Regulating Underwater Superoleophobicity to Superoleophilicity on

Article pubs.acs.org/Langmuir

Regulating Underwater Superoleophobicity to Superoleophilicity on Hierarchical Structured Copper Substrates through Assembling n‑Alkanoic Acids Defeng Li,†,‡ Ang Wu,‡ Guangyin Xu,*,†,‡ Hua Lai,§ Zhongjun Cheng,*,∥ and Yuyan Liu§ †

Collaborative Innovation Center of Biomass Energy and ‡College of Mechanical and Electrical Engineering, Henan Agricultural University, Zhengzhou 450002, Henan Province, China § School of Chemical Engineering and Technology and ∥Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, P. R. China S Supporting Information *

ABSTRACT: In this paper, we report a simple method based on assembling n-alkanoic acids on hierarchical structured copper toward preparing surfaces with tunable oil wetting performance in water. Surface wettability from superoleophobicity to superoleophilicity in water can be regulated through tuning the chain length of n-alkanoic acids. Importantly, even in strongly acid and basic water, such phenomena can still be observed. The cooperation between the hierarchical structures and the surface chemical composition variation is responsible for the controllability. Meanwhile, the tunable ability is universal and the controllability is suitable for various oils including silicon oil, n-hexane, and chloroform. Moreover, the method was also used on copper mesh substrates, and we reported the related application of selective oil/water separation. This paper provides a flexible strategy toward preparing surfaces with tunable oil wetting performances, which can also be suitable for other materials, and offers some fresh ideas in manipulating underwater oil wetting performances on surfaces.

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INTRODUCTION In nature, many animals have a special antifouling ability to oil, such as fish (fish scales). Recently, Jiang et al. have found that the underwater superoleophobicity on the scales is responsible for the antifouling ability, which is the result of the cooperation between the micro/nanostructures and the hydrophilic chemical compositions on the fish scales.1 Taking inspiration from this finding, a lot of materials with underwater superoleophobicity have been reported2−16 and applied in many fields such as bioadhesion,17−19 microfluidic technology,20−22 and so forth.23,24 On the other hand, materials with underwater superoleophilicity are also significant in many applications.25−31 For instance, Jin et al. prepared an underwater superoleophilic organosilane surface and applied it successfully for the removal of oil in water.25 Given the difference in wettability, which would allow the materials to be used in different fields, it is expected that surfaces with tunable oil wetting performances would be more favorable for practical applications. In spite of many studies having been done toward controlling the underwater solid/oil interaction,32−39 most research can only display the transition in a narrow range between general oleophobicity and superoleophobicity. Reports about surfaces with controlled wetting performances between superoleophobicity and superoleophilicity in water are still rare,40−43 and advancing a simple strategy toward the © 2016 American Chemical Society

preparation of surfaces with tunable oil/solid wettability in a wide range between the two extreme states is still highly desired. Herein, we advance a simple way for the preparation of such surfaces that have controlled oil wetting performances in water from superoleophobicity to superoleophilicity. The surfaces were fabricated through assembling n-alkanoic acid molecules on copper substrates with special hierarchical structures. In detail, the hierarchical structured Cu was fabricated first on copper substrates; then, n-alkanoic acids with various chain lengths were used to modify these substrates. By simply controlling the chain length of the n-alkanoic acids, a series of surfaces with tunable wetting performances from superoleophobicity to superoleophilicity in water can be obtained. The cooperation between the hierarchical structures and the surface chemistry variation is responsible for the good controllability. Meanwhile, the as-prepared surfaces have particular acid/basic resistance in water solution from pH 1 to 14; the controllability can also be observed. Finally, the method was also used on the copper mesh substrates and the Received: October 16, 2016 Revised: November 10, 2016 Published: November 25, 2016 13493

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Figure 1. (a−c) SEM images of the surface at different magnified scales and (d) cross-sectional view of the hierarchical structures.

and the microstructure.46−50 To achieve superoleophobicity and superoleophilicity, a rough structure is often necessary to be prepared on the substrates.1 In this work, copper that has good resistance to corrosion was chosen as the substrate, and the hierarchical structures were produced through an immersion (details see Experimental Section and Figures S1 and S2) and H2 reduction process,41,45 which can be further explained by the following equations

application of controllable oil/water separation was also demonstrated.

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EXPERIMENTAL SECTION

Materials. n-Alkanoic (n = 1−12) (Aldrich, Germany), polydimethylsiloxane (silicon oil), (NH4)2S2O8, 1,2-dichloroethane, oil red (Sudan III), alcohol, sodium hydroxide, CHCl3, petroleum ether, methylene blue, n-hexane, copper foils, (99.9%), and copper mesh substrates were obtained from Aladdin Chemical Reagent Co. Growth of Hierarchical Structures on the Substrates. The hierarchical structure was prepared according to the previously reported procedure.44 Briefly, the copper substrates were first ultrasonically cleaned to remove the pollutes on the surface and then put into the reaction solution of (NH4)2S2O8 (0.1 M) and NaOH (2.5 M). After approximately 1 h, these substrates were washed with abundant deionized water and dried under N2. Finally, these substrates were heated at 180 °C for 2 h and further at 200 °C in H2 for approximately 12 h. The similar process was carried out for the copper mesh films. Assembly of n-Alkanoic Acids onto the Hierarchical Structured Copper Substrates. The substrates were further immersed into the ethanol solution containing different n-alkanoic acids (0.001 M) for approximately 12 h.45 Finally, the surfaces were cleaned using ethanol and dried under N2. Separation of Oil/Water Mixture. The separating membrane was fixed between two tubes, and the mixture of petroleum ether and water was added into the upper tube. Corresponding photos were recorded by a camera (Canon 100D). Instrumentation and Characterization. The surface morphologies were investigated using a scanning electron microscope (Hitachi, SU8000). The oil contact angles (OCAs) were measured using a JC 2000D5 contact angle measure meter (Shanghai Zhongchen Digital Technology Apparatus Co., Ltd, for the examining process see Scheme S1). X-ray photoelectron spectroscopy (XPS) data were recorded using a K-Alpha electron spectrometer from Thermo Fisher Scientific Company using Al Kα (1486.6 eV) radiation. X-ray diffraction data (XRD) were obtained on an X-ray diffractometer (D8 Advance, Bruker) with Cu Kα radiation (λ = 1.5418 Å). Acid and basic water solutions were prepared by diluting HCl and NaOH in water, respectively, and a pH meter (PB-10, Sartorius) was used to test the solution pH.

Cu + 4NaOH + (NH4)2 S2O8 → Cu(OH)2 + 2Na 2SO4 + 2NH3 ↑ + 2H 2O

(1)

Δ

Cu(OH)2 → CuO + H 2O↑

(2)

Δ

CuO + H 2 → Cu + H 2O↑

(3)

Figure 1a displays the scanning electron microscopy (SEM) image of the microstructure on the substrate. As shown, on the surface, lots of microflowers stand on the nanowires, and the average diameter of the microflowers is approximately 4 μm (Figure 1b). Further amplifying the image, it can be seen that the microflowers are composed of nanograins with diameters ranging from 80 to 500 nm (Figure 1c). From the crosssectional image, one can observe that the whole thickness of the hierarchical structure is approximately 21 μm (Figure 1d). Figure 2 displays the XRD result of the film; after comparison with the standard spectrum, one can find that all peaks can be ascribed to Cu, which is in good agreement with the energy dispersive spectroscopy (EDS) result (Figure S3). After production of hierarchical structured Cu, the substrates were added to a solution of ethanol and n-alkanoic acid. The nalkanoic acid molecules would adsorb onto the copper surfaces,51 which can be proved using the XPS results. From Figure 3, panel a, it can be found that on the substrate modified by n-octanoic acid, elements Cu, C, and O can be seen clearly. On the high-resolution C 1s spectra, more detailed information can be found (Figure 3b). The C 1s peak assigned to C−C/C− H at 284.8 eV is greatly enhanced compared with the original Cu. Meanwhile, a peak at 288.2 eV ascribed to −COO arises, indicating that the n-octanoic acid molecules have been assembled onto the copper film successfully. From the above

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RESULTS AND DISCUSSION It is well-known that surface wetting performance is simultaneously governed by the surface chemical composition 13494

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Figure 2. XRD pattern of the surface.

Figure 4. Shapes of an oil droplet on the surfaces modified by npropanoic acid (a) and n-octanoic acid (b). (c and d) Relationship between underwater OCAs and the chain length on hierarchical structured and flat copper substrates, respectively.

be seen that the two surfaces show underwater superoleophobicity (with an OCA of approximately 162°) and superoleophilicity (with an OCA of approximately 0°), indicating that the assembly of different n-alkanoic acids can effectively adjust the oil wettability in water. Through detailed research, it can be found that the OCAs are strongly dependent on the chain length of the n-alkanoic acids (Figure 4c). For a surface modified by n-alkanoic acids with a short chain length (with carbon number equal or less than 4), the surface is underwater superoleophobic. The OCAs for 1,2-dichloroethane droplets (4 μL) are higher than 150°. Increasing the chain length (with carbon number of 5−7) would decrease the OCA. As the molecular chain length is further increased (with carbon number equal or larger than 8), the obtained surfaces would be underwater superoleophilic, and the OCA of an oil droplet on all of these surfaces is nearly 0°. Herein, smooth copper foils were also used as substrates and assembled with the same molecules. However, the OCA change is restricted in a narrow range between 136° and 65° (Figure 4d). These results indicate that the wetting performance for oil droplets on the hierarchical structured surfaces can be regulated in a wide range by varying the chain length of the n-alkanoic acids.46−50 In addition to 1,2-dichloroethane, the surface underwater oil wettability was also tested by other oils including n-hexane, silicon oil, petroleum ether, and chloroform. For these oils,

Figure 3. (a) XPS survey spectrum of the sample modified by noctanoic acid; (b) high-resolution C 1s XPS spectra of the copper surface and after modification by n-octanoic acid.

results, it is reasonable to conclude that the method used here is effective for assembling n-alkanoic acids on the copper surface. The surface wettability was examined accurately using a contact angle measure meter. Figure S4 shows the results of water contact angles (WCAs) and OCAs measured in air (1,2dichloroethane was used as the test oil). It can be observed that all of the obtained surfaces are superoleophilic with OCAs of nearly 0°. Noticeably, for water, a remarkable variation in the WCA can be observed, which is increased from superhydrophilicity to superhydrophobicity with increasing molecular chain length. The results are similar to that of Wang’s report,45 and such increase in the WCAs can further confirm that the n-alkanoic acids have been assembled on the copper films. In this work, underwater oil wetting performances are our major interest. Figure 4a,b displays shapes of an oil droplet on the surfaces modified by n-propanoic and n-octanoic acid. It can 13495

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increased. As reported by Tao,51 the structure of a selfassembled monolayer can be influenced prominently by the chain length of the n-alkanoic acids. On surfaces modified by nalkanoic acids with a short chain length, the molecular chains are scattered and disordered, a lot of high-energy metal surface is exposed (Figure 7a), and the surface shows hydrophilicity

controllable oil wetting performances can also be observed (Figure 5), indicating that the tunability of the surface wetting

Figure 5. Statistics of the OCAs for different oils on the surfaces modified by n-propanoic acid and n-octanoic acid.

is universal. Furthermore, in practical applications for underwater devices, the resistance to corrosive solutions, for example, strongly acid/basic solutions, is also significant. Noticeably, our films have such a particular anticorrosive ability. As shown in Figure 6, in a water solution with pH from 1 to 14,

Figure 7. Schematic illustration of the oil/solid interactions in water. For surface modified by n-alkanoic acids with a short chain length, the molecular chains are disordered and scattered (a). The surface shows hydrophilicity, and an oil droplet resides in the composite Cassie state because water can enter into the hydrophilic nanostructures (c); thus, the surfaces show underwater superoleophobicity. When the chain length is increased, the packing of molecular chains becomes more ordered and denser (b), the substrate is mainly covered by alkyl groups with hydrophobicity/oleophilicity; thus, only oil can enter into the gaps among the microstructures, and the surface displays superoleophilicity in water (d).

(Figure S4). With the increase in the chain length, the packing of molecular chains becomes more ordered and denser (Figure 7b); as a result, surfaces become hydrophobic, as shown in Figure S4. Thus, it is easy to understand that the increase in the chain length would enhance the surface hydrophobicity, which means that the θw would increase (Figure S5). As γoa, γwa, and γow are fixed, and the change of θo is negligible (Figure S5), according to eq 4, the θow would decrease as the chain length is increased.54 For surfaces with hierarchical structures, the variation tendency of the OCA is similar to that on flat surfaces; however, an apparently amplified effect can be seen (Figure 4c). As to the surfaces modified by n-alkanoic acids with a short chain length, as described above, the molecular chains are disordered and scattered, a lot of high-energy metal surface is exposed, and the surface shows hydrophilicity or even superhydrophilicity (Figure S4). After putting these hydrophilic/superhydrophilic surfaces in water, water occupies the whole gaps among the microstructures. On these surfaces, the oil droplet is prone to reside in the Cassie state (Figure 7c). The high OCAs can be clarified by the following modified Cassie equation1,55

Figure 6. Statistics of the OCAs for the surfaces modified by npropanoic acid and n-octanoic acid in solutions with different water pH values.

controllability of surface wettability can still be observed. Surfaces modified with a short chain length show underwater superoleophobicity, whereas those modified with a long chain length show underwater superoleophilicity. Meanwhile, after storage in water or air without special protection for at least 1 month, the oil wetting controllability can still be observed, which means a good chemical stability for our surfaces. The mechanism affecting the OCAs was carefully analyzed, which is favorable for us to understand the controllability of oil wetting performances. As reported, the OCA for an oil droplet on a smooth substrate in water can be obtained as follows52,53 cos θow =

′ = f cos θow + f − 1 cos θow

γoa cos θo − γwa cos θw γow

(5)

where, f represents the area fraction of the copper surface in contact with oil, θ′ow and θow are the OCAs on the hierarchical structured and flat copper surfaces in water, respectively. Taking the surface modified by n-propanoic acid as an example, θ′ow = 162°, θow = 102° (Figure S6), and f = 0.062 means that the water/oil contact area takes up more than 93% contact area on the hierarchical structured surface. Thus, the underwater superoleophobic wetting performance can be observed.

(4)

where θw and θo are the contact angles (CAs) of water and oil in air, respectively. θow represents the OCA. γow, γwa, and γoa are the interface tensions of the oil/water, water/air, and oil/air interfaces, respectively. In this work, the decrease of the OCA on flat copper surfaces is due to the change in surface chemistry as the length of the molecular chain for n-alkanoic acids is 13496

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red and methylene blue, respectively). As the film modified by molecules with long chain length was used, for instance, noctanoic acid (Figure S8c−e), the result was just opposite and only oil could get through (Figure 8c). These results indicate that by simply tuning the length of the molecular chain of nalkanoic acids, the obtained separating films can be applied in the controllable oil/water separation.

With the increase in the chain length, the packing of molecular chains becomes more ordered and denser (Figure 7b), and the surfaces become hydrophobic and even superhydrophobic. These surfaces cannot be wetted by water while they can be easily wetted by oil. As explained by the Wenzel equation,56 the surface oleophilicity can be intensified by the rough structures. As displayed in Figure 1, the hierarchical structures composed of microflowers/nanowires can increase the surface area and the surface roughness remarkably. As a result, the oil can be absorbed into the rough structures because of the 3D capillary effect (Figure 7d). Thus, the surface shows underwater hydrophilicity and even superoleophilicity as the chain length is increased. Herein, what needs to be stressed is that, although the method reported here has been used to regulating the surface wettability in ref 57, the results are different. In the previous report,57 by controlling the chain length of the n-alkanoic acids, all surfaces show underwater superoleophobicity, while different oil-adhesions can be observed. In the present work, surfaces can vary from underwater superoleophobicity to superoleophilicity. Different wetting performances between the reference and the present work can be ascribed to different microstructures on the substrates: nanostructures in the reference and hierarchical structures in the present work (for a more detailed discussion see Supporting Information). In addition to copper foils, this method was also used on copper mesh substrates and demonstrated controllable separation of oil/water mixture. As shown in Figure S7, after a similar immersion and H2 reduction process, nanostructures can also be produced. On further modification with different nalkanoic acids, similar tunable underwater oil wetting performances can also be observed. Importantly, these films can be applied to separate the mixture of oil and water.58−60 As shown in Figure 8a, the separating device was obtained by fixing the

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CONCLUSIONS In conclusion, a simple method based on the self-assembly of nalkanoic acids on copper substrates with hierarchical structures was reported toward regulating underwater oil wetting performances. Surfaces with wetting performances from superoleophobicity to superoleophilicity in water can be prepared through tuning the chain length of the n-alkanoic acids. The cooperation between the hierarchical structures and the variation of surface chemistry is responsible for the controllability, especially the hierarchical structures that can provide better control of wetting performances in a wide range. Moreover, the method was also used on copper mesh substrates and applied in the controllable oil/water separation. The method reported here may provide some fresh ideas in regulating surface oil wetting performances and has wide applications in antipollution, microfluidic devices, and bioadhesive materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03771. SEM images of substrates after immersion for different times; variation of the microstructure before and after heating, H2 reduction; EDS of the microstructure; dependence of WCA and OCA on the chain length of n-alkanoic acids in air; dependence of WCA and OCA on the chain length of n-alkanoic acids on flat substrates in air; dependence of underwater OCAs on the chain length of n-alkanoic acids on the flat copper surface; SEM images of the copper mesh film; wetting performances on the copper mesh films; and schematic illustration of the OCA examination (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.X.). *E-mail: [email protected] (Z.C.). ORCID

Zhongjun Cheng: 0000-0001-5550-2989

Figure 8. (a) Optical photograph of the oil/water separation device; (b) using the films modified by n-propanoic acid (b) and n-octanoic acid (c) as the separating membrane, only water or oil can permeate the film, demonstrating that controllable separation of oil/water mixture can be realized.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

film between two glass tubes. For films modified by molecules with a short chain length, for example, n-propanoic acid, the film is superhydrophilic and underwater superoleophobic (Figure S8a,b). When the oil/water mixture (water and petroleum ether) was added into the upper glass tube (the film was prewetted by water), only water could permeate the film (Figure 8b, the color of oil and water for the addition of oil

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ACKNOWLEDGMENTS This work was supported by the assisted project by Henan Postdoctoral Funds for scientific research initiation (no. 2015067); science and technology research plan of the Henan province (no. 16210211012); the National Natural 13497

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Science Foundation of China (NSFC Grant no. 21304025); and the assisted project by Heilong Jiang Postdoctoral Funds for scientific research initiation (LBH-Q13063).

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DOI: 10.1021/acs.langmuir.6b03771 Langmuir 2016, 32, 13493−13499

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DOI: 10.1021/acs.langmuir.6b03771 Langmuir 2016, 32, 13493−13499