Superhydrophobic Cellulose Nanofiber-Assembled Aerogels for

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Superhydrophobic Cellulose Nanofiber-Assembled Aerogels for Highly Efficient Water-in-Oil Emulsions Separation Sukun Zhou, Tingting You, Xueming Zhang, and Feng Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00079 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Applied Nano Materials

Superhydrophobic Cellulose Nanofiber-Assembled Aerogels for Highly Efficient Water-in-Oil Emulsions Separation Sukun Zhou, Tingting You, Xueming Zhang, and Feng Xu* Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China * Corresponding author, E-mail: [email protected] (F. Xu); Tel/fax: +86 10 62337993

Abstract Here, we reported a facile strategy to create superhydrophobic aerogels via freeze-drying of silylated cellulose nanofibers and silica nanoparticles mixed suspensions. The as-prepared aerogels possessed a hierarchical porous structure with high roughness and low surface energy. The hierarchical rough structure and low surface energy endowed the resultant aerogels with superhydrophobicity (water contact angle up to 168.4°). Importantly, the composite aerogels could separate surfactant-stabilized water-in-oil emulsions without external pressure, with high separation efficiency (> 99%) and high flux (1910 ± 60 L m-2 h-1). The aerogels were easily recyclable and showed great antifouling performance, which could meet the requirements for long-term use. We also assembled a simple device to collect oil directly from water-in-oil emulsions with the obtained aerogel and a self-priming pump. The fabrication of the composite aerogels in our work provides a versatile way to fabricate cellulose composite materials for water-in-oil emulsions separation. Keywords: Cellulose nanofibers, Silica nanoparticles, Aerogels, Superhydrophobic, Emulsions separation

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Introduction Oil-water separation has become a worldwide problem because of increasing industrial oily water pollution, which is posing great threat to the environment and health.1 Various methods have been developed for separation of immiscible oil-water mixtures, including absorption, flotation, flocculation, electrocatalysis,

and

biofiltration.2-4

However,

they

are

invalid

for

the

separation

of

surfactant-stabilized oil-water emulsions, especially the emulsions with droplet sizes less than 20 µm, because the surfactant-stabilized emulsion droplets have lower surface energy than those for immiscible and surfactant-free ones.5 Traditional demulsification methods mainly rely on introducing an electric field or adding chemicals,6-7 but these methods cannot meet the requirement of low energy consumption, high separation efficiency, and no secondary pollution. Physical filtration method is a promising method for oil-water emulsions separation, and this method usually bases on separation materials. Commercial two-dimensional (2D) micro- and ultrafiltration membranes, which show high separation efficiency (> 98%) for emulsions have been applied to industrial wastewater separation.8-15 However, these membranes are easily fouled by surfactant and oil due to low porosity and short permeation channels. Three-dimensional (3D) bulk aerogel is a kind of ideal material for the separation of oil-water emulsions due to its high porosity and long permeation channels.2, 16-17 Some functional aerogels made from carbon nanotube,18 graphene,17 polymers,19-22 boron nitride,23 and nanocellulose,24-26 have been applied to oil-water separation. However, most of these current research mainly focused on the immiscible oil-water mixtures separation,26-30

and oil absorption capacity of the aerogels,20, 24, 31-32 while little attention on the oil

recovery procedures and the water-in-oil emulsions separation. Therefore, it is necessary to develop a low cost and environmentally friendly oil-water emulsions separation aerogel. 2


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Cellulose nanofiber (CNF) disintegrating from wood or other agricultural byproducts is widely used for the fabrication of functional materials due to its attractive properties, including abundant resource, biodegradability, renewability, high Young's modulus and high aspect ratio. Benefiting from its crystal structure, CNF is also a kind of rigid nanoscale building block for fabricating functional aerogels.33-34 CNF aerogels hold the advantages of high porosity (over 99%), low density (0.27-103 mg/cm3), and high specific surface areas (10-284 m2/g).35-37 The design of aerogels for oil-water emulsions separation needs to meet two important requirements, porous structure and hydrophobicity.38 The pores in the aerogels govern the permeation rate of the liquid passing through the aerogels, while the hydrophobicity affects the interception and demulsification of the mixture. As mentioned above, the pristine CNF aerogels possess highly porous structure, which is a basic requirement for the oil-water emulsions separation materials. As for the requirement of hydrophobicity, the CNF can be easily modified due to the large amount of hydroxyl groups on its surface.39 Besides, the nanoparticles such as SiO2 and TiO2 can be deposited on the surface of CNF, which would improve the surface area and roughness of the CNF aerogels. Thus, the CNF aerogels can meet the two basic requirements for the separation of oil-water emulsions. Here, we exhibited a simple way to fabricate superhydrophobic cellulose nanofiber-silica nanoparticle composite aerogels for the filtering separation of surfactant-stabilized water-in-oil emulsions. The obtained aerogels were highly efficient in separating surfactant-stabilized water-in-oil emulsions without external pressure, with a high permeation flux. We also assembled an oil recycling device consisting of the obtained aerogels and a self-priming pump to recycle oil from stabilized water-in-oil emulsions in a direct and continuous way.

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Experimental Section Materials. Cellulose was purchased from Northwood Pulp and Timber Limited (Canada). TEMPO, sodium bromide, sodium hypochlorite, ethanol, methyltrimethoxysilane (MTMS), silica (SiO2) nanoparticles with diameter about 30 nm, span 80, petroleum ether, trichloromethane, toluene, hexane, dichloromethane, isooctane, gasoline, motor oil, and other chemicals were purchased from Beijing Chemical Factory. Preparation of CNF-based Composite Aerogels. CNF suspensions were fabricated refering to a previously literature.40 The procedure for preparation of CNF composite aerogels was shown in Figure 1. Firstly, different amount of SiO2 nanoparticles (Table 1) were added to the obtained CNF suspension (100 mL) and stirred at 1000 rpm for 20 min, and then 2 mL MTMS were added dropwise to the mixed suspension with further stirring for 1 h at 1000 rpm. Finally, the composite aerogels were obtained by freeze-drying of the suspensions.

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Figure 1. Schematic illustration for the composite aerogels preparation. Table 1. Reagents dosage in the fabrication process.

Samples

CNF (g)

MTMS (mL)

SiO2 (mg)

A0

0.4

0

0

AM0

0.4

2

0

AM1

0.4

2

5

AM2

0.4

2

10

AM3

0.4

2

15

AM4

0.4

2

20

Emulsions Separation Experiments. The surfactant-stabilized water-in-oil emulsions were prepared referring to a previous literature.10 Briefly, 0.1 wt% span 80 was firstly mixed with different kinds of oils (petroleum ether, trichloromethane, toluene, hexane, dichloromethane, isooctane, soybean oil, gasoline, motor oil, and

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silicone oil), and 2 wt% water was added into the oils. Following, the mixtures were stirred by an IKA T15 blender (10000 rpm for 10 min) and further stirred at 500 rpm for 3 h to form milky white solutions. The surfactant-stabilized water-in-oil emulsions were stable for more than 1 month without demulsification. The droplet sizes of the emulsions were observed by an optical microscopy (Leica DM IL). In the separation process, the obtained aerogels were placed into a glass funnel, and then freshly prepared emulsions were poured into the glass funnel and the oils permeated automatically. The fluxes of the aerogels were calculated from the volumes of the filtrate within 5 min according to eq 1.

Flux = ⁄

eq 1

where V is the volume of filtrate, A is the valid filtration area of the aerogel and t is the testing time. The separation efficiency was calculated by the water rejection efficient (E) using eq 2.

= 1 − ⁄ × 100%

eq 2

where and stands for the water concentration in the filtrate and the oil-water mixture, respectively. Characterizations. The apparent density of the aerogel was determined by measuring its weight and volume. The weight of aerogel was measured by an electronic balance with a readability of 1µg and the dimension was measured using a digital caliper. Aerogel porosity was calculated by the aerogel apparent density ( ) and solid density ( ) using eq 3.

P = 1 − ⁄ × 100%

eq 3

Compared with the weight percentage of cellulose nanofibers, the weight fraction of silica nanoparticles in the composite aerogels is small, thus we used the density of cellulose (1500 kg/m3) as the solid density, based on literature data.41 6


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Scanning electron microscopy (SEM) images were examined using a Hitachi S8010 field-emission SEM system. FTIR spectra were obtained by an infrared spectrophotometer (Nicolet iN10-MX, ThermoScientific) in the range of 4000-400 cm-1. X-ray photoelectron spectroscopy (XPS) was conducted on an X-ray Photoelectron Spectrometer (EscaLab 250Xi, ThermoFisher). Water contact angles (WCAs) were measured by a contact angle meter (SL200KS, KINO) and the average value was obtained from three measurements for each sample. N2 adsorption-desorption isotherms were examined by a Gemini V (Tristar II 3020, Micromeritics). The specific surface areas were determined by BET method from obtained adsorption isotherm, and the pore diameter distribution between 2-100 nm were obtained from the absorption isotherm by the Barrett-Joyner-Halenda (BJH) method. A mercury porosimeter (Auto Pore IV9500, Micromeritics) was used to analyze the pore diameter distribution between 0.1-100 µm for the aerogels. Stress-strain curves were obtained by a Zwick 005 Materials Tester with a 50 N load cell and the specific parameters referred to a previously published method35.

Results and Discussion Characterizations of the aerogels. The preparation procedure for superhydrophobic aerogels was shown schematically in Figure 1. CNF was chosen as building blocks for the construction of aerogels due to their high aspect ratio and high Young modulus. Silica nanoparticles were added to improve the surface roughness and the porosity of the obtained materials. We used methyltrimethoxysilane (MTMS) as hydrophobic modification reagent to decrease the surface energy of cellulose nanofibers and silica nanoparticles. Moreover, MTMS could form covalent bonds with both cellulose and silica, which avoids the silica 7



leakage during filtration process. As a reference, pure aerogel was also prepared by freeze-drying of pristine cellulose nanofibers suspension without silica nanoparticles and MTMS modification. The densities, porosities as well as specific surface areas for all the aerogels were presented in Table S1. Compared with the pure CNF aerogel (A0), the densities and specific surface areas of MTMS modified aerogels increased because of the addition of silica nanoparticles. However, the change of the porosity could be ignored after the modification process, and all the aerogels exhibited high porosity above 99%. According to the Cassie model, 42-44 high surface roughness and low surface energy are beneficial to obtaining hydrophobic surface. Therefore, the construction of micro- and nanoscale hierarchical structure is important for obtaining hydrophobic surface.45-46 Figure 2b-d presented the SEM images with different magnifications. The images showed that the aerogel exhibited a hierarchical three-dimensional porous structure composed of thin sheets, nanofilaments and nanoparticles. The thin sheets in Figure 2b were derived from the agglomeration of cellulose nanofibers during the freezing process, and there were numerous pores with diameter sizes about dozens of microns between these sheets. Compared with the pores between the thin sheets, the pores between the nanofilaments were relatively small (Figure 2c). Image with high magnification (Figure 2d) presented that the silica nanoparticles were deposited on the surface of thin sheets, confirming the successful formation of nanoscale roughness in the composite aerogels. The SEM images for other aerogels were shown in Figure S1. As expected, with increasing the silica nanoparticles amount, the specific surface area of the obtained aerogel significantly increased from 28.3 to 108.6 m2 g-1 (Table S1). Figure 3 exhibited the pore size distribution of the aerogel (AM4) and the results were in accord with the morphology analysis. The pore systems around 20 µm and 50-60 µm were derived from the pores formed by the thin sheets 8


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in Figure 2b, and the pores with diameters around 8 µm might stem from the nanofilaments. Finally, the pores with diameters about 6-40 nm resulted from the silica nanoparticles. The enhanced surface roughness and hierarchical porous structure were favorable for the efficient oil-water emulsions separation.

Figure 2. (a) Photograph of the obtained aerogel (AM4). (b-d) SEM images of aerogel (AM4) at different

magnifications.

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Figure 3. (a) Pore diameter (2-100 nm) distribution obtained from the absorption isotherm by the BJH method and (b) Pore diameter (0.1-100 µm) measured by mercury intrusion method for the obtained aerogel (AM4).

Apart from the high surface roughness, low surface energy is also crucial to the surface wettability.47 The surface chemical composition of the aerogels was characterized by FTIR and XPS spectra (Figure 4). Pure cellulose nanofiber aerogel showed several strong absorption peaks in FTIR spectra, which derived from the stretching vibrations of –OH (3430 cm-1), C–H (2920 cm-1) and C–O (1000-1130 cm-1). In addition to these peaks, several new peaks at 750 cm-1, 2980 cm-1, 1250 cm-1 and 1000–1130 cm-1 appeared in the MTMS modified aerogel FTIR spectra (Figure 4a). According to the literature, 35 these peaks were corresponded to the vibrations of CH3 in silicane, Si–CH3 bonds, and Si– O–Si bonds respectively. As shown in Figure 4b, the XPS spectra demonstrated that pure cellulose aerogel mainly contained carbon and oxygen elements, while after MTMS modification, two new peaks were observed at 101.0 (Si 2p) and 152.0 eV (Si 2s). The high resolution C 1s peak for the MTMS modified aerogel showed three peaks at 282.9 eV (C–C and CH3 bonds), 285.0 eV (C–Si bonds), and 286.7 eV (C–O bonds).48 All these results confirmed the successful modification and the incorporation of Si element endowed the material with low surface energy.

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Figure 4. (a) FTIR spectra and (b) XPS spectra of the pure cellulose aerogel (A0) and MTMS modified aerogel (AM0), (c) the high resolution Si 2p spectrum and (d) high resolution C 1s spectrum of the MTMS modified aerogel (AM0).

Benefiting from the high surface roughness and low surface energy, the aerogels showed superhydrophobicity and superoleophilicity. As shown in Figure 5a, the obtained aerogel (AM4) could absorb the oil droplet on its surface, while leave the water droplets standing on its surface. The water contact angles (WCAs) for aerogels were exhibited in Figure 5b. As expected, the MTMS modified aerogel (AM0) was hydrophobic with a WCA of 129.9°, which could be ascribed to successful

silanizing modification. A remarkable increase of WCA up to 168.4° was achieved with increasing the amount of silica nanoparticles, demonstrating the superhydrophobicity of the obtained composite 11



aerogels. Figure 5c exhibited photos of water and oil droplets (5 µL) contacting aerogel surfaces. The liquid droplets were forced to fully touch the composite aerogel surface even when the droplets were squeezed. The resultant photos presented that the water droplet could recover to its original shape after leaving the surface, demonstrating good hydrophobicity and low water adhesion. On the contrary, when the oil droplet contacted the aerogel surface, it would be absorbed by the aerogel immediately. These results were consistent with previous literature, which concluded that the parameters of the porous structure significantly influence the wettability of the surfaces and oils could penetrate into the composite aerogels.49 The superhydrophobic and superoleophilic properties ensured that the oil could permeate and move through the aerogels, which are good for the oil-water separation performance.

Figure 5. (a) Water (blue) and oil (orange) on the surface of AM4. (b) WCAs for aerogels with different amount of silica nanoparticles. (c) Photos of the process of water (top) and oil (bottom) contacting the surface of AM4.

The compressive stress–strain curves for the aerogels were exhibited in Figure 6. All the aerogels showed similar curves, indicating that the modification and addition of SiO2 nanoparticles did not influence the mechanical property of the nanocellulose aerogels. All these aerogels could be compressed to more than 70% without mechanical failure due to the flexibility of cellulose nanofibers. The aerogels presented about 276 kPa at 70% stain, and this value was higher than the values of

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hydrophobic nanocellulose aerogels in previous literature.24-25 The high compressive stress would be beneficial for the integrity maintenance in practical applications.

Figure 6. Mechanical properties for the aerogels.

Water-in-oil emulsions separation. To evaluate the separation performance of the obtained aerogels, the as-prepared aerogels were cut into appropriate shapes (20 mm in diameters and 3 mm in height) and placed into glass funnels, as shown in Figure 7a. Then, the freshly prepared water-in-oil emulsions were poured onto the aerogels to execute the separation. As expected, the liquid could spontaneously permeate through the aerogel, and the filtrate was clear, indicating the successful separation of water-in-oil emulsions (Figure 7a, right, Movie S1). The optical microscopic images exhibited that the diameters of water droplets before filtration were in the range of 5-10 µm, while no water droplets were observed in the whole image after filtration (Figure 7b and c). The diameter distribution of the water droplets was calculated by Image-J software, and the results showed that the water droplet diameter distribution for the

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surfactant-stabilized water-in-oil emulsions was around 8 µm, while after filtration, no droplets were observed around this range (Figure 7d and e). The water weight percentage in the filtrate was further measured by a SF-5 trace moisture meter (Table S2), and the filtrate showed a low water concentration for less than 100 ppm, demonstrating the high separation efficiency of the aerogels. As a reference, the freshly prepared emulsions were also poured into another funnel with the pure aerogel (A0, Figure 7a, left), but the filtrate was still milky white due to the amphiphilic property of the pure cellulose aerogel. This phenomenon indicated that the superhydrophobic aerogel was the key factor for the successful water-in-oil

emulsions

separation.

Figure 7. (a) Gravity-driven separation of water-in-oil emulsions with the pure aerogel (A0, left) and composite aerogel (AM4, right), water-in-petroleum ether emulsion was taken as an example. (b, c) The optical images of the emulsions before and after filtration separation. (d, e) Diameter distribution of water droplet in the emulsions before and after filtration separation.

We used three parameters to assess emulsions separation performance of the obtained aerogels, the initial separation fluxes, the total separation amounts and the separation efficiency. The initial 14


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separation fluxes were obtained by measuring the volumes of filtrate collected within 5 min from per area of the aerogels (the height of the emulsions in the funnel was kept at 3 cm). The total separation amount was defined as the total volume of filtrate collected from per valid area of aerogels, from the beginning of separation until no liquid could pass through the aerogel, and the calculation of separation efficiency was explained in the experimental section. As shown in Figure 8, the initial separation flux for AM0 aerogel could reach up to 6840 ± 120 L m-2 h-1, but the total separation amount was small (2140 ± 40 L m-2). As mentioned above, AM0 possessed relatively low surface hydrophobicity, thus some of water droplets would permeate into the aerogel and block the oil transportation channels in a relatively short time. This could also be proved by the lower separation efficiency (96.1%) for AM0 compared to that for AM4 (99.5%). With increasing silica nanoparticles amount in the composite aerogels, the initial separation flux decreased because of the pore size reduction, while the separation flux kept stable upon running time. As a result, AM4 showed a comparable initial separation flux of 1910 ± 60 L m-2 h-1, but an extremely high total separation amount of 6547 ± 210 L m-2 with separation efficiency up to 99.5%. Given the low density of the aerogels, we also calculated the mass of the emulsions that could be separated by per gram of aerogel, and the results showed that about 129.7 kg emulsions could be separated by per gram of aerogel (AM4), which has rarely been reported in previous separation materials. Additional separation data of the aerogels for other water-in-oil emulsions separation were presented in Figure 9. These results demonstrated that the composite aerogels could separate various surfactant-stabilized water-in-oil emulsions with good performance.

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Figure 8. Initial separation flux, total separation amount and separation efficiency for different samples.

Figure 9. Initial separation flux and separation efficiency of the aerogels (AM4) for various water-in-oil emulsions.

It is also important that the high fluxes and high separation efficiency can be maintained for repeated use. The aerogel (AM4) was applied to separate emulsions for 20 cycles and the flux as well as separation efficiency for each cycle were presented in Figure 10a. For each cycle, the aerogel separated emulsions for 20 min, and then the aerogel was washed with ethanol to recover the flux. One can note that the decrease for flux and efficiency was negligible. Besides, the aerogel surface was still 16


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hydrophobic even after 20 cycles with water contact angle above 140°, demonstrating the excellent antifouling performance of the aerogel for recycling use. The mechanical property of the recycled aerogels was also measured and the compressive stress–strain curve was shown in Figure S2. Compared with the original aerogels, the recycled aerogels possessed similar compressive stress–strain curve, the decrease of compressive stress was negligible, indicating that the mechanical properties could be maintained during the recycle process. In addition, we developed an oil recycling device by combination of the composite aerogel and a self-priming pump to collect oil from the emulsions continuously. The pure oil in the emulsions could flow through the aerogel to the collection beaker, leaving water in the original liquid (Figure 10b, Movie S2). This device made the separation process simpler, faster, and closer to the practical applications.

Figure 10. (a) Changes of the initial separation flux and separation efficiency over 20 cycles of AM4, and water-in-petroleum ether emulsion was used to do this experiment. (b) The oil recycling device assembled by the obtained composite aerogel (AM4) and a self-priming pump.

Proposed separation mechanism. In general, the emulsions separation mechanism of the aerogel is based on the coalescence and size-sieving effect. The separation process consists of two steps, demulsification and separation (Figure 17



11a). When the emulsions are poured onto the aerogels, they will contact with the aerogels sufficiently because of the hierarchical porous structure of composite aerogels. Then the intermolecular forces between the superhydrophobic aerogels and the hydrophobic surface of emulsified water-in-oil droplets will serve as the driving force to achieve demulsification of surfactant-stabilized water-in-oil emulsions. The hierarchical porous structure combines with the superhydrophobic properties of the aerogels lead to the high demulsification rate. Then the separated oil will permeate through the aerogels to the collection container, while the separated water droplets will coalesce into large sizes in the tortuous microchannels and store in the pores. When all the aerogel pores are filled with water droplets and surfactant, the separation process will be stopped and the aerogel will be washed with ethanol to recover the separation process. The separation mechanism for commercial two-dimensional micro- and ultrafiltration membranes is on the basis of size-sieving filtration because there are large amount of nanopores on the membranes.50 But the filtration membranes with nanopores cannot accomplish the emulsions separation solely by gravity because of the small pore size (< 0.3 µm). To accelerate the process, the separation usually need external pressure to force the liquid to pass through the membranes. Moreover, the small pore size and short permeation channels of membranes will lead to quick reduction of separation performance due to the surfactant absorption and pore plugging, which will cause a significant reduction in permeation flux. Compared with the membranes, the aerogels possess high porosity and long permeating channels, which can guarantee a long running time without pore blocking. The high demulsification rate and high porosity endow the aerogels with high separation fluxes. Figure 11b showed the comparison of the initial separation flux of our work and previous reported works. Our work exhibited a comparable initial separation flux at a low driven pressure. The materials with high separation flux were filtration membranes, which needed higher driven pressure. 18


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Moreover, the aerogels obtained in our work could maintain a stable flux in a relatively long time. In the view point of separation performance and energy conservation, the aerogels fabricated in this work are promising candidate for the emulsions separation and fuel purification applications.

Figure 11. (a) Illustration of emulsions separation process. (b) Comparison of initial separation flux of our work and previous reported works.

Conclusion In conclusion, we have successfully constructed superhydrophobic aerogels from silylated cellulose nanofibers and silica nanoparticles via a simple freeze-drying method. The as-prepared aerogels showed hierarchical porous structure with high porosity (≥ 99.60%), low density (≤ 6.43 mg/cm3), and high superhydrophobicity with water contact angle up to 168.4°. The obtained composite aerogels could separate surfactant-stabilized water-in-oil emulsions without external pressure, with high flux (1910 ± 60 L m-2 h-1) and high efficiency (99.5%) for various mixtures. More importantly, the aerogels maintained stable separation fluxes and efficiency even after 20 separation cycles. We also assembled a simple device to collect pure oil in suit from the emulsions with the obtained aerogels and

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a self-priming pump. These superhydrophobic aerogels show great potential for applications such as organic solvents purification, and oil-water emulsions separation. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Density, porosity and specific area of the aerogels, water concentration in the oil-water mixture (C0) and filtrates (C1), and SEM images of the aerogels. (PDF). The separation process of water-in-oil emulsions using the aerogel as the filtration material, and the oil recycling apparatus made up of the obtained aerogels and a self-priming pump (AVI). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Feng Xu: 0000-0003-2184-1872 Notes The authors declare no competing financial interest. Acknowledgments The authors gratefully thank the grants from the National key R&D Program of China (2017YFD0600204) and the Fundamental Research Funds for the Central Universities (BLYJ201619).

References

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Antiwear,

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Robust

Fluorine-Free

Superhydrophobic

PDMS-Ormosil@Fabrics

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