Superhydrophobic Methylated Silica Sol for Effective Oil–Water Separation

Superhydrophobic methylated silica with a core–shell structure was successfully fabricated by a sol-gel process. First, a pristine silica gel with an average particle size of ca. 110 nm was prepared, using tetraethylorthosilicate (TEOS) as a precursor, ethanol as a solvent, and NH4OH as a catalyst. Then, the superhydrophobic methylated silica sol was prepared by introducing methyltrimethoxysilane (MTMS), to graft the surface of the pristine silica gel with methyl groups. The structure and morphology of the methylated silica sol were characterized by Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), and transmission electron microscope (TEM). The characterization results showed that methyl groups were successfully grafted onto the surface of the pristine silica, and the diameter of the methylated silica was increased by 5–10 nm. Various superhydrophobic surfaces on glass, polyethylene terephthalate (PET) fabric, cotton, open-cell polyurethane (PU) foam, and polypropylene (PP) filter cloth were successfully constructed by coating the above substrates with the methylated silica sol and reached with a maximum static water contact angle and slide angle of 161° and 3°, respectively. In particular, the superhydrophobic PP filter cloth exhibited promising application in oil–water separation. The separation efficiency of different oil–water mixtures was higher than 96% and could be repeated at least 15 times.

Colloidal silica spheres have been widely used in many important industrial products, including catalysts, pigments, personal care products, and pharmaceuticals [24,25]. Spherical silica nanoparticles can be synthesized by a sol-gel method from alcohol solutions of silicon alkoxides in the presence The weight uptakes of the coatings for the PET fabric, medical cotton, PU foam, and PP filter cloth are ca. 2.5%, 3.5%, 2.0%, and 1.0%, respectively.

Characterization
The ultraviolet-visible (UV-Vis) spectra of the PET film, with and without coating, were recorded using a UV-Vis spectrophotometer (Lambada 900, PerkinElmer, Inc., Massachusetts, United States. The surface functional groups of the superhydrophobic silica sol were analyzed by a Fourier transform infrared (FTIR, Nicolet 5700, Thermo Electron Scientific Instruments Corp., Massachusetts, United States) spectrophotometer. The thermal degradation of the silica was characterized by a thermogravimetric analyzer (TGA, model Q600, TA Instruments, Delaware, Unite State), under a nitrogen atmosphere, at a heating rate of 10 • C/min.
The contact angle (CA) and slide angle (SA) of the samples were measured, using a contact angle analyzer (DSA-25, German Kruss Scientific Instrument Co., Ltd., Hamburg, German), at 25 • C, with 6 µL of deionized water or hexadecane droplets on the sample surface. The CA was measured 10 times at different locations, and an average of the five data within ±2 • were recorded for each sample.
The particle shape and size of the silicas were characterized by a field emission scanning electron microscope (SEM, 7800F, JEOL Ltd., Tokyo, Japan). The morphology of single silica particles was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan). The silica sols of 0.1 wt.% were dropped on copper grids coated with carbon and dried at room temperature for TEM imaging.

Reaction Mechanism of Superhydrophobic Silica Sol
As Figure 1 shows, methylated silica sol was prepared by a facile surface treatment, according to the Stöber method [43]. In the first step, pristine silica gel was prepared by a sol-gel method, using tetraethylorthosilicate (TEOS) as the precursor, ethanol as the solvent, and NH 4 OH as the catalyst. Under base catalysis, Si(OH) 4 was produced by hydrolyzing the ethoxy groups of TEOS [44,45]. Pristine silica sol was then prepared through de-hydration and de-alcohol condensation between TEOS/partially hydrolyzed TEOS and pristine silica. Then, methylated silica sol was prepared by reacting with MTMS to graft methyl groups on the silica surface. MTMS was selected as the modifier here because it has one methyl and three methoxy functional groups. When MTMS was added to the pristine silica sol, de-hydration and de-alcohol took place under the catalysis of ammonia. As a result, most hydrophilic hydroxyl groups on the silica surface were converted to hydrophobic methyl groups [46].

Characterization
The ultraviolet-visible (UV-Vis) spectra of the PET film, with and without coating, were recorded using a UV-Vis spectrophotometer (Lambada 900, PerkinElmer, Inc., Massachusetts, United States. The surface functional groups of the superhydrophobic silica sol were analyzed by a Fourier transform infrared (FTIR, Nicolet 5700, Thermo Electron Scientific Instruments Corp., Massachusetts, United States) spectrophotometer. The thermal degradation of the silica was characterized by a thermogravimetric analyzer (TGA, model Q600, TA Instruments, Delaware, Unite State), under a nitrogen atmosphere, at a heating rate of 10 °C/min.
The contact angle (CA) and slide angle (SA) of the samples were measured, using a contact angle analyzer (DSA-25, German Kruss Scientific Instrument Co., Ltd., Hamburg, German), at 25 °C, with 6 µL of deionized water or hexadecane droplets on the sample surface. The CA was measured 10 times at different locations, and an average of the five data within ±2° were recorded for each sample.
The particle shape and size of the silicas were characterized by a field emission scanning electron microscope (SEM, 7800F, JEOL Ltd., Tokyo, Japan). The morphology of single silica particles was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan). The silica sols of 0.1 wt.% were dropped on copper grids coated with carbon and dried at room temperature for TEM imaging.

Reaction Mechanism of Superhydrophobic Silica Sol
As Figure 1 shows, methylated silica sol was prepared by a facile surface treatment, according to the Stöber method [43]. In the first step, pristine silica gel was prepared by a sol-gel method, using tetraethylorthosilicate (TEOS) as the precursor, ethanol as the solvent, and NH4OH as the catalyst. Under base catalysis, Si(OH)4 was produced by hydrolyzing the ethoxy groups of TEOS [44,45]. Pristine silica sol was then prepared through de-hydration and de-alcohol condensation between TEOS/partially hydrolyzed TEOS and pristine silica. Then, methylated silica sol was prepared by reacting with MTMS to graft methyl groups on the silica surface. MTMS was selected as the modifier here because it has one methyl and three methoxy functional groups. When MTMS was added to the pristine silica sol, de-hydration and de-alcohol took place under the catalysis of ammonia. As a result, most hydrophilic hydroxyl groups on the silica surface were converted to hydrophobic methyl groups [46]. The chemical composition of the pristine and methylated silica was confirmed by FTIR, as shown in Figure 2a. The pristine silica and methylated silica sol show two strong absorption bands at 1095 and 452 cm −1 , which can be assigned to the stretching and bending vibration of Si-O-Si bonds, respectively [47]. The absorption bands of 3340 and 954 cm −1 for Si-OH groups and 1640 cm −1 for the adsorbed water decreased significantly after MTMS modification. In the spectrum of the methylated silica sol, the peaks at 2975 and 1274 cm −1 can be attributed to the C-H in -CH3 groups and Si-C groups, respectively [46]. In brief, the FTIR characterization supports that MTMS molecules were successfully grafted onto the surface of silica, producing silica-CH3 surface functional groups. The chemical composition of the pristine and methylated silica was confirmed by FTIR, as shown in Figure 2a. The pristine silica and methylated silica sol show two strong absorption bands at 1095 and 452 cm −1 , which can be assigned to the stretching and bending vibration of Si-O-Si bonds, respectively [47]. The absorption bands of 3340 and 954 cm −1 for Si-OH groups and 1640 cm −1 for the adsorbed water decreased significantly after MTMS modification. In the spectrum of the methylated silica sol, the peaks at 2975 and 1274 cm −1 can be attributed to the C-H in -CH 3 groups and Si-C groups, respectively [46]. In brief, the FTIR characterization supports that MTMS molecules were successfully grafted onto the surface of silica, producing silica-CH 3 surface functional groups. As shown in Figure 2b, the TGA thermograms demonstrated a clearly different thermal degradation between the pristine and methylated silica. For the pristine silica, the first weight loss step from 30 to 150 °C and the second weight loss step from 300 to 600 °C were attributed to the evaporation of the adsorbed water and the dehydration condensation between -OH groups, respectively. Different from the pristine silica, due to the superhydrophobic nature, there is virtually no water loss in the methylated silica before 100 °C. The weight loss from 100 to 600 °C results from the combination of the dehydration condensation between the -OH groups and the degradation of the -CH3 groups [44]. From the weight-loss steps of the methylated silica, the graft ratio of -CH3 groups is estimated to be ca. 8 wt.%.

Morphology of the Pristine and Methylated Silica Coatings
For the formation of a superhydrophobic coating, usually, both the modification of surface chemistry and surface roughness enhancement are critical [48], with surface morphology typically as the more dominating factor. The resultant superhydrophobic silica sol was used to fabricate coatings on various substrates. To observe the morphology of the silica particles, the synthesized sol was diluted to 0.1 wt.%, to avoid particle aggregation. Figure 3a shows the pristine silica particles of ca. 100 nm in diameter, with a narrow size distribution and virtually no aggregation. After MTMS treatment, the formed methylated silica was covered with a thin layer of 5~10 nm (inset of Figure 3b). The surface of the methylated silica is rougher than the pristine silica. Figure 3c shows the surface morphology of the methylated silica coating on a PET film, whose surface is rugged, with each domain composed of many small particles. The image indicates that a high level of porosity on the coating surface was formed. Overall, the characterization results suggest that the methylated silica coating has a hierarchical micro/nano structure, similar to the structure of the lotus leaf [49], and these hierarchical pinecone-like structures generate numerous grooves, in which air can be trapped, leading to a more hydrophobic surface [44]. At the same time, the surface was covered with a layer of low polarity -CH3 groups. The formation of a rough surface with low surface energy is expected to endow the coating with both superhydrophobic and superoleophilic properties [50]. As shown in Figure 2b, the TGA thermograms demonstrated a clearly different thermal degradation between the pristine and methylated silica. For the pristine silica, the first weight loss step from 30 to 150 • C and the second weight loss step from 300 to 600 • C were attributed to the evaporation of the adsorbed water and the dehydration condensation between -OH groups, respectively. Different from the pristine silica, due to the superhydrophobic nature, there is virtually no water loss in the methylated silica before 100 • C. The weight loss from 100 to 600 • C results from the combination of the dehydration condensation between the -OH groups and the degradation of the -CH 3 groups [44]. From the weight-loss steps of the methylated silica, the graft ratio of -CH 3 groups is estimated to be ca. 8 wt.%.

Morphology of the Pristine and Methylated Silica Coatings
For the formation of a superhydrophobic coating, usually, both the modification of surface chemistry and surface roughness enhancement are critical [48], with surface morphology typically as the more dominating factor. The resultant superhydrophobic silica sol was used to fabricate coatings on various substrates. To observe the morphology of the silica particles, the synthesized sol was diluted to 0.1 wt.%, to avoid particle aggregation. Figure 3a shows the pristine silica particles of ca. 100 nm in diameter, with a narrow size distribution and virtually no aggregation. After MTMS treatment, the formed methylated silica was covered with a thin layer of 5~10 nm (inset of Figure 3b). The surface of the methylated silica is rougher than the pristine silica. Figure 3c shows the surface morphology of the methylated silica coating on a PET film, whose surface is rugged, with each domain composed of many small particles. The image indicates that a high level of porosity on the coating surface was formed. Overall, the characterization results suggest that the methylated silica coating has a hierarchical micro/nano structure, similar to the structure of the lotus leaf [49], and these hierarchical pinecone-like structures generate numerous grooves, in which air can be trapped, leading to a more hydrophobic surface [44]. At the same time, the surface was covered with a layer of low polarity -CH 3 groups. The formation of a rough surface with low surface energy is expected to endow the coating with both superhydrophobic and superoleophilic properties [50].   Figure 4 shows the transparency of the PET films coated with pristine or methylated silica sol. In general, hydrophobicity and transparency are conflicted from the viewpoint of surface roughness, because a rough surface typically induces light scattering, and thus decreases transparency. Figure 4 shows that the PET film coated with the pristine silica exhibits a high transparency (>90% at 400~800 nm), virtually the same as the non-coated PET. The PET film coated with the methylated silica sol exhibits a slightly lower transparency because of the rough surface, as shown in Figure 3c. However, the reduction in transparency is very marginal, which is probably because of the very low surface roughness of the methylated silica coating in the micro-/nanometer range. This is very desirable for applications requiring both high transparency and superhydrophobicity.

Wetting Behavior and Corresponding Application Demonstrations
To evaluate the wetting behavior of the coatings, the images of water droplets on the coatings on glass slides are shown in Figure 5. After MTMS modification, polar (-OH) groups were replaced by nonpolar and thermally stable -CH3 groups, forming a rough low-surface-energy superhydrophobic and superoleophilic surface [47]. The static water CA was changed from 13.5° (pristine silica coating, Figure 5a) to 161° (methylated silica coating, Figure 5b). Figure 5c shows a digital picture of water drops on a glass slide coated with methylated silica. The virtually spherical drops proved the superhydrophobicity of the methylated silica coating. The hexadecane CA of the methylated silica coating was tested to be only 4.5°, which shows that the methylated silica coating is superoleophilic (Figure 5d). Figure 6 and Video S1 show that a drop of water can roll and bounce  Figure 4 shows the transparency of the PET films coated with pristine or methylated silica sol. In general, hydrophobicity and transparency are conflicted from the viewpoint of surface roughness, because a rough surface typically induces light scattering, and thus decreases transparency. Figure 4 shows that the PET film coated with the pristine silica exhibits a high transparency (>90% at 400~800 nm), virtually the same as the non-coated PET. The PET film coated with the methylated silica sol exhibits a slightly lower transparency because of the rough surface, as shown in Figure 3c. However, the reduction in transparency is very marginal, which is probably because of the very low surface roughness of the methylated silica coating in the micro-/nanometer range. This is very desirable for applications requiring both high transparency and superhydrophobicity.   Figure 4 shows the transparency of the PET films coated with pristine or methylated silica sol. In general, hydrophobicity and transparency are conflicted from the viewpoint of surface roughness, because a rough surface typically induces light scattering, and thus decreases transparency. Figure 4 shows that the PET film coated with the pristine silica exhibits a high transparency (>90% at 400~800 nm), virtually the same as the non-coated PET. The PET film coated with the methylated silica sol exhibits a slightly lower transparency because of the rough surface, as shown in Figure 3c. However, the reduction in transparency is very marginal, which is probably because of the very low surface roughness of the methylated silica coating in the micro-/nanometer range. This is very desirable for applications requiring both high transparency and superhydrophobicity.

Wetting Behavior and Corresponding Application Demonstrations
To evaluate the wetting behavior of the coatings, the images of water droplets on the coatings on glass slides are shown in Figure 5. After MTMS modification, polar (-OH) groups were replaced by nonpolar and thermally stable -CH3 groups, forming a rough low-surface-energy superhydrophobic and superoleophilic surface [47]. The static water CA was changed from 13.5° (pristine silica coating, Figure 5a) to 161° (methylated silica coating, Figure 5b). Figure 5c shows a digital picture of water drops on a glass slide coated with methylated silica. The virtually spherical drops proved the superhydrophobicity of the methylated silica coating. The hexadecane CA of the methylated silica coating was tested to be only 4.5°, which shows that the methylated silica coating is superoleophilic (Figure 5d). Figure 6 and Video S1 show that a drop of water can roll and bounce

Wetting Behavior and Corresponding Application Demonstrations
To evaluate the wetting behavior of the coatings, the images of water droplets on the coatings on glass slides are shown in Figure 5. After MTMS modification, polar (-OH) groups were replaced by nonpolar and thermally stable -CH 3 groups, forming a rough low-surface-energy superhydrophobic and superoleophilic surface [47]. The static water CA was changed from 13.5 • (pristine silica coating, Figure 5a) to 161 • (methylated silica coating, Figure 5b). Figure 5c shows a digital picture of water drops on a glass slide coated with methylated silica. The virtually spherical drops proved the superhydrophobicity of the methylated silica coating. The hexadecane CA of the methylated silica coating was tested to be only 4.5 • , which shows that the methylated silica coating is superoleophilic (Figure 5d). Figure 6 and Video S1 show that a drop of water can roll and bounce on the surface of the methylated silica coating on a glass slide, and the water slide angle (WSA) is only 3 • . The photos captured from 0 to 1.50 s in Figure 6 show the bouncing of a drop of water on the surface of the coating, which distinctly demonstrates the water repellence of the coating.
Materials 2020, 13, x FOR PEER REVIEW 6 of 11 on the surface of the methylated silica coating on a glass slide, and the water slide angle (WSA) is only 3°. The photos captured from 0 to 1.50 s in Figure 6 show the bouncing of a drop of water on the surface of the coating, which distinctly demonstrates the water repellence of the coating.  Superhydrophobic and superoleophilic surfaces were also achieved on PET fabric, medical cotton, PU form, and PP filter cloth by impregnating the methylated silica into their lacuna, and they were subsequently dried at 80 °C for 10 min, to remove the solvent. The SEM images of the methylated silica coating on the PP filter cloth are shown in Figure 7. As shown in Figure 7a,b, the surface of the PP filter cloth was coated with a layer of methylated silica. Since methyl groups of the methylated silica improve the interaction between the methylated silica and the PP surface, it enhances the adhesion between them and thus the durability of the treated filter cloth [51]. As Figure  7c shows, the surface of the PP filter cloth is not flat, since the coating was fabricated by a facile dipping process. Similar morphology was observed on the surface of PET fabrics and PU foams. The overall morphology of the coatings is of multilevel roughness, which is beneficial for achieving superhydrophobicity [48,52]. on the surface of the methylated silica coating on a glass slide, and the water slide angle (WSA) is only 3°. The photos captured from 0 to 1.50 s in Figure 6 show the bouncing of a drop of water on the surface of the coating, which distinctly demonstrates the water repellence of the coating.  Superhydrophobic and superoleophilic surfaces were also achieved on PET fabric, medical cotton, PU form, and PP filter cloth by impregnating the methylated silica into their lacuna, and they were subsequently dried at 80 °C for 10 min, to remove the solvent. The SEM images of the methylated silica coating on the PP filter cloth are shown in Figure 7. As shown in Figure 7a,b, the surface of the PP filter cloth was coated with a layer of methylated silica. Since methyl groups of the methylated silica improve the interaction between the methylated silica and the PP surface, it enhances the adhesion between them and thus the durability of the treated filter cloth [51]. As Figure  7c shows, the surface of the PP filter cloth is not flat, since the coating was fabricated by a facile dipping process. Similar morphology was observed on the surface of PET fabrics and PU foams. The overall morphology of the coatings is of multilevel roughness, which is beneficial for achieving superhydrophobicity [48,52]. Superhydrophobic and superoleophilic surfaces were also achieved on PET fabric, medical cotton, PU form, and PP filter cloth by impregnating the methylated silica into their lacuna, and they were subsequently dried at 80 • C for 10 min, to remove the solvent. The SEM images of the methylated silica coating on the PP filter cloth are shown in Figure 7. As shown in Figure 7a,b, the surface of the PP filter cloth was coated with a layer of methylated silica. Since methyl groups of the methylated silica improve the interaction between the methylated silica and the PP surface, it enhances the adhesion between them and thus the durability of the treated filter cloth [51]. As Figure 7c shows, the surface of the PP filter cloth is not flat, since the coating was fabricated by a facile dipping process. Similar morphology was observed on the surface of PET fabrics and PU foams. The overall morphology of the coatings is of multilevel roughness, which is beneficial for achieving superhydrophobicity [48,52]. In order to test the performance of the methylated silica coating on various surfaces of a polar polymer, biomaterial, elastomer, and nonpolar polymer, the methylated silica was coated on the surfaces of a PET fabric, medical cotton, PU foam, and PP filter cloth by a direct soaking and drying process. Water (dyed to blue color by methyl blue) and dichloromethane (dyed to red color by tony red) are used as polar and nonpolar solvents (oil), respectively. As Figure 8a and Videos S2 and S3 show, when placed on water, the methylated-silica-treated PET fabric and medical cotton could not be wetted at all and could float on the water's surface. In strong contrast, the untreated samples were quickly wetted, and they precipitated into the water, after absorbing water. Due to their superhydrophobicity, when the methylated silica treated samples were inserted into water, a negative meniscus was formed on the solid-liquid-vapor interfaces that would prevent water from entering the substrate [50]. Figure 8b and Video S4 demonstrate the performance of the superhydrophobicity and superoleophilicity of the methylated silica coating. The treated PET fabric repelled water and absorbed dichloromethane immediately, while the untreated PET fabric absorbed both water and dichloromethane quickly.
As Figure 8c and Video S5 show, a methylated-silica-coated PU foam cube could absorb virtually all dichloromethane from a dichloromethane-water mixture (vol/vol = 1:1), which can be potentially applied to recycle spilled oil from water. Similarly, the application of the superhydrophobic and superoleophilic methylated silica coating on a PP filter cloth for oil-water separation is presented in Figure 8d and Video S6. When a mixture of dichloromethane-water (vol/vol = 1:1) was poured into the separator, virtually all dichloromethane (20 mL) quickly permeated through the treated PP filter cloth by gravity, and virtually all water (20 mL) was retained. The efficiency of oil-water separation for a series of different oil-water mixtures (vol/vol = 1:1), including vegetable oil, hexane, gasoline, petroleum ether, and dodecane, were studied. As shown in Figure 9a, the separation efficiency was higher than 96.7% for all of the abovementioned oil-water mixtures, suggesting that the coating can be used for the separation of a broad range of oils from water. After oil-water separation, the treated PP filter cloth was recycled, to test the reusability by separating the mixture of dichloromethanewater (vol/vol = 1:1). As Figure 9b shows, the oil-separation efficiency remained higher than 96.3% after 15 cycles of operation. The testing result indicates that the methylated silica coating has superhydrophobic and superoleophilic performance, with a high separation efficiency and excellent durability for oil-water separation. All of these result from the hierarchical structure and nonpolar surface of the methylated silica. In order to test the performance of the methylated silica coating on various surfaces of a polar polymer, biomaterial, elastomer, and nonpolar polymer, the methylated silica was coated on the surfaces of a PET fabric, medical cotton, PU foam, and PP filter cloth by a direct soaking and drying process. Water (dyed to blue color by methyl blue) and dichloromethane (dyed to red color by tony red) are used as polar and nonpolar solvents (oil), respectively. As Figure 8a and Videos S2 and S3 show, when placed on water, the methylated-silica-treated PET fabric and medical cotton could not be wetted at all and could float on the water's surface. In strong contrast, the untreated samples were quickly wetted, and they precipitated into the water, after absorbing water. Due to their superhydrophobicity, when the methylated silica treated samples were inserted into water, a negative meniscus was formed on the solid-liquid-vapor interfaces that would prevent water from entering the substrate [50]. Figure 8b and Video S4 demonstrate the performance of the superhydrophobicity and superoleophilicity of the methylated silica coating. The treated PET fabric repelled water and absorbed dichloromethane immediately, while the untreated PET fabric absorbed both water and dichloromethane quickly.
As Figure 8c and Video S5 show, a methylated-silica-coated PU foam cube could absorb virtually all dichloromethane from a dichloromethane-water mixture (vol/vol = 1:1), which can be potentially applied to recycle spilled oil from water. Similarly, the application of the superhydrophobic and superoleophilic methylated silica coating on a PP filter cloth for oil-water separation is presented in Figure 8d and Video S6. When a mixture of dichloromethane-water (vol/vol = 1:1) was poured into the separator, virtually all dichloromethane (20 mL) quickly permeated through the treated PP filter cloth by gravity, and virtually all water (20 mL) was retained. The efficiency of oil-water separation for a series of different oil-water mixtures (vol/vol = 1:1), including vegetable oil, hexane, gasoline, petroleum ether, and dodecane, were studied. As shown in Figure 9a, the separation efficiency was higher than 96.7% for all of the abovementioned oil-water mixtures, suggesting that the coating can be used for the separation of a broad range of oils from water. After oil-water separation, the treated PP filter cloth was recycled, to test the reusability by separating the mixture of dichloromethane-water (vol/vol = 1:1). As Figure 9b shows, the oil-separation efficiency remained higher than 96.3% after 15 cycles of operation. The testing result indicates that the methylated silica coating has superhydrophobic and superoleophilic performance, with a high separation efficiency and excellent durability for oil-water separation. All of these result from the hierarchical structure and nonpolar surface of the methylated silica.

Conclusions
In summary, a convenient, economical, environmentally friendly, and scalable method to prepare multifunctional superhydrophobic and superoleophilic silica sol was reported. Superhydrophobic and superoleophilic surfaces can be directly constructed on hard and soft substrates by facile coating methods. The treated substrates exhibited excellent performance of water repellence, oil absorption, and oil-water separation. These properties can be practically applied in related fields.
Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/4/842/s1. Video S1: A drop of water can bounce and roll off on the coating of the methylated silica. Video S2: Comparison of water-repellent ability of the treated (left) and untreated (right) PET fabric. Video S3: Comparison of water-repellent ability of the treated (left) and untreated (right) Medical cotton. Video S4: Comparison of the superwettability of treated (top) and untreated PET fabric (bottom) for dichloromethane (dyed red) and water (dyed blue). Video S5: Treated sponge absorbing dichloromethane (dyed red) in dichloromethane-water (dyed blue) mixture. Video S6: Gravity-driven separation of dichloromethane (dyed red) and water (dyed blue) mixture by the treated PP filter cloth (mesh number, 400).