layered oil/water mixture and water-in-oil emulsion, and both the oil flux and separation efficiency are carefully investigated.

Experimental:


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1. Materials The polymer PVB, fluorescein and rhodamine B were all obtained from Sinopharm Chemical Reagent Company. Absolute ethanol was purchased from Tianjin Zhi Yuan Reagent Co., Ltd. Liquid paraffin was obtained from Tianjin Tianli Chemical Reagents Ltd. Methylene blue was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Oil red was obtained from Aladdin Reagent Company.

2. Fabrication of nanofibrous mats with tunable pore sizes In a typical preparation procedure, 0.4 g PVB powders were added into 5 mL of ethanol, followed by magnetic stirring for 0.5 h at room temperature to obtain a transparent spinning solution. Then, the solution was carefully transferred into a 10 mL syringe for the further electrospinning. The stainless steel wire meshes with different mesh numbers (12#, 20#, 30#, 40#) were employed as collectors to obtain nanofibrous mats with tunable pore sizes and a stainless steel sheet was also selected as control, and the corresponding nanofibrous mats were denoted as S-12, S-20, S-30, S-40 and S-N, respectively. The electrospinning parameters were shown as follows. The applied voltage was fixed at 6.80 kV and the working distance was 10 cm. The feeding rate of the solution was controlled as 1.0 mL·h-1. The resultant nanofibrous mats were carefully peeled off from collectors for the further using.

3. Characterization of nanofibrous mats The morphology of the nanofibrous mats was characterized by scanning electron microscopy (SEM, Hitachi, S4800). The average pore size and the pore size distribution were evaluated by utilizing a microscopy (Nikon Eclipse Ti-U), firstly, the nanofibers in the same focal plane were focused by the microscopy, then a included software of microscopy was used to measure the diameter of a virtual inscribed circle among the nanofibers, finally the diameter of the virtual inscribed circle was recorded as the pore size of the nanofibrous mat. For each sample more than 50 virtual inscribed circles were selected, and the average pore size was denoted as mean ± standard deviation. Both the water contact angle (WCA) and oil contact


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angle (OCA) of the nanofibrous mats were determined by using the sessile-drop technique at 25 °C 35. Briefly, a glass cell containing the test liquid (water or paraffin oil) was covered with aluminum foil. Then, a microsyringe was inserted through the aluminum foil, and a small droplet was dripped on sample surface from the microsyringe. After that, the droplet size was gradually increased by dripping with some new liquid into the deposited droplet, and the final diameter of droplet was fixed to 8 mm. Each value was obtained by measuring at least five different positions of the same sample and the results were expressed as mean ± standard deviation. In order to test the pH stability of the nanofibrous mat, the water contact angle measurements of the water droplets with different pH values (1-13) on the nanofibrous mat were all carried out.

4. Oil/water separation In the process of oil/water separation, the nanofibrous mat was fixed between two glass tubes with the diameter of 25 cm. Both the layered oil/water mixtures and water-in-oil emulsion were separated. The layered oil/water mixture was prepared by mixing of water and oil (liquid paraffin) with the volume ratio of 1:9, and the water-in-oil emulsion was fabricated by vigorously stirring the mixture of paraffin oil and water (100:1, v:v) for 2 h. A total of 20 mL of the mixture was poured into the tube quickly and the separation was achieved by gravity. The oil flux was calculated by using the equation: Flux=V/S*T where V was the volume of the separated oil (L), S was the effective area of the nanofibrous mat (m2), and T was the separation time (h). To evaluate the separation efficiency, the quantity of oil before and after separation was weighted and calculated according to the following equation: η=(M1/M0)×100% where M0 and M1 were the weight of oil before and after separation, respectively. The results were expressed as mean ± standard deviation (n = 5). To test the durability of the nanofibrous mat, the oil/water mixture separation


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experiment was repeated for 30 times, and the stability of the nanofibrous mat was also evaluated by using the water with different pH values ranging from 1 to 13.

Results and discussion: 1. Morphology of the nanofibrous mats with controllable pore sizes Figure 1 showed the digital images, SEM images and the corresponding pore size distribution of the resultant nanofibrous mats. As depicted in Figure1 (A1, A2, A3), the nanofibrous mat using the conventional stainless steel sheet as collector was in a nonwoven structure, and the nanofibers were randomly distributed and closely stacked. By contrast, with utilizing stainless steel wire mesh as collector all the nanofibrous mats showed patterned structures and the nanofibers were preferentially deposited on the metal wires rather than the interspaces between the wires (Figure1 (B1-E1; B2-E2)). Moreover, it was also found that as the mesh number of stainless steel wire mesh decreased (the interspaces between the wires became larger) fewer nanofibers were deposited in the interspaces (Figure1 (B3-E3)). Finally, pore size of each mat was quantified, and it could be observed that the average pore size of the five kinds of nanofibrous mats were about 20, 25, 30, 37 and 47 µm, respectively, which indicated the pore size of the nanofibrous mats could be controlled by the mesh number of the stainless steel wire mesh.

2. The wettability of the nanofibrous mats with controllable pore sizes Figure 2 displayed the wetting behaviors of water and oil on the surface of nanofibrous mats. It was found that the water rested on the nanofibrous mat was in the form of quasi-spherical morphology and the WCA was about 140°; by contrast, the nanofibrous mat was quickly and completely wetted by the red liquid paraffin with the OCA of about 0° (Figure 2 (A, C)), which indicated the highly hydrophobic and superoleophilic behavior of the nanofibrous mat. The affinity of nanofibrous mat to both water and oil was further investigated, and the results indicated that negligible water was attached onto the surface of the sample; however, the nanofibrous mat was completely wetted after immersing into the oil (Figure 2 (B, D)). In order to


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systematically investigate the wettability of all the five kinds of nanofibrous mats, both the WCA and OCA were measured. Interestingly, three kinds of fiber arrangements were found in the nanofibrous mat with using the stainless steel wire mesh as collector (Figure 2E, F insert). Therefore, the WCA and OCA of all the three different positions in the five kinds of nanofibrous mats were measured, and it was observed that the WCA and OCA of the samples were about 140° and 0°, respectively, which was independent of the kind of nanofibrous mat and the position of the nanofibrous mat. All the results demonstrated that the nanofibrous materials showed distinct opposite affinities towards oil and water, which implied the commendable application in oil/water separation.

3. Oil/water mixture separation Due to the significantly different affinity to water and oil, the nanofibrous mats with different pore sizes were then applied to separate the oil/water mixture. Figure 3A showed the setup of oil/water separation, and the nanofibrous mat was tightly fixed between two glass tubes. Both the oil flux and separation efficiency of the four kinds of nanofibrous mats with different pore sizes were measured (S-12; S-20; S-30; S-40), and the oil/water separation property of the nonwoven nanofibrous mat (S-N) was also investigated as control. As depicted in Figure 3B, the oil flux of the nanofibrous mat S-40, S-30, S-20 and S-12 were about 4890, 5180, 5820 and 6110 L·m-2·h-1, respectively; by contrast, the oil flux of the nonwoven mat S-N was only 550 L·m-2·h-1. Therefore, the maximum oil flux of the nanofibrous mat could be improved up to 10 times as compared with the nonwoven mat S-N. In addition, the separation efficiency of all the five kinds of nanofibrous mats were measured, and they were 99.7%, 99.4%, 99.5%, 99.5% and 99.5%, respectively. As can be seen that, all the nanofibrous mats showed very high separation efficiencies, and the pore size did not greatly influence the separation efficiency. In order to intuitive judgment of the oil/water separation ability of the nanofibrous mat, a layered oil/water mixture was prepared and the water was labeled with methylene blue (Figure 3C), and it could be found that after the separation only oil passed through the nanofibrous mat, and no


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blue trace could be visibly observed (Figure 3D), which further conformed the highly effective separation of layered oil/water mixture of the nanofibrous mat. The obtained nanofibrous mats were then used to separate water-in-oil emulsion, and the results were shown in Figure 4. It was clearly observed that the oil flux was increased with the increasing of pore size of the nanofibrous mat, and the maximum oil flux for the sample S-12 was about 11 times higher than the nonwoven one S-N. Additionally, it was also found that pore size had no obvious influence on the separation efficiency, and all the separation efficiencies were high up to 99.4% or the above (Figure 4A). Figure 4B and 4C showed the digital and optical images of the water-in-oil emulsion before and after separation, and the results demonstrated that after separation the blue turbid emulsion turned to be transparent; in addition, from the optical images it could be found that the emulsion droplets with the average size about 22.12 µm were all disappeared after separation, which indicated the nanofibrous mat could also be used to rapidly and efficiently separate the water-in-oil emulsion. For practical applications both the recyclability and stability of the filtration materials were of great importance. It was found that after 30 times of continuous oil/water separation, both the oil flux and separation efficiency changed slightly, which were kept about 5500 L·m-2·h-1 and 99.5%, respectively (Figure 5A). The water contact angle measurements of the water droplets with different pH values on the nanofibrous mat were all carried out to test the pH stability of the nanofibrous mat, and the results demonstrated that the nanofibrous mat showed highly hydrophobic property in a wide range of pH value from 1 to 13 (Figure 5B). Moreover, the stability of the nanofibrous mat was also evaluated by using the water with different pH values ranging from 1 to 13, the results indicated that the oil flux remained high in acid and neutral environment, and a slight decrease in the strong base environment (pH 13) was observed (Figure 5C). Nonetheless, the separation efficiency always remained unchanged. These results indicated the excellent recyclability and stability of the nanofibrous mat. Recent studies had demonstrated that the nanofibrous mat showed great potential in oil/water separation.18-29 However, due to the relative small and uncontrollable pore


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size the flux of the nanofibrous mats was still less than optimal. In this study, in order to greatly improve the oil flux, the nanofibrous mats with tunable pore sizes were fabricated by employing the stainless steel wire mesh as collector. Previous studies had demonstrated that the topography of the collectors greatly affected the deposition of electrospun nanofibers, and the electrospun nanofibrous mats could replicate the configuration of the collectors.36 By using this feature, in this study, we ingeniously introduced stainless steel wire mesh as collector because of its unique structure (Figure 6), and it was found the nanofibers were inclined to deposit on the conductive wire but not the interspaces between wires. Therefore, the low density of nanofibers could be achieved in the interspaces, and the pore size of the nanofibrous mat was thus increased. Base on this, we further employed various stainless steel wire meshes with different mesh numbers to construct nanofibrous mats with controllable pore sizes. As it is known to all, both the wettability and pore size of the filtration materials greatly influence the oil/water separation. In this study, it was found that there were no obvious difference of WCA and OCA for all the five kinds of nanofibrous mats, thus pore size became the major factor to influence the oil flux and separation efficiency. For the separation process, due to the highly hydrophobic and superoleophilic properties of the nanofibrous mat, oil would quickly wet and permeate into the pores of the nanofibrous mat by gravity while the water would remain on the surface by the strong water repulsive force. As the pore size of nanofibrous mat increased, the oil was easier to pass through the nanofibrous mat; therefore, the oil flux was gradually increased. Although the pore sizes of nanofibrous mats S-12, S-20, S-30 and S-40 were all larger than the size of the emulsion droplet when separating the water-in-oil emulsion, it was found the separation efficiencies for all the samples were still very high. The reason was that, during the electrospinning process the nanofibers were deposited layer upon layer,37 it was very difficult for the small emulsion droplet to pass through all the nanofiber layers and the droplets would be blocked and intercepted by the nanofibers in the underlying layers. Another reason was that, the emulsion was not stable and the size of the emulsion droplets increased


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over time, therefore these emulsion droplets could be filtered by the fibrous network. Therefore, the separation efficiency of the nanofibrous mat with large pore size was still high. In our study the stainless steel wire mesh with mesh number of 6#, which had a larger pore size as compared with other wire meshes used in this study (12#; 20#; 30#; 40#), was further chosen to fabricate nanofibrous mat with a larger pore size. However, it was found that very few nanofibers were deposited in the voids of the wire mesh, which resulted in a poor mechanical property of the prepared nanofibrous mat. After pouring the oil/water mixture on the nanofibrous mat, the structure of the nanofibrous mat was destroyed and the oil/water mixture could not be separated. As seen from previous publications it could be found that the oil flux of the conventional nanofibrous mats was relatively low.22,25,26 For example, Zhang reported a novel superhydrophobic-superoleophilic nanofibrous mat by electrospinning for the separation of oil/water mixtures, and the results demonstrated that the oil flux was only 2890 L·m-2·h-1.22 Similarly, Zhang prepared polybenzoxazine/TiO2 nanofibrous mat with superhydrophobicity and superoleophilicity properties for oil/water separation, and the oil flux was about 3000 L·m-2·h-1.26 By contrast, in this study the oil flux of the prepared nanofibrous mats was more than 6000 L·m-2·h-1, which was much higher than the mat reported in other literatures. The main reason for this was that, previous studies focused on the design and construction of nanofibrous mats with special wettability. However, less attention had been made on the pore size of the nanofibrous mat which was also vital during the oil/water mixtures separation process. This phenomenon was also found in preparation of other kinds of filtration materials for oil/water separation. For example, Lee et al. prepared a novel multi-walled carbon nanotubes coated stainless steel mesh for oil/water separation, due to the superhydrophobic and superoleophilic property, this filtration material showed excellent capacity to separate the oil from the oil/water mixture.38 The similarity between the multi-walled carbon nanotubes coated stainless steel mesh and the nanofibrous mat fabricated in our study was that both the two filtration materials showed special wettability. Besides the wettability, previous studies had also


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demonstrated the pore size of the filtration materials was also vital for oil/water separation. However, in Lee’s study, the controllable fabrication of filtration materials with different pore sizes was not involved. By contrast, in our study, the special wettable nanofibrous mat based filtration materials with the controllable pore sizes were prepared, which was favor for optimizing the nanofibrous mat with the best oil flux. One must be mentioned that, the breakthrough pressure of the nanofibrous mat might be relative low due to its relatively large pore size. In our further study, the nanofibrous mat would be integrated with other mechanical robust porous substrates (such as stain steel wire meshes, fabrics), and the nanofibrous mat would be used as a core layer to efficiently separate the oil/water mixture, and the underlying substrates would be acted as a supporting layer to prevent the damage of the nanofibrous mat. Much effort would be paid to investigate the oil/water separation property of the integrated device, especially the breakthrough pressure.

Conclusions: In summary, an ingenious design to fabricate hydrophobic and superoleophilic nanofibrous mats with controllable pore sizes for the oil/water separation was presented. The results indicated that the pore size of the nanofibrous mat could be finely controlled by using the stainless steel wire mesh with different mesh numbers and all the nanofibrous mats showed highly hydrophobic and superoleophilic properties. The oil/water separation results demonstrated that the nanofibrous mats could selectively and rapidly separate oil from both the layered oil/water mixtures and water-in-oil emulsion, and the enlarged pore size could greatly improve the oil flux without obvious influence on the separation efficiency. The designed nanofibrous mats with controllable pore sizes showed large oil flux, high separation efficiency, excellent durability and recyclability for oil/water separation, which made them great potential in oily water management.


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Acknowledgements This research was supported by grants from NFFTBS (No.: J1103311 and J1210057), Natural Science Basic Research Plan in Shaanxi Province of China (No.: 2015JQ5158), Scientific Research Program Funded by Shaanxi Provincial Education Department (No.: 14JK1726), and China Postdoctoral Science Foundation (No.: 2014M560800; 2013M542376).

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Soft Matter 2014, 10 (40), 7985-7992. (17) Huang, K.; Yeh, S.; Huang, C. Surface Modification for Superhydrophilicity and Underwater Superoleophobicity: Applications in Antifog, Underwater Self-Cleaning, and Oil-Water Separation. Acs Applied Materials & Interfaces 2015, 7 (38), 21021-21029. (18) Lee, M. W.; An, S.; Latthe, S. S.; Lee, C.; Hong, S.; Yoon, S. S. Electrospun Polystyrene Nanofiber Membrane with Superhydrophobicity and Superoleophilicity for Selective Separation of Water and Low Viscous Oil. Acs Applied Materials & Interfaces 2013, 5 (21), 10597-10604. (19) Tai, M. H.; Gao, P.; Tan, B. Y. L.; Sun, D. D.; Leckie, J. O. Highly Efficient and Flexible

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Figure captions: Figure 1. Digital images, SEM images with low magnification, SEM images with high magnification and the corresponding pore size distribution of the nanofibrous mats (A1-A4: S-N; B1-B4:S-40; C1-C4:S-30; D1-D4:S-20; E1-E4:S-12). Figure 2. (A, C) Digital images of water droplets stained with methylene blue and liquid paraffin droplets stained with oil red on the surface of nanofibrous mat. (B, D) Digital images of the nanofibrous mat after immersing into the water labeled with methylene blue and liquid paraffin labeled with oil red. (E, F) The WCA and OCA of the different positions of the five kinds of nanofibrous mats. (Inserts were digital images of the liquid droplets rested on the different positions and SEM image of the corresponding positions) Figure 3. (A) The setup of oil/water mixture separation. (B) The oil flux and separation efficiency of the five kinds of nanofibrous mats. Digital images of the layered oil/water mixture (C) before and (D) after separation (Water was dyed with methylene blue). Figure 4. (A) The oil flux and separation efficiency of the five kinds of nanofibrous mats for separation of water-in-oil emulsion. Digital images of the water-in-oil emulsion (B1) before and (C1) after separation (Water was also dyed with methylene blue), and the optical images of the water-in-oil emulsion (B2) before and (C2) after separation.) Figure 5. (A) The recyclability of the nanofibrous mat after 30 times of oil/water mixture separation, (B) WCA of the nanofibrous mat towards the droplets with different pH values, (C) the oil flux and separation efficiency of the nanofibrous mat for separation of oil/water mixtures with different pH values of water. Figure 6. Schematic illustration of the process of fabrication of nanofibrous mats with different pore sizes and the corresponding mechanisms for separation of oil/water mixture.


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TOC graphic


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Figure 1. Digital images, SEM images with low magnification, SEM images with high magnification and the corresponding pore size distribution of the nanofibrous mats (A1-A4: S-N; B1-B4:S-40; C1-C4:S-30; D1D4:S-20; E1-E4:S-12). 199x119mm (300 x 300 DPI)


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Figure 2. (A, C) Digital images of water droplets stained with methylene blue and liquid paraffin droplets stained with oil red on the surface of nanofibrous mat. (B, D) Digital images of the nanofibrous mat after immersing into the water labeled with methylene blue and liquid paraffin labeled with oil red. (E, F) The WCA and OCA of the different positions of the five kinds of nanofibrous mats. (Inserts were digital images of the liquid droplets rested on the different positions and SEM image of the corresponding positions) 150x86mm (300 x 300 DPI)


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Figure 3. (A) The setup of oil/water mixture separation. (B) The oil flux and separation efficiency of the five kinds of nanofibrous mats. Digital images of the layered oil/water mixture (C) before and (D) after separation (Water was dyed with methylene blue). 150x54mm (300 x 300 DPI)


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Figure 4. (A) The oil flux and separation efficiency of the five kinds of nanofibrous mats for separation of water-in-oil emulsion. Digital images of the water-in-oil emulsion (B1) before and (C1) after separation (Water was also dyed with methylene blue), and the optical images of the water-in-oil emulsion (B2) before and (C2) after separation.) 199x73mm (300 x 300 DPI)


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Figure 5. (A) The recyclability of the nanofibrous mat after 30 times of oil/water mixture separation, (B) WCA of the nanofibrous mat towards the droplets with different pH values, (C) the oil flux and separation efficiency of the nanofibrous mat for separation of oil/water mixtures with different pH values of water. 150x35mm (300 x 300 DPI)


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Figure 6. Schematic illustration of the process of fabrication of nanofibrous mats with different pore sizes and the corresponding mechanisms for separation of oil/water mixture. 150x92mm (300 x 300 DPI)


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Highly Hydrophobic and Superoleophilic Nanofibrous Mats with

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