Polyaniline Nanofibers: Their Amphiphilicity and Uses for Pickering

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Polyaniline Nanofibers: Their Amphiphilicity and Uses for Pickering Emulsions and On-Demand Emulsion Separation Ping Zhou, Jing Li, Wenwen Yang, Lihua Zhu, and Heqing Tang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04353 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Polyaniline Nanofibers: Their Amphiphilicity and Uses for Pickering Emulsions and OnDemand Emulsion Separation Ping Zhou,† Jing Li,*,‡ Wenwen Yang,† Lihua Zhu,*,§ Heqing Tang*,† †

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and

Ministry of Education, College of Resources and Environmental Science, South-Central University for Nationalities, 182 Minyuan Road, Wuhan 430074, People’s Republic of China ‡

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, People’s Republic of China §

College of Chemistry and Chemical Engineering, Huazhong University of Science and

Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China * E-mail addresses: [email protected] (H. Tang), [email protected] (J. Li), [email protected] (L. Zhu).

ACS Paragon Plus Environment

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ABSTRACT: The wetting property of nanomaterials is of great importance to both fundamental understanding and potential applications. However, the study on the intrinsic wetting property of nanomaterials is interfered by organic capping agents, which are often used to lower the surface energy of nanomaterials and avoid their irreversible agglomeration. In this work, the wetting property of nanostructured polyaniline that requires no organic capping agents is investigated. Compared to hydrophilic granular particulates, polyaniline nanofibers are amphiphilic and have an excellent capability of creating Pickering emulsions at a wide range of pH. It is suggested that polyaniline nanofibers can be easily wetted by water and oil. Furthermore, the amphiphilic polyaniline nanofibers as building blocks can be used to construct filtration membranes with small pore size. The wetting layer of the continuous phase of emulsions in the porous nanochannels efficiently prevents the permeation of the dispersed phase, realizing highefficiency on-demand emulsion separation.

KEYWORDS: polyaniline; nanostructures; amphiphilicity; Pickering emulsions; on-demand emulsion separation

INTRODUCTION Polyaniline as one of the most useful conducting polymers has received great attention due to its easy synthesis, reversible acid/base doping/dedoping capability, and environmental stability. Since polyaniline nanofibers were developed via an interfacial polymerization,1 it is continuously being demonstrated that its nanostructures can lead to better performance in already established areas such as sensors,1-6 and can create new opportunities such as flash welding.7 Besides, polyaniline nanofibers can be well and stably dispersed in water by purifying product and


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controlling the pH (about 3) without any surface modifier or steric stabilizer.8 For irregular granular particulates synthesized via the conventional method, the sedimentation in water is quickly observable after standing.9 The stable colloids of polyaniline nanofibers make it very convenient to operate using low-cost and environmental-friendly solution-processing techniques. Taking advantage of the good dispersibility in water, Pd nanoparticles supported on polyaniline nanofibers as semi-heterogeneous catalysts in water have been also exploited.10 Routinely, polyaniline has been regarded as hydrophilic to broadly improve the performance and antifouling ability of the membranes.11-13 Recently, it is reported that graphene oxide (GO) should be viewed as an amphiphile in spite of its water dispersity and is capable of creating Pickering emulsions.14 Because polyaniline has similar conjugated structures with GO,15 polyaniline would be amphiphilic to stabilize Pickering emulsions. More importantly, Pickering emulsions stabilized by solid particles have been developed for emulsion polymerization,16-20 biphasic catalysis,21-24 SERS detection,25 separation,26 preparation of various functional materials including porous,27-29 Janus,30-32 and molecular imprinting33,34 materials. For example, carbon nanotube-SiO2 hybrid nanoparticles deposited with Pd can stabilize emulsion droplets and serve as microreactors to catalyze biofuel upgrade reactions at water-oil interface.21 Therefore, there is a need to further study the intrinsic wetting property of polyaniline especially its nanostructures. In this work, we prepared polyaniline nanofibers and granular particulates to investigate their wetting property. It is found that polyaniline nanofibers show excellent amphiphilic property. Compared to hydrophilic granular particulates, polyaniline nanofibers can act as stabilizers at various oil-water interfaces to form Pickering emulsions. In addition, amphiphilic polyaniline


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nanofibers can be also used as building blocks to construct filtration membranes with underwater superoleophobicity and underoil superhydrophobicity for on-demand emulsion separation.

EXPERIMENTAL SECTION Materials. All chemicals were commercial available and used as received. Deionized water (18.2 MΩ cm) was used throughout the work. Organic microporous filtration membranes (0.45 µm) as the support were supplied by Shanghai Xingya Purifying Materials Factory (China). Preparation of Polyaniline Nanofibers and Granular Particulates. Aniline (0.1 M) and ammonium peroxydisulfate (0.05 M) were dissolved in 20 mL HCl (2 M) and pure water, respectively. After rapid mixing and vigorous shaking for 1 min, the reaction proceeded for 1 h without stirring. Subsequently, the product was washed by centrifugation and then redispersed in water for further use. To prepare polyaniline granular particulates, the reaction concentrations of aniline and ammonium peroxydisulfate were increased to 0.4 and 0.2 M, respectively. The reaction was performed at 0 ºC under stirring. The concentration of the stored polyaniline was determined by a definite volume of polyaniline aqueous solutions and the mass measurement of polyaniline by depositing on the organic membranes via vacuum filtration. Preparation of Polyaniline Thin Films. The suspensions of polyaniline nanofibers and granular particulates were vacuum filtrated through commercial organic microporous filtration membranes, followed by drying in an oven at 50 ºC. Preparation of GO and GO-Coated Mesh. Natural flake graphite (0.5 g) and NaNO3 (0.5 g) were added into concentrated sulfuric acid (23 mL) under stirring in an ice-water bath. After KMnO4 (3 g) was slowly added, the reaction system was transferred to a 35 ºC oil bath and stirred for about 1 h. Afterward, 40 mL of water was slowly added dropwise and the temperature


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of oil bath was raised to 90 ºC. The reaction was conducted for another 30 min. Finally, 100 mL of water and 5 mL of H2O2 (30%) were added. The product was washed with water by centrifugation and then dispersed into 200 mL of water, followed by low-power sonication for 30 min (Branson 1510E-MT). To prepare GO-based film, stainless steel mesh (300 mesh size) was immersed into the GO aqueous solution for 10 min, followed by drying in an oven at 50 ºC. The dip-coating procedure was repeated for 5 times. Study on the Activity of Polyaniline and GO at Oil-Water Interfaces. The aqueous solutions of polyaniline and GO (0.5 g/L) were mixed with oils (such as hexane, toluene, nhexadecane, chloroform, and 1,2-dichloroethane) at the same volume by vigorous shaking. The pH was adjusted by HCl and NaOH. Preparation of Oil-in-Water and Water-in-Oil Emulsions. Oils included hexane, toluene, nhexadecane, chloroform, and 1,2-dichloroethane. Tween 80 and Span 80 were selected to stabilize oil-in-water and water-in-oil emulsions, respectively. Water and oil (100:1 or 1:100 v/v) were mixed by using a high-speed mixer (5000 r/min), forming highly stable oil-in-water and water-in-oil emulsions for months. Emulsion Separation. Emulsion separations were tested in a filtration system. Pressure difference (0.1 bar) was controlled by a vacuum pump (Millipore). Flux was determined according to the equation: flux = V/St, where V is the volume of filtrates, S is the area of membrane, and t is the separation time (5 min). Characterization. Optical and microscopy photographs were taken by a digital camera (SONY DSC-T99DC) and a microscope (Nikon 50i), respectively. UV-vis absorption spectra were recorded by a spectrophotometer (Thermo Evolution201). Hydrodynamic diameters of


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emulsions were measured by a Zetasizer Nano ZNE3690 (Malvern, UK). Contact angles were measured on a JC20001 system (Zhongchen digital equipment Co., Ltd. Shanghai, China). The morphology of the samples was analyzed by transmission electron microscopy (TEM, TECNAI G2 20S) and scanning electron microscopy (SEM, Hitachi SU8010). TEM and SEM samples were prepared by dropping the polyaniline suspensions on a copper TEM grid and conductive adhesive, respectively, and then dried at room temperature. The three-dimensional profiles of the prepared polyaniline thin films were imaged using an atomic force microscope (AFM, Agilent 5500) in the contact tapping mode. XPS analysis was conducted on a Multilab2000 X-ray photoelectron spectrometer. Raman spectra were obtained on a DXR Raman microscope with 532 nm wavelength laser light (Thermo Scientific). XPS and Raman samples were prepared by vacuum filtrating polyaniline suspensions through porous alumina membranes (0.2 µm, Whatman). Organic contents in the filtrates were studied by a total carbon analyzer (Elementar vario TOC). Water contents in the filtrates were detected by a Karl Fischer titrator (Metrohm 831 KF). The conductivity of polyaniline films in the doped state (pH 2) was measured by a fourprobe conductivity test meter (Mitsubishi Chemical MCP-610). The hydrophilic-lipophilic balance (HLB) was estimated according to the reported method, in which Span 80 and Tween 80 emulsifiers were used as standard samples.35 Water was slowly dropped into isopropyl-toluene solutions (5.8 mL, 100:15 v/v) containing polyaniline nanofibers until the emulsions were formed. The HLB values were calculated by the consumption volume of water.

RESULTS AND DISCUSSION Polyaniline nanofibers and granular particulates were prepared according to our previous work by controlling aniline polymerization parameters.36 Dilute reactants (0.05 M aniline), higher


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temperature (room temperature), and no stirring are advantageous to one-dimensional growth of polyaniline under the protection of the electric double layers. In contrast, the preparation conditions of polyaniline granular particulates are 0.4 M aniline, 0 ºC, and stirring. Figure 1 shows TEM and SEM images of the prepared polyaniline nanofibers and granular particulates. Polyaniline nanofibers present uniform diameters of 20-40 nm. The lengths of the nanofibers range from 500 nm up to several micrometers, exhibiting an interconnected network. When the electric double layers are disturbed and destroyed, the resulting particulates that consist of short nanofibers are highly and irreversibly aggregated. The size is up to several micrometers. We used the samples to investigate the structure-wettability relationship of polyaniline without organic capping agents such as organic doping acids.37 Figure 2 shows UV-vis absorption spectra of the suspensions of polyaniline nanofibers in water. At pH 2, polyaniline nanofibers are completely doped (Figure S1), showing absorption bands at about 350, 400, and 800 nm. Above pH 6, the absorption bands shift to about 320 and 600 nm owing to the dedoping of polyaniline. Polyaniline nanofibers have much better dispersibility in water than granular particulates (Figure S2), which is consistent with the reported results.9 To further estimate the hydrophilic degree of polyaniline nanofibers and granular particulates, we studied their activity at various oil-water interfaces. It is found that polyaniline nanofibers can behave like a colloidal surfactant to adsorb at the oil-water interfaces and lower the surface tension, forming Pickering emulsions that can be highly stable for months (Figure 3). Clearly, the droplets are uniformly coated by green doped or blue dedoped polyaniline nanofibers. The activity of polyaniline nanofibers at oil-water interfaces is obviously superior to GO. For example, GO stabilizes aromatic solvents (such as toluene) more efficiently than aliphatic solvents (such as hexane) and the pH must be decreased down to 2 (Figure S3).15 Interestingly, Pickering emulsions can be created at various


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oil-water interfaces using polyaniline nanofibers at a wide range of pH values from 2 to 10. In contrast, both doped and dedoped polyaniline granular particulates stay in the water phase or climb on the hydrophilic vessel wall and cannot stabilize Pickering emulsions even after vigorous shaking (Figure 4).38 Therefore, polyaniline nanofibers are more hydrophobic and possess stronger capability to form Pickering emulsions than granular particulates and GO. Compared to granular particulates, polyaniline nanofibers with higher hydrophobicity have better dispersibility in water. Similarly, liquid exfoliated graphene with fewer layers exhibits better dispersibility.39-41 Here we recommend Zobel’s research, in which solvents universally restructure around nanoparticles and a solvation shell from the molecule size up to 2 nanometers contributes to the enhanced reactivity of solvated nanoparticles.42 In other word, polyaniline nanofibers can be easily wetted by water and oil, leading to the improved dispersibility in water and the formation of Pickering emulsions. We measured contact angles of thin films constructed by polyaniline nanofibers and granular particulates to further evaluate the wetting property of polyaniline. Polyaniline aqueous solutions were vacuum filtrated through a commercial membrane (0.45 µm), forming the thin films (about 0.4 g/m2). Figures 5 and S4 show SEM images of the support and thin film of polyaniline nanofibers. Polyaniline nanofibers are uniformly deposited on the support with a thickness of about 1 µm. The water and oil contact angles in air on the thin films of polyaniline nanofibers are 0º (Figure 6a). Excluding the influence of the support on the contact angle measurements (Figure S6), the thin films of polyaniline nanofibers in the doped and dedoped states show underwater superoleophobic and underoil superhydrophobic (Figure 6b). The underoil water contact angles and underwater oil contact angles are more than 150º. Tian and co-workers reported that water intrinsic contact angles needed to be in the range of 56-74º to produce the unusual dual


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superlyophobic surfaces with the re-entrant structures.43 Indeed, polyaniline nanofibers as a single component can be used to build the superwetting surfaces in the solid-water-oil system. The thin film of polyaniline granular particulates is underwater superoleophobic and only underoil hydrophobic. The underoil water contact angle is less than 150º (Figure S7). Under hexane, when we take a water droplet to contact the thin films of polyaniline nanofibers, the droplet distorts under pressure and can completely detach after dropping sample stage. In contrast, the water droplet strongly adheres on the thin film of granular particulates (Figure S8). Inspired by fish scales and clam shells, GO as a building block was used to construct biomimetic film.44 The GO-coated stainless steel mesh is underwater superoleophobic. Under oil, the GO coating selectively absorbs water, resulting in a low water contact angle (about 65º, Figure S10), which is ascribed to the high hydrophilic degree of GO. Reliably, the contact angle results agree with our viewpoint that polyaniline nanofibers are more hydrophobic than granular particulates and GO. Hence, the amphiphilic property of polyaniline nanofibers contributes to the excellent capability of forming Pickering emulsions and the construction of superwetting surfaces in the solid-water-oil systems. We deposited polyaniline nanofibers and granular particulates onto the surfaces of porous alumina membranes (0.2 µm, Whatman) and investigated their structure difference by using XPS and Raman analysis. As shown in Figure 7, the high-resolution XPS C1s spectrum of polyaniline nanofibers exhibits one single peak at about 284.6 eV. The granular particulates have the similar XPS spectrum except for the appearance of small tails at the higher binding-energy region (around 289 eV). For Raman spectra, over 1000 cm-1, six peaks at about 1164, 1219, 1333, 1476, 1558, and 1592 cm-1 are ascribed to C-H bending (Q), C-H bending (B), C-N stretching, C=N stretching, C-C stretching, and C=C stretching, respectively. Differently, the intensity ratios


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(including IC=C:IC-C and IC=N:IC-N) of polyaniline nanofibers are obviously higher than that of granular particulates. Moreover, the granular particulates show an additional peak at about 572 cm-1, which is attributed to amine deformation. According to the proposed theoretical models,9,36,45 nanofiber is the intrinsic morphology of polyaniline. As polyaniline takes place homogeneous nucleation, the initial nanofibers have enough positive charges on the chains, limiting the side-toward diffusion of reactive species by electrostatic repulsion forces. Because the active reactants are available only at the end of the nanofibers, one-dimensional growth advantageously and continuously proceeds. The head-to-tail polymerization favors the formation of highly π-conjugated structures of polyaniline nanofibers with high conductivity (4.39 S/cm), leading to one main XPS C1s peak and enhanced hydrophobicity. When stirring destroys the protection of the electric double layers around the initial nanofibers, the reactive species easily arrive at the surface nucleation sites and take place heterogeneous growth, forming irregular granular particulates with low conductivity (0.007 S/cm). Reasonably, these renascent polyaniline could have different chemical structures, such as more quinoid units and amine groups verified by XPS and Raman data, resulting in the hydrophilicity of the granular particulates. Besides, according to the reported method,34 the measured HLB value of polyaniline nanofibers is about 16 (Figure S12). Together, amphiphilicity is the intrinsic wettability of polyaniline nanofibers with hydrophobic πconjugated structures and hydrophilic nitrogen components. Amphiphilic nanomaterials especially one- and two-dimensional nanomaterials that can stabilize Pickering emulsions would be expected for on-demand emulsion separation without any continuous external stimulus.46-51 One- and two-dimensional nanomaterials as building blocks can be used to construct filtration membranes with smaller pore sizes than that of emulsion


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droplets, such as ultrafiltration and nanofiltration membranes.52-54 The continuous phase (water or oil) of emulsions can wet amphiphilic nanomaterials. The wetting layer in porous nanochannels will efficiently prevent the permeation of the dispersed phase, resulting in ondemand emulsion separation. To test the hypothesis, we estimated the capability of thin film of polyaniline nanofibers to separate various emulsions. Unless otherwise stated, 1 g/L Tween 80 and 1 g/L Span 80 were selected to prepare highly stable oil-in-water and water-in-oil emulsions using a high-speed mixer, respectively. Oils included n-hexane, toluene, n-hexadecane, chloroform, and 1,2-dichloroethane. The applied force was 0.1 bar. Figure 8 shows dynamic light scattering (DLS) and photographs (insets) of Tween 80 stabilized oil-in-water emulsions and Span 80 stabilized water-in-oil emulsions before and after separation. Clearly, the droplet sizes of the prepared emulsions are in range of 70-1000 nm, whereas only DLS signals at several nanometers of the collected filtrates can be observed. Note that the DLS signals at several nanometers also appear in the emulsifier aqueous solutions. Moreover, the turbid emulsions become transparent after separation and no droplets are observed in the collected filtrates from microscopy images (Figures S13 and S14), indicating that filtration membranes made of polyaniline nanofibers can be applied for high-efficiency emulsion separation, even nanoemulsion separation. In addition, the flux of the emulsion separation is strongly dependent on the droplet sizes of emulsions (Tables S1 and S2), for example, nanoemulsion separation takes a long time. Total organic carbon (TOC) was measured to detect organic contents in the collected filtrates after the separation of oil-in-water emulsions. For water-in-oil emulsions, the oil purities of the collected filtrates were analyzed by a Karl Fischer titrator. As a result, the TOC values in the collected filtrates for oil-in-water emulsions are about 300 mg/L (Figure 9a). Furthermore, we


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tested the TOC values of 1 g/L emulsifier aqueous solutions and the collected filtrates after separating the emulsifier stabilized oil-in-water emulsions, in which emulsifiers included Tween 80, sodium dodecyl sulfonate (SDS), and cetyltrimethyl ammonium bromide (CTAB). The TOC values of the collected filtrates are all less than that of emulsifier aqueous solutions (Table S3), indicating that the filtrates mainly contain emulsifiers. On the other hand, over 99.95% purities of all collected oils can be achieved for water-in-oil emulsions. Finally, we studied the recyclability of the thin film of polyaniline nanofibers for on-demand emulsion separation. Tween 80 (1 g/L) stabilized hexane-in-water emulsion and Span 80 (1 g/L) stabilized water-indichloroethane emulsion were employed as feed emulsions. In every cycle, ethanol was used to wash the membrane, which facilitated the switch of solvent molecules in the porous nanochannels. As shown in Figure 9b, the fluxes for the separation of both oil-in-water and water-in-oil emulsions remain unchanged. These results demonstrate that amphiphilic polyaniline nanofibers are promising building blocks to construct filtration membranes for ondemand emulsion separation, which would be extended to other amphiphilic nanomaterials with the capability of creating Pickering emulsions.

CONCLUSIONS In summary, the intrinsic wetting property of polyaniline has been studied. It is found that polyaniline granular particulates synthesized via the conventional method are hydrophilic. In contrast, polyaniline nanofibers are amphiphilic and have an excellent capability of creating Pickering emulsions at a wide range of pH from 2 to 10. XPS and Raman analysis confirm that one-dimensional growth favors the formation of highly π-conjugated structures of polyaniline nanofibers with high conductivity, resulting in the enhanced hydrophobicity. The filtration


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membranes constructed by the amphiphilic polyaniline nanofibers as a single component can be efficiently used for on-demand emulsion separation. The discovery will explore polyaniline nanofibers more opportunities taking advantages of Pickering emulsions and will extend to other conductive nanomaterials. Wishfully, more amphiphilic nanomaterials that can stabilize Pickering emulsions will be developed for the applications in on-demand emulsion separation.


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Figure 1. (a, b) TEM and (c, d) SEM images of polyaniline nanofibers (a, c) and granular particulates (b, d).


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Figure 2. UV-vis absorption spectra of polyaniline nanofibers in aqueous solution at different pH.


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Figure 3. Polyaniline nanofibers at various oil-water interfaces. The top and bottom rows show microscopy images of oil droplets stabilized by doped (pH 2) and dedoped (pH 10) polyaniline nanofibers, respectively. The bar is 200 µm.


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Figure 4. Polyaniline granular particulates at hexane-water interface.


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Figure 5. (a-c) Top-view and (d) cross-section SEM images of the thin film of polyaniline nanofibers.


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Figure 6. (a) Photograghs of a water droplet in air or an oil droplet (1,2-dichloroethane) in air on the thin films of polyaniline nanofibers. (b) Photograghs of an oil droplet (1,2-dichloroethane) in water or a water droplet in oil (hexane) on the thin films of doped and dedoped polyaniline nanofibers.


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Figure 7. (a) High resolution XPS C1s and (b) Raman spectra of polyaniline nanofibers and granular particulates.


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Figure 8. (a-c) DLS and photographs (insets) of 1 g/L Tween 80 stabilized oil-in-water emulsions before and after separation. (d-f) DLS and photographs (insets) of 1 g/L Span 80 stabilized water-in-oil emulsions before and after separation. The used oils were n-hexane (a), toluene (b, f), n-hexadecane (c), chloroform (d), and 1,2-dichloroethane (e).


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Figure 9. (a) TOC in the filtrates after separating 1 g/L Tween 80 stabilized oil-in-water emulsions and oil purity of the filtrates after separating 1 g/L Span 80 stabilized water-in oil emulsions. Oil-in-water emulsions included hexane-in-water (1), toluene-in-water (2), and hexadecane-in-water (3) emulsions. Water-in-oil emulsions included water-in-chloroform (1'), water-in-dichloroethane (2'), and water-in-toluene (3') emulsions. (b) Cycling performance of filtration membranes of polyaniline nanofibers for switchable hexane-in-water and water-indichloroethane emulsion separation.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Dispersibility of polyaniline nanofibers and granular particulates in aqueous solutions, interfacial activity of GO, SEM, AFM images and contact angles of the support and polyaniline thin films, SEM images, contact angles and oil-water separation of GO-coated mesh, HLB measurement, typical microscopy images of emulsions before and after separation, and Tables S1-S3. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Tang). *E-mail: [email protected] (J. Li). *E-mail: [email protected] (L. Zhu). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China (no. 21377169 and 21203217). REFERENCES (1) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. Polyaniline Nanofibers: Facile Synthesis and Chemical Sensors. J. Am. Chem. Soc. 2003, 125, 314–315. (DOI: 10.1021/ja028371y)


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Table of Contents Graphic and Synopsis


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Polyaniline Nanofibers: Their Amphiphilicity and Uses for Pickering

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