Water Separation Performances of Superhydrophobic and

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Oil/Water Separation Performances of Superhydrophobic and Superoleophilic Sponges Qingping Ke,*,† Yangxin Jin,† Peng Jiang,† and Jian Yu‡ †

College of Chemistry and Materials Engineering, Wenzhou University, Zhejiang 325035, P. R. China College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China

‡

S Supporting Information *

ABSTRACT: Superhydrophobic and superoleophilic sponges were fabricated by immersion in an ethanol solution of octadecyltrichlorosilane. The resulting coating strongly adheres to the sponges after curing at 45 °C for 24 h. Absorption capacities of 42−68 times the polymerized octadecylsiloxane sponge weight were obtained for toluene, light petroleum, and methylsilicone oil. These adsorption capacities were maintained after 50 cycles.

1. INTRODUCTION Oil/water separation is becoming more topical, because of the increasing industrial application of oil/water emulsifications and recent high-profile oil spills.1−4 Conventional methods, including gravity separation, burning, and air flotation, are hindered by low separation efficiencies.4−7 An alternative separation process involving the selective absorption of oil and repellency of water was recently reported for oil/water separation.8−19 This process requires special wettability materials (SWMs), which are superhydrophobic [water contact angles (WCAs) >150°], superoleophilic (oil CAs 150° after 100 cycles, suggesting that the superhydrophobicity of the PODSmodified sponge was robust. Sufficient adhesion between the coating and sponge is important for ensuring that the coating is maintained under working conditions. Materials including graphene and activated carbon are often added to polydimethylsiloxane as cementing agents, to improve adhesion between the coating and sponge.34,35,46−48 The PODS loading of the PODS-modified sponge decreased by 0.2% (from 10.1 to 9.9 wt %) after 100 cycles. This indicated that PODS was strongly adheres to the sponge. 3.2. Oil/Water Separation Ability of the As-Prepared Materials. Figure 4a shows one cycle of the adsorption separation process of toluene from toluene/water by the PODS-modified sponge. An unmodified sponge was also studied for comparison (Figure 4b). As expected, toluene was absorbed by the PODS-modified sponge within 3 seconds, and no water was observed during squeezing according to the separate layer in squeezed liquid (as presented in the fourth image of Figure 4a, and video S2 in the Supporting Information). The unmodified sponge was stained by blue ink after immersing in toluene/water, and the amounts of water were observed after squeezing (as presented in the fourth image of Figure 4b, and video S3 in the Supporting Information). Trapped air plays an important role in the wettability of sponges, as their water repellency may decrease when trapped air is expelled by the absorbed liquid. This gradually affects the separation ability of the sponge. The absorption capacity and separation selectivity are important factors in evaluating the oil/ water and organic solvent/water separation abilities of sponges. Oils were removed from water surfaces as shown in Figure 4 and video S2 in the Supporting Information. In a typical absorption process, PODS-based sponges were immersion in the oil/water mixtures, left for three seconds to saturate, and then weighed immediately to avoid evaporation of solvents or oils. The sponge was squeezed and the absorbed mixture was collected. The PODS-based sponge was washed with ethanol and dried in an oven (50 °C) for reuse after every five cycles. The separation selectivities were calculated from the oil/water weight ratios for a total of 50 cycles. Figure 5 shows the absorption capacities of the PODS-modified sponge for toluene, light petroleum, and methylsilicone oil with increasing cycle number. The absorption capacities of the PODS sponge for toluene, light petroleum, and methylsilicone oil are 42−68 times its own weight, depending on the density and viscosity of

Figure 2. SEM images of the (a) unmodified and (b) PODS-modified sponges. WCAs of the (c) PODS-modified and (d) ODS SAMmodified sponges. (e−g) OCA measurement for the PODS-modified sponge. The WCA values in the images are the average values that are calculated from the 5 data points.

pore size of 56.2 μm (the pore size distribution is shown in Figure S2b in the Supporting Information). The unmodified sponge was superhydrophilic, owing to its strong water absorption. The WCA of the ODS SAM-modified sponge was 91 ± 1° (Figure 2c, the plus/minus sign identify the standard deviation), which increased to 153 ± 1° after PODS modification (Figure 2d, the plus/minus sign identify the standard deviation). The wettability of a solid surface is typically governed by its surface components and morphology. It can often be described by the Wenzel and Cassie equation,43,44 which suggests that the wettability of sponges is enlarged by the existence of pores. The WCA value of 91 ± 1° for the ODS SAM-modified sponge was less than reported values of 100−110° for ODS SAM-modified flat surfaces.45 Octane was absorbed immediately upon contact with the PODS-modified sponge (Figure 2e−g). The PODS-modified sponge exhibited superoleophilicity (video S1 in the Supporting Information shows the measurement process). The robustness of the superhydrophobicity of the PODSmodified sponge was evaluated by cyclic compression measurements. The compression ratio (CR) was maintained at 83.3%, which was calculated using the relationship CR = (D0 − D1)/D0 × 100%

(1)

where D0 is the sponge thickness, namely 2 cm. Figure 3 shows the WCA of the PODS-modified sponge on repeated 13139

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Figure 4. Toluene/water collection process with the PODS-modified sponge (a) and unmodified sponge (b). A few drops of ink were added into the mixture for clear observation.

Figure 5. Absorption capacities of PODS-modified sponge for different oils. Data points were calculated as average absorption capacities of the previous 10 cycles. The oil adsorption capacity, k, of the sponges was calculated from k = [(Wai − Wb0)/Wb0] × 100% (0 ≤ i ≤ 50), where Wai refers to the weight of the saturation recycledsponges after immersing in the oil/water mixture for 3 s and Wb0 refers to the weight of the fresh PODS-modified sponges. When i = 0, Wai is the weight of the saturation adsorbed sponges after immersing in the oil/water mixture for 3 s. The weight Wai of the saturation adsorbed sponges was determined immediately after removing the sponge from the mixutre.

Figure 6. Optical images of the 1,2-dichloroethane/water separation process designed using PODS-modified sponge as membrane. (a) 1,2Dichloroethane/water mixture in the glass container before permeation; (b) and (c) separation of water and 1,2-dichloroethane; (d) collection of permeated 1,2-dichloroethane in another glass container.

the oil. In contrast, the absorption capacity of the unmodified sponge is 84−90 times its weight for the pure toluene. The oils could be readily collected after 50 cycles by mechanical squeezing. The robustness of the high adsorption capacity of the PODS-modified sponge was attributed to its elasticity, porosity and the strong adhesion of the coating to the sponge. Separation selectivities of 98.8%, 97.5%, and 99.3% were obtained for toluene, light petroleum, and methylsilicone oil, respectively, for the PODS-modified sponge, respectively. In addition, although slightly deformations of the PODS-modified sponges were observed after 50 cycles, the sponges were not stained by ink (Figure S4 in the Supporting Information). For oils with densities greater than that of water, the oil and water were separated by direct permeation. Figure 6 shows the separation process of a 1,2-dichloroethane/water mixture over the PODS-modified sponge. Ten milliliters of 1,2-dichloroethane was absorbed or permeated by the sponge within 15 s of the mixture contacting the sponge. After 15−30 s, no water

drops were observed in the 1,2-dichloroethane vessel or sponge. The absorbed 1,2-dichloroethane was readily collected by squeezing; 9.8 mL of 1.2-dichloroethane were collected after separation.

4. CONCLUSIONS Superhydrophobic and superoleophilic sponges were fabricated by a scalable method based on PODS coatings. The PODS coating strongly adheres to the sponge, which resulted in robust oil/water separation after 50 cycles. Absorption capacities of 42−68 times the PODS-sponge weight were obtained for toluene, light petroleum, and methylsilicone oil. The process for fabricating superhydrophobic and superoleophilic sponges has potential applications for treating industrial oil/water mixtures and environmental oil spills. 13140

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra of PODS- and ODS-SAM-modified sponges (Figure S1), pore size distribution of the um-modified sponge (Figure S2), measurement process for the WCA on unmodified sponge (Figure S3), PODS-modified sponge and unmodified sponge before and after absorption of mixture of toluene/water (Figure S4), water absorption capacity of PODS-modified sponges after each ten oil/water mixture cycles (Figure S5), OCA measurement process (video S1), and collection of oils from the PODS-modified sponge (video S2), and unmodified sponge (video S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +086-137 3635 0110. E-mail: [email protected]. Author Contributions

Q.K. is the lead scientist, and prepared this manuscript, with feedback from Y.J., P.J., and J.Y. Y.J., P.J., and J.Y. performed the experiments and measurements. All authors approve submission of this final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Zhejiang Nature Science Foundation of China (No. Y13B030004) and the National Nature Science Foundation of China (No. 21306142). Q.K. thanks Jingfeng Zhang (from Wenzhou University) for SEM measurements and the anonymous reviewers for the invaluable advices.

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ABBREVIATIONS PODS, polymerized octadecylsiloxane; OTS, octadecyltrichlorosilane; ODS, octadecylsiloxane; SWM, special wettability materials; WCA, water contact angle; OCA, octane contact angle; W, weight; SAM, self-assembled monolayers

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