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One-step coating towards multifunctional applications: oil/water mixtures and emulsions separation and contaminants adsorption Yingze Cao, Na Liu, Weifeng Zhang, Lin Feng, and Yen Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11226 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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ACS Applied Materials & Interfaces

One-step coating towards multifunctional applications: oil/water mixtures and emulsions separation and contaminants adsorption Yingze Cao†,‡, Na Liu†, Weifeng Zhang†, Lin Feng*†, and Yen Wei† †

Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China

‡

Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing,

China *Corresponding author: [email protected] KEYWORDS: mussel-inspired chemistry, special wettability, oil/water separation, emulsion separation, contaminants adsorption

ABSTRACT: Here, a method that can simultaneously separate oil/water mixtures and remove water soluble contaminants has been developed. Various substrates with different pore size were coated by polydopamine and polyethylenepolyamine co-deposition films. The as-prepared materials were superhydrophilic and superoleophobic, in air and under water.The materials can separate a range of different oil/water mixtures (including immiscible oil/water mixtures and surfactant-stabilized emulsions) in a single unit operation, with >99.6% separation efficiency and high fluxes. Copper ion and methyl blue can be effectively absorbed from water when it

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permeates through the materials. This method can be applied on organic and inorganic substrates and used in preparing large-scale product. Therefore, the simple and facile method has excellent potential in practical application and creates a new field for oil/water separation materials with multifunctional applications.

1. Introduction Environmental issues have always been a great concern to humankind. The abundant discharge of industrial wastewater and frequent oil spill accident generated a lot of oil/water mixtures and emulsions.1-5 Oil/water mixtures and emulsions are harmful to ecotope like the habitat for migratory birds and related to people’s production and living.6-10 Thus, materials can effectively separate immiscible oil/water mixtures and emulsions are in great need. In the past decade, immiscible oil/water mixtures separation materials with special wettability have been broadly studied and are divided into two types: oil-removing and water-removing materials.11-14 Oilremoving materials refer to materials with superhydrophobicity and superoleophilicity that can let oil permeate the materials and block the water. Oil-removing mesh was first reported by Jiang and co-workers in 2004 through spraying polytetrafluoroethylene (PTFE) on stainless-steel mesh.15 Water-removing materials usually refer to materials with superhydrophilicity and underwater-superoleopholicity and can only let water permeate. Polyacrylamide (PAM) hydrogel was first used to fabricate water-removing materials.16 Chemical composition and micro-nano composite structure is found to be two main keys to develop oil/water separation materials. Other materials have been fabricated for immiscible oil/water mixtures separation by using silica, titanium dioxide, chitosan, palygorskite etc.17-22 Recently, with further research, special wettability has been used for the separation of oil/water emulsions.23-25 A. Tuteja and co-workers


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reported the fluorodecyl polyhedral oligomericsilsesquioxane and cross-linked poly(ethylene glycol) diacrylate) blend coated membrane and can it is only effective for separation of the oil/water emulsions with droplet sizes larger than 1 µm.26 An inorganic nanowire haired copper mesh has been made to separate non-surfactant oil-in-water emulsion.27 The functional silica coated mesh can separate surfactant-stablized water-in-oil emulsion, while the flux is not fast which limit its application.28 Water purification requires not only the insoluble oil can be separated from water through the sepacial wettability of the materials, but also the removal of the water soluble contaminants.29-33 Wastewater from textile, printing, leather, cosmetic industry usually contains a lot dyes which has to be removed before discharging.34-35 Copper contamination is also considered great concern on account of its high toxicity and non-biodegradability. Massive emissions of Cu2+ containing wastewater have been generated from electrical and semiconductor industries and need to be purified.36 However, the materials reported are mainly effective for either immiscible oil/water mixtures or emusions and usually simple funtional. Therefore, there is a critical need to develop a method for both immiscible oil/water mixtures and stablized emulsions separation with more funtion such as dyes or metal ion contamninates adsorption. Herein, by simply vary the substrates, simultaneous oil/water mixtures separation and water soluble contaminants removal can be successsful realized. Different substrates with different pore size were coated with the adhesive film by immersion in an aqueous solution of dopamine(DA) and polyethylenepolyamine (PEPA). Dopamine is a hydrophilic molecule and was reported by Messersmith’s group as a molecular structural mimic of the Mytilus edulis foot protein 5 (Mefp-5). Dopamine through self-polymerization can form strong adhesive films on inorganic and orangic substrates in the presence of Tris.37 PEPA, an amino-rich polymer, is


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hydrophilic and low-cost and can react with dopamine via Michael addition or Shiff base reaction between amine and catechol.38, 39 As shown in Scheme 1, the co-deposition of PDA and

Scheme 1. Schematic description of the preparation of PDA/PEPA modified materials and the process of oil/water mixtures separation and Cu2+ and methyl blue adsorption. PEPA form an amnio-rich and superhydrophilic films on different substrates such as mesh, sponge, and microfiltration membrane. For stainless steel mesh with relatively large pore size (approximately 10 µm), the PDA/PEPA coated mesh exhibits superhydropholicity/underwatersuperoleophibicity and can separate varous immiscible oil/water mixtures with high efficiency. For PVA sponge with micrometer-sized pores and large specific surface area, it can separate oil/water mixtures and adsorb Cu2+ in the meantime. The Cu2+ can be attracted by the amino group in the surface with high flux. At last, for mixed cellulose ester membrane with relatively small pore size (less than 1µm), it can realize separation of surfactant-free and surfactant-


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stabilized emulsions and adsorption of dye. Methyl blue was used in this work and can be adsorbed on the membrane. Besides, the membrane has high fluxes up to 30000 L m−2h−1bar−1 and is capable of separating diesel-in-water emulsions despite of diesel’s the complex composition and high viscosity. The as-prepared materials was multifunctional, cost saving and large-scale producible. More importantly, they were obtained by simple, facile and one-step immersion method. Therefore, the materials have promising prospect in industry application.

2. Experimental 3.1 Chemicals Dopamine hydrochloride (Sangon Biotech Co. Ltd., Shanghai, China) and polyethylenepolyamine, methyl blue (Aladdin Industrial Inc., Shanghai, China) was used as purchased. Other reagents from Sinopharm Chemical Reagent are of analytical grade and used without further purification. 3.2 Preparation of the PDA/PEPA Films on Multiple Substrates Stainless steel mesh and PVA sponge were cut into 4×4 cm2, then together with the mixed cellulose ester membrane were immersed in a beaker containing 250 mL deionized water and 0.75 g DA. After sonicating for 5 minutes, a 15 mL aqueous solution containing 0.25 g PEPA was dropwise added into the solution. Then the beaker was sealed and placed in ambient temperature for 40 h. The as-prepared materials were cleaned by water and stored in water for further characterization. 3.3 Oil/Water Mixtures and Emulsions Separation Experiments


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The as-prepared mesh or sponge (pre-wetted before use) was fixed between two Teflon fixtures which were attached with glass tube. The diameter of the glass tube was 30 mm. The oil/water mixtures (1:1 by volume) were poured onto the mesh and the separation was achieved by the force of gravity. Three kinds of surfactant-free oil-in-water emulsions have been prepared. Each emulsion contains 1 mL hexane (or toluene, diesel) and 100 mL water was sonicated for 30 min and was named as SFH, SFT and SFD, respectively. Three kinds of surfactant-stabilized oil-in-water emulsions have been made by adding 0.05 g Tween20 in the aforementioned mixtures and stirred for 2 h. They were correspondingly names as SSH, SST and SSD. The MCE membrane was placed in a vacuum filtration device. Emulsions were poured onto the membrane to achieve demulsification and separation. 3.4 Instruments and Characterization Scanning electron microscopy (SEM) images were obtained by field emission scanning electron microscope (SU-8010, Hitachi Limited, Japan) and environment scanning electron microscope (FEI Quanta 200). Contact angles were measured on an OCA20 machine (Data-Physics, Germany) at ambient temperature and each was the average of measuring five different positions on one sample. Optical microscopy images of emulsions and filtrates were taken on a Nikon ECLIPSE LV100POL polarizing optical microscope. Droplet size distribution was measured by a dynamic light scattering (DLS) measurement (ZetaPlus, Brookhaven Instruments, Holtsville, NY).The oil content in the filtrates was extracted by CCl4 and then tested by infrared spectrometer oil content analyzer (OIL 480). The concentration of Cu2+ and methyl blue was measured with a Perkin Elmer Lambda-750 UV spectrometer.


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3. Results and Discussions 3.1 Morphology and Wettability of the As-Prepared Materials

Figure 1. SEM images of the as-prepared materials before and after modification: a and b) The cross-knitted stainless steel mesh change from a smooth and clear surface to numerous hairy nanoparticles coated surface. c and d) The PVA sponge has relatively large pore sizes and the inset figure demonstrate the films has been modified. e and f) The pores on the membrane were less than 1µm and numerous nanoparticles can be observed in the right image. g) Water and dichloroethane (DCE) contact angles on the as-prepared materials and photographs of water droplets spread and permeate the mesh, the sponge and the membrane. h) Hexane, toluene and diesel contact angles of the as-prepared materials underwater. The SEM images of stainless steel mesh, PVA sponge, and mixed cellulose ester membrane (MEC membrane) before and after modification were shown in Figure 1. Figure 1a presents the cross-knitted stainless steel mesh has a smooth and clear surface. After coated with PDA/PEPA films, numerous hairy nanoparticles were observed on the wires in Figure 1b. Figure 1c and d exhibit that PVA sponge has relatively large pore sizes (from 10 to 100 µm), and the inset


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demonstrate the films has been modified. Figure 1e and f show that the pores on the membrane were less than 1µm. Same with the other two substrates, the PDA/PEPA coated membranes is rougher and owns more nanoparticles. Due to the hydrophilicity of DA and PEPA, the as-prepared materials all show superhydrophilic and underwater-superoleophilic properties. Figure 1g shows the water contact angles of the asprepared materials were about 0 ° and the oil (dichloroethane in this case) contact angles on the mesh, the sponge and the membrane were 151.8 ± 2.0 °, 154.0 ± 2.9 °, and 159.7 ± 4.0 °. The insets illustrate that water can quickly spread and permeate the mesh, the sponge and the membrane while the oil droplets stay spheroidal on the materials . Figure 1h shows that other types of oils including hexane, toluene and diesel have been tested on the as-prepared materials, and all the oil contact angles are above 150 ° demonstrating the underwater-superoleophobicity of the materials. 3.2 PDA/PEPA Coated Mesh for Immiscible Oil/Water Mixtures Separation

Figure 2. a and b) The photographs of diesel/water mixtures separation process. c) Separation efficiency PDA/PEPA modified stainless steel mesh for a selection of oil/water mixtures.


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A series of oil/water mixtures were prepared to test the performance of the PDA/PEPA coated stainless steel mesh. The oil/water separation experiment procedure was performed as shown in Figure 2a, b. The as-prepared mesh was fixed between two Teflon fixtures which were attached with glass tubes. As pouring the diesel/water mixture onto the mesh, water permeated the mesh quickly while diesel was kept in the upper side. The separation efficiency for immiscible oil/water mixtures by the mesh was shown in Figure 2c. Three kinds oil/water mixtures including hexane, toluene, and diesel were successfully separated by the as-prepared mesh and all separation efficiency are higher than 99.9%. The oil content in mixtures after separation was tested by the infrared spectrometer oil content analyzer. The separation efficiency was calculated by oil rejection coefficient (R (%)) according to: 

CP 



C0 

R(%) = 1 −

 ×100

where C0 and CP are the oil concentration of the original oil/water mixtures and the collected water after the first separation. After the separation, the oil can stand on the mesh for at least 80 minutes without permeating which demonstrate the excellent stability of mesh and give the operator enough time to collect water and oil separately. (Figure S1) In addition, the as-prepared meshes can be easily cleaned for reuse. The meshes maintain the high separation efficiency after 20 times use by taking hexane/water mixture as an example (Figure S2). 3.3 PDA/PEPA Coated Sponge for Immiscible Oil/Water Mixtures Separation and Cu2+ Adsorption


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Figure 3. a) Hexane/water mixtures separation efficiency by PDA/PEPA modified PVA sponge, the inset is the photograph of separation of hexane/water mixtures. b) UV-Vis spectra of Cu2+ solution (Treated with Copper Reagent and extracted by CCl4) before and after adsorption by PDA/PEPA modified PVA sponge. The PVA sponge shows the same capacity in separation of oil/water mixtures. As shown in the inset of Figure 3a, the PVA sponge was fixed between two Teflon features. Hexane (dyed with oil red) and water mixtures were poured onto the sponge, water pass through the sponge quickly while hexane was kept in the upper side. After separation, there was no visible oil can be seen in the water and the separation efficiency can be up to 99.99%. Cu2+ is a toxic soluble contaminate in industrial wastewater. The strong coordination between Cu2+ and amino group made it possible for the material to adsorb copper ions. 0.01 M Cu2+ solution was prepared and poured onto the PDA/PEPA modified PVA sponge. Figure 3b is the UV-Vis spectra of Cu2+ solution (Treated with Copper Reagent and extracted by CCl4) before and after adsorption. The concentration of Cu2+ has also been measured by atomic absorption spectroscopy and the results have been summarized in Table S1. It shows a decrease of copper ions about 13% before and after adsorption due to the large specific surface area of the sponge. 3.4 PDA/PEPA Coated Membrane for Oil-in-Water Emulsions Separation and Methyl Blue Adsorption


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Figure 4. Separation results for surfactant-free and surfactant-stabilized toluene-in-water emulsions. a and c) Droplet size distribution of the surfactant-free and surfactant-stabilized toluene-in-water nanoscale emulsions by DLS. b and d) Optical microscopy images of SFT and SST, respectively, before and after separation.

When the method was applied on substrates with smaller pore sizes, the materials was able to separate oil-in-water emulsions. A series of emulsions including hexane, toluene, and diesel-inwater emulsion have been prepared to test the PDA/PEPA coated MCE membrane. The asprepared membrane was fixed in a vacuum filtration to fullfil the separation. Figure 4 gives the results of surfactant-free (above) and surfactant-stabilized (below) toluene-in-water emulsions as examples. DLS spectra and optical microscopy images demonstrate that these two kinds of emulsions both contain mciro-/nano-sized oil droplets (Figure 4a and c). The photographs of the emulsions and filtrates before and after separation shows that original milky feed emulsions the filtrates became transparent after separation. The optical microscopy images also demonstrate the effective separation that no oil droplets can be observed in the fields (Figure 4b and d). Hexanein-water and diesel-in-water emulsions have been separated and the results are given in Supporting Information (Figure S3 and S4). Besides, the membrane was proved to be stable in acid media as shown in Figure S5 which give it a broader application.


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The water purity after one time separation was calculated by the content of water in filtrate using the infrared spectrometer oil content analyzer. As summarized in Table S2, the oil contents in water were all less than 35 ppm which leads to an extremely high separation efficiency. Figure 5a demonstrates that the separation efficiencies of all six emulsions are higher than 99.6%. The separation efficiency was the average value of at least three measurements with different samples. Membrane permeability is another evaluation for emulsions separation materials. The fluxes for emulsions of the membrane were determined by calculating the permeate volume in unit time and in unit area under the pressure difference of ~ 0.01 MPa. As illustrated in Figure 5b, the fluxes of surfactant-free hexane, toluene, and diesel-in-water emulsions are very high: 34096, 32486, and 27232 L m-2h-1bar-1. Although the fluxes of surfactant-stabilized water-in-oil emulsions have an obvious decrease, about 5000 L m-2h-1bar-1 for surfactant-stabilized toluene and diesel-in-water emulsions. The flux of surfactant-stabilized hexane-in-water emulsions can still be higher than 10000 L m-2h-1bar-1. Two sets of data has proved the theory discussed in previous work that the permeate flux of the membrane is related with the density of oils instead of viscosity when dealing with oil-in-water emulsions.40 The density and viscosity of all the oils employed in this work are summarized in Table S3.


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Figure 5. a) Separation efficiency and b) permeate flux for various emulsions of the PDA/PEPA coated MCE membrane. c) UV-Vis absorption spectra of the original methyl blue (red) solution and after one time adsorption filtrate (black). The inset photographs illustrate that the original methyl blue solution became clear and colourless after adsorption. With good performance in emulsions separation, the as-prepared membrane also possess water soluble dye adsorption property. In the dye adsorption experiment, 5 mg/L of methyl blue was prepared and passed through the PDA/PEPA coated MCE membrane. As exhibited in Figure 5c, the photographs of original methyl blue solution was in the color of blue, and after one time permeate, the solution became clear and colorless. The solutions before and after adsorption were analyzed using UV-Vis absorption spectrometer. In the initial methyl blue solution spectra, the maximum adsorption appears at a wavelength of 628 nm while the peaks were disappeared after one time adsorption by the as-prepared MCE membrane. The photograph of MB adsorbed on the used membrane was shown in Figure S6. We have also test the absorption on the blank membrane and find out that the membrane has a weak ability of dye absorption. In the same condition, the methyl blue can be completely absorbed by the as-prepared membrane. It proves the coating played a major role in the dye absorption process. (Figure S7) We assumed that the


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adsorption was owing to the hydrogen bond formed between sulfonic acid group in MB and abundant amino group on the membrane.

4. Conclusions In summary, a one-step coating method has been developed to realize immiscible oil/water mixtures and emulsion separation and contaminants adsorption. The method is simple and facile and can be applied on organic or inorganic substrates. Different porous substrates including stainless steel mesh, PVA sponge and MCE membrane were coated with PDA/PEPA films. The as-prepared materials were superhydrophilic and underwater-superoleophobic. The modified stainless steel mesh and PVA sponge with relatively large pore size can be used to separate oil/water mixtures with high efficiency. The MCE membrane with relatively small pore size can separate various surfactant-free and surfactant-stabilized emulsions with high efficiency and high flux. All separation efficiencies are higher than 99.6% and the as-prepared materials can be reused. More importantly, the abundant amino groups on the surface can be used for contaminants adsorption. The PVA sponge and MCE membrane can effectively adsorb Cu2+ and methyl blue from the wastewater which allows the as-prepared materials are capable of oil/water separation and water purification at the same time. Therefore, the simple and facile method has excellent potential to be used in practical application such as purifying wastewater from industry and daily life, and dealing with oil spill accidents. ASSOCIATED CONTENT Supporting Information. Separation results of surfactant-free and surfactant-stabilized hexanein-water and diesel-in-water emulsions and DLS data are given in the supporting information.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT The authors are grateful for financial support from the National Natural Science Foundation (51173099, 21134004). The authors also thank Dr. Xiaoyong Zhang for his help in this study. REFERENCES 1.

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26. Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A., Hygro-Responsive Membranes for Effective Oil-Water Separation. Nat. Commun. 2012, 3, 1025. 27. Zhang, F.; Zhang, W. B.; Shi, Z.; Wang, D.; Jin, J.; Jiang, L., Nanowire-Haired Inorganic Membranes with Superhydrophilicity and Underwater Ultralow Adhesive Superoleophobicity for High-Efficiency Oil/Water Separation. Adv. Mater. 2013, 25 (30), 4192-4198. 28. Cao, Y.; Chen, Y.; Liu, N.; Lin, X.; Feng, L.; Wei, Y., Mussel-Inspired Chemistry and Stöber Method for Highly Stabilized Water-in-Oil Emulsions Separation. J. Mater. Chem. A 2014, 2 (48), 20439-20443. 29. Gao, C.; Sun, Z.; Li, K.; Chen, Y.; Cao, Y.; Zhang, S.; Feng, L., Integrated Oil Separation and Water Purification by a Double-Layer TiO2-Based Mesh. Energy Environ. Sci. 2013, 6 (4), 1147-1151. 30. Mohan, D.; Sarswat, A.; Ok, Y. S.; Pittman, C. U., Organic and Inorganic Contaminants Removal From Water with Biochar, a Renewable, Low Cost and Sustainable Adsorbent–a Critical Review. Bioresour. Technol. 2014, 160, 191-202. 31. Ding, J.; Li, B.; Liu, Y.; Yan, X.; Zeng, S.; Zhang, X.; Hou, L.; Cai, Q.; Zhang, J., Fabrication of Fe3O4@Reduced Graphene Oxide Composite via Novel Colloid Electrostatic SelfAssembly Process for Removal of Contaminants from Water. J. Mater. Chem. A 2015, 3 (2), 832-839. 32. Li, B.; Cao, H.; Yin, G.; Lu, Y.; Yin, J., Cu2O@ Reduced Graphene Oxide Composite for Removal of Contaminants from Water and Supercapacitors. J. Mater. Chem. 2011, 21 (29), 10645-10648.


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Insert Table of Contents Graphic and Synopsis Here

A method that can simultaneously separate oil/water mixtures and remove water soluble contaminants has been developed.


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Scheme 1. Schematic description of the preparation of PDA/PEPA modified materials and the process of oil/water mixtures separation and Cu2+ and methyl blue adsorption. 156x113mm (300 x 300 DPI)


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Figure 1. SEM images of the as-prepared materials before and after modification: a and b) The cross-knitted stainless steel mesh change from a smooth and clear surface to numerous hairy nanoparticles coated surface. c and d) The PVA sponge has relatively large pore sizes and the inset figure demonstrate the films has been modified. e and f) The pores on the membrane were less than 1µm and numerous nanoparticles can be observed in the right image. g) Water and dichloroethane (DCE) contact angles on the as-prepared materials and photographs of water droplets spread and permeate the mesh, the sponge and the membrane. h) Hexane, toluene and diesel contact angles of the as-prepared materials underwater. 140x94mm (300 x 300 DPI)



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Figure 2. a and b) The photographs of diesel/water mixtures separation process. c) Separation efficiency PDA/PEPA modified stainless steel mesh for a selection of oil/water mixtures. 19x8mm (600 x 600 DPI)


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Figure 3. a) Hexane/water mixtures separation efficiency by PDA/PEPA modified PVA sponge, the inset is the photograph of separation of hexane/water mixtures. b) UV-Vis spectra of Cu2+ solution (Treated with Copper Reagent and extracted by CCl4) before and after adsorption by PDA/PEPA modified PVA sponge.



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Figure 4. Separation results for surfactant-free and surfactant-stabilized toluene-in-water emulsions. a and c) Droplet size distribution of the surfactant-free and surfactant-stabilized toluene-in-water nanoscale emulsions by DLS. b and d) Optical microscopy images of SFT and SST, respectively, before and after separation.


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Figure 5. a) Separation efficiency and b) permeate flux for various emulsions of the PDA/PEPA coated MCE membrane. c) UV-Vis absorption spectra of the original methyl blue (red) solution and after one time adsorption filtrate (black). The inset photographs illustrate that the original methyl blue solution became clear and colourless after adsorption. 145x75mm (300 x 300 DPI)


Water Mixtures and Emulsions Separation and ... - ACS Publications

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