underwater oil capture by a 3d network organosilane surface

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2011, XX, 1–4 1

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

wileyonlinelibrary.com

Meihua Jin , * Jing Wang , Xi Yao , Mingyi Liao , Yong Zhao , * and Lei Jiang

Underwater Oil Capture by a Three-Dimensional Network Architectured Organosilane Surface

Dr. M. H. Jin , J. Wang , Prof. M. Y. Liao Department of Materials Science and EngineeringDalian Maritime UniversityDalian, 116026, P. R. China E-mail: [email protected] Dr. X. Yao , Dr. Y. Zhao , Prof. L. Jiang Beijing National Laboratory for Molecular Sciences (BNLMS)Institute of ChemistryChinese Academy of SciencesBeijing, 100190, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201101048

Oil pollution caused by shipping or offshore oilfi elds oil leakage has become an urgent global issue. One of the routine anti-oil pollution methods is based on oleophilic materials that can absorb oil pollution. However, such oleophilic materials are one-offs because the oil absorbed is hard to remove, which results in secondary pollution during the post-treatment process as well as a waste of both absorbed oil and oleophilic materials. [ 1 ] It is thus desirable to develop new reusable oil absorbent mate-rials with special wettability that could controllably capture and release oil pollution repeatedly. Natural biological species such as lotus leaves, water strider’s legs, gecko’s feet, and desert bee-tle’s backs, exhibit some amazing wettabilities in order to adapt to nature. [ 2 ] For instance, the desert beetle [ 2 e] that can live in a water-defi cient desert, relies on its bumpy back, which consists of alternating wax-coated hydrophobic regions and non-waxy hydrophilic regions. Such a special bumpy surface can capture drinking water from fog-laden wind. However, it is noticed that nearly all studies are carried out in air–liquid–solid systems, [ 3 ] while efforts on underwater wettability, a vitally important topic, no less so than normal wettability in air, is seldom consid-ered. [ 4 , 5 ] We have created an underwater superoleophobic water/solid interface inspired by fi sh scales. [ 6 ] This work blazes a new area of functional interfacial materials with special wettability. Nevertheless, research on underwater interface characteristics remains a challenge. Here, a novel organosilane surface, which is superamphiphobic in air and superoleophilic under water, is successfully prepared through a phase separation reaction. Such a surface mimics the “desert beetle effect” underwater and could repeatably capture and collect oil droplets in water.

Organosilanes with the general formula R n SiX 4− n (where X is a hydrolyzable group) have been widely used as effective surface-modifying agents for different substrates because of their strong Si–O linkage between silane and the surface or between adjacent silane molecules forming highly cross-linked discrete structures or networks. [ 7 ] Hence surfaces derived from organosilanes show excellent thermal and chemical stability. [ 8 ]

Here we used 1 H ,1 H ,2 H ,2 H -perfl uorodecyltrichlorosilane (FTS) to graft to a substrate through hydrolysis and polyconden-sation, and fi nally an interesting 3D network architectured sur-face was obtained.

The organosilane surfaces were synthesized by a simple phase separation reaction and grafted to glass substrates. [ 9 ] Hydrolysis of the Si–Cl organosilane monomer yielded Si–OH and polycondensation formed highly cross-linked 3D networks. Figure 1 is the scanning electron microscopy (SEM) image of

Figure 1 . Typical SEM images of the FTS-derived rough surface. a) The surface is composed of nanofi bers and microbumps, which criss-cross each other and form a network architecture. b) A magnifi ed image showing the nanobranches on the nanofi bers.

2


© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, XX, 1–4

CO

MM

UN

ICATI

ON


mainly applies to ideal fl at surfaces. Through Young’s equation, a formula for calculating the contact angle of a liquid–liquid–solid three-phase system is: [ 6 , 14 ]

cos2OW =(OA cos2O − (WA cos2W

(OW

(1)

where γ OA , γ WA , and γ OW are surface tensions of the oil/air, water/air, and oil/water interfaces, respectively. θ OW , θ O , and θ W are the contact angle of oil in water, oil in air, and water in air, respectively. According to this formula, a smooth solid surface that is amphiphobic in air might be oleophobic ( γ OA cos θ O < γ WA cos θ W ) or oleophilic ( γ OA cos θ O > γ WA cos θ W ) under water. [ 14 ]

Using Equation (1) we are able to explain why the FTS-derived smooth surface, which is amphiphobic in air (Figure S1, Supporting Information), becomes oleophilic in water. Taking 1,2-dichloroethane as an example, its interfacial tension with air ( γ OA ) is 24.15 mN m − 1 , and the water surface tension ( γ WA ) is 72.0 mN m − 1 . [ 15 ] In air, the 1,2-dichloroethane contact angle on the FTS-derived smooth surface ( θ O ) was 102.1 ° , and the water contact angle ( θ W ) was 110.4 ° . As a result, γ OA cos θ O – γ WA cos θ W > 0, which indicates that the smooth surface is oleophilic in water. The calculation result is consistent with the experiments ( θ OW = 40.3 ° ).

In previous studies, we have created a superhydrophilic rough surface in air. If it is immersed in water, the water is trapped in the rough grooves of the surface. This state is equivalent to an underwater Cassie state of wetting. In this case, the rough structure of the solid surface plays a key role in improving the oleophobicity under water. [ 6 ] It has been reported that air can be trapped inside the rough grooves of the superhydrophobic surface under water. [ 16 ] The as-prepared FTS-derived rough sur-face is superamphiphobic in air. Air will be trapped inside the grooves when the rough surface is immersed in water. A com-plex interfacial system, i.e., a four phase system (air–olid–oil–water), will then be formed when an oil droplet contacts such a surface.

In order to evaluate the durability, the surfaces were placed in air and in water repeatedly several times. The wettability could be changed from being superamphiphobic in air to oil droplets spreading on the surfaces under water ( Figure 3 a). This process was repeated for several cycles. The repeatability remained even after the samples had been put aside without special protection for at least two months. In addition the surfaces were immersed into water (immersion depth of 20 cm) for different immersion times and the contact angle then measured. It was found that the surface was superoleophilic even after 24 h of immersion (Figure 3 b), which demonstrates that the oleophilic property is relatively stable under water.

The FTS-derived surface with the properties reported here can be used as an “oil capture system” to collect oil droplets in water. To more clearly reveal the advantages of the surfaces, an experiment was carried out to determine the capability to cap-ture oil on the surface. As seen in Figure 4 , we sprayed a layer of oil droplets (1,2-dichloroethane) under water in a quartz con-tainer (step 1). In order to observe the process conveniently, the oil was colored yellow beforehand. We then used a FTS-derived glass tube to touch and collect the oil drops underwater (step 2).

the as-prepared surface, it shows that the surface is composed of nanofi bers and microbumps, which criss-cross each other and form a 3D network architecture. From the magnifi ed image (Figure 1 b), the nanofi bers are observed clearly with an average diameter of ≈ 90 nm and a few nanobumps, which is consistent with the condensation reaction and reaction time.

It is well known that the wettability of a surface is gov-erned by the surface free-energy as well as the surface rough-ness. [ 10–12 ] The as-obtained organosilane-grafted surface is of a relatively low free-energy and high roughness, its wettability is thereby evaluated by a contact angle system. For the FTS-derived surface in air, the contact angle of water ( θ W ) and oil ( θ O , e.g., 1,2-dichloroethane) are 168.2 ± 1.3 ° and 148.1 ± 2.1 ° , respectively ( Figure 2 a, b). This result confi rms that the surface presents a typical superamphiphobic property in air. It is known that the contact angle of water and oil on a –CF 3 aligned smooth surface, the lowest surface energy material so far, can merely reach 119 ° and 107 ° , respectively. [ 13 ] As for the FTS-derived surface, the rough structure formed by the 3D nanofi bers and microbumps contributes greatly to an increase in the amphiphobicity, and makes the surface amphihydro-phobic. The θ O is a little smaller than θ W because oil has a lower surface tension than water.

However, when the FTS-derived surface was placed in water, its oil wettability varied so signifi cantly that an oil droplet spread out quickly once it contacted the surface (Figure 2 c). As such, the contact angle of oil under water ( θ OW ) of the FTS-derived surface is about 0 ° under water, which seems to confl ict with its superoleophobic property in air.

The contact angle is commonly defi ned as the angle between the liquid/air interface and the liquid/solid interface, which is formed when a droplet contacts a solid surface in air. Young’s equation, one of the essential equations of surface chemistry,

Figure 2 . The wettability of the FTS-derived surface in air or in water. Photograph of a) a water and b) an oil droplet on the FTS-derived rough surface in air, θ W = 168.2 ± 1.3 ° , θ O = 148.1 ± 2.1 ° , showing the super-amphiphobicity in air of the surface. c) Photograph of an oil droplet in contact with the surface under water, which spread out quickly with θ OW = 0 ° , indicating the surface is superoleophilic under water.

3


© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2011, XX, 1–4

CO

MM

UN

ICATIO

N


were then extracted from the water surface easily, thus the oil droplets were captured by the FTS-derived tube (step 4–6).

In order to show the excellent features of the superam-phiphobic surface in air, another superhydrophobic/superole-ophilic surface in air (a fl uorine-free methylchlorosilane-derived surface, Figure S2, Supporting Information) was prepared and its underwater wettability was studied. Such surfaces can also capture oil droplets underwater. However, the surface was com-pletely wetted and polluted by oil after collecting oil underwater and the oily surface could not to be reused (Figure S3, Sup-porting Information). So the FTS-derived superamphiphobic surface in air possesses repeatable underwater oil absorption properties as opposed to a normal superhydrophobic surface in air. After the FTS-derived surface was removed from the water surface, the superamphiphobic surface in air was easily cleaned by fl ushing. In other words, the oil droplet collected underwater could fall off from the surface automatically and quickly.

In summary, we have successfully created a FTS-derived sur-face with nanofi bers and microbumps intertwined in a 3D net-work structure. The cooperation of low free-energy and micro/nanoscale roughness makes the surface superamphiphobic in air. More importantly, such a superamphiphobic surface shows a contrary superoleophilic property under water. The surfaces can be used to capture and collect oil droplets in water. This study opens up a new strategy to the disposal of oily wastewater, which is certainly signifi cant for future industrial applications.

Experimental Section Materials : Glass wafers were obtained from SMALL BRAND and were

cut into smaller 1.0 cm × 1.0 cm squares. FTS (Alfa Aesar, America) was handled under water-free conditions. Anhydrous toluene (AT) was distilled before use. A water-saturated toluene solution (WT) was the upper solution of a mixture of anhydrous toluene and deionized water, which was left for 2 d under normal temperature and pressure. Other reagents were used as received.

Synthetic Method : A perfectly hydrophobic organosilane surface had previously been prepared on a silicon wafer using a phase separation method. [ 9 ] The method is simple and reproductive. Here, a novel wettable surface on glass wafers was prepared using the same method. Initially, the glass substrates were ultrasonicated for 30 min in a cleanser solution, rinsed with deionized water several times, and dried in a clean

oven at 120 ° C for 20 min. The glass substrates were then soaked in piranha solution (H 2 SO 4 /H 2 O 2 = 7/3 by volume) at 50 ° C for 2 h. After activation by the piranha solution, the substrates were rinsed with copious amounts of deionized water and dried in a clean oven at 120 ° C for 20 min. The wafers were then immersed into glass vials that contained toluene (4 mL, WT/AT = 3/2 by volume), and FTS (0.005 mL) was added immediately to the solvent for silanization with a calibrated pipette. The glass vials were closed to the air during the reaction but exposed to the environment during the solution and sample introductions. After the reaction, the substrates underwent a series of rinses with solvents in the following sequence several times: toluene, ethanol, ethanol/water (1/1), and water. Finally, the substrates were dried in an oven at 120 ° C for 10 min. The synthesis of the smooth surface was same as that of the rough surface, but the silanization was carried out in AT alone.

As the glass tube was moved underwater, oil droplets gathered at the bottom of the glass tube by the liquid spreading and the oil droplets coalescing together (step 2 and 3). The oil droplets

Figure 3 . a) The repeatable wettability of the FTS-derived surface showing the stable super amphiphobicity in air and oleophilicity underwater. b) The wetting behavior of an oil droplet on the surface at different immer-sion times, which exhibits the durability and the stability of the surface.

Figure 4 . The oil capture and collection process with a FTS-derived glass tube. Process 1: a layer of oil droplets is sprayed in bottom of a water container. Process 2: FTS-derived glass tube touches and captures the oil drops underwater. Process 3: as the glass tube moves, oil drops are gathered together. Process 4–6: the oil droplets are sucked off from the water.

4


© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, XX, 1–4

CO

MM

UN

ICATI

ON


[ 3 ] a) L. Jiang , Y. Zhao , J. Zhai , Angew. Chem. Int. Ed. 200 4 , 43 , 4338 ; b) F. Shi , Z. Wang , X. Zhang , Adv. Mater. 2005 , 17 , 1005 ; c) M. H. Jin , X. J. Feng , L. Feng , T. L. Sun , J. Zhai , T. J. Li , L. Jiang , Adv. Mater. 2005 , 17 , 1977 ; d) L. Zhai , M. C. Berg , F. C. Cebeci , Y. Kim , J. M. Milwid , M. F. Rubner , R. E. Cohen , Nano Lett. 2006 , 6 , 1213 .

[ 4 ] J. Genzer , K. Efi menko , Biofouling 2006 , 22 , 339 . [ 5 ] M. J. Liu , Y. M. Zheng , J. Zhai , L. Jiang , Acc. Chem. Res. 2010 , 43 ,

368 . [ 6 ] M. J. Liu , S. T. Wang , Z. X. Wei , Y. L. Song , L. Jiang , Adv. Mater. 2009 ,

21 , 665 . [ 7 ] E. G. Rochow , J. Am. Chem. Soc. 1945 , 67 , 963 . [ 8 ] a) A. Y. Fadeev , T. J. McCarthy , Langmuir 2000 , 16 , 7268;

b) G. R. J. Artus , S. Jung , J. Zimmermann , H.-P. Gautschi , K. Marquardt , S. Seeger , Adv. Mater. 2006 , 18 , 2758 .

[ 9 ] L. Gao , T. J. McCarthy , J. Am. Chem. Soc. 2006 , 128 , 9052 . [ 10 ] R. N. Wenzel , Ind. Eng. Chem. 1936 , 28 , 988 . [ 11 ] A. B. D. Cassie , S. Baxter , Trans. Faraday Soc. 1944 , 40 ,

546 . [ 12 ] L. Feng , S. H. Li , Y. S. Li , J. Zhai , Y. L. Song , B. Q. Liu , L. Jiang ,

D. B. Zhu , Adv. Mater. 2002 , 14 , 1857 . [ 13 ] T. Nishino , M. Meguro , K. Nakamae , M. Matsushita , Y. Ueda ,

Langmuir 1999 , 15 , 4321 . [ 14 ] Y. C. Jung , B. Bhushan , Langmuir 200 9 , 25 , 14165 . [ 15 ] D. R. Lide , CRC Handbook of Chemistry and Physics , 73rd ed. ,

CRC Press , Boca Raton 1992 . [ 16 ] a) A. Marmur , Langmuir 2006 , 22 , 1400 ; b) M. S. Bobji , S. V. Kumar ,

A. Asthana , R. N. Govardhan , Langmuir 2009 , 25 , 12120 ; c) R. Poetes , K. Holtzmann , K. Franze , U. Steiner , Phys. Rev. Lett. 2010 , 105 , 166104 .

Measurements : SEM images were measured with a fi eld-emission scanning electron microscope at 3 kV (JSM-6700F, Japan). CAs were measured on a JC2000D machine (POWEREACH, China) at ambient temperature. The oil droplets were placed carefully onto the FTS-derived surfaces, which were immersed in water. The CA values were obtained by measuring more than fi ve different positions on the same sample.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Natural Science Foundation of China (20703007, 20801057) and by the Fundamental Research Funds for the Central Universities (2009QN049).

Received: March 22, 2011 Published online:

[ 1 ] I. Park , K. Efi menko , J. Sjoblom , J. Genzer , J. Disp. Sci. Technol. 200 9 , 30 , 318 .

[ 2 ] a) W. Barthlott , C. Neinhuis , Planta 1997 , 202 , 1 ; b) X. F. Gao , L. Jiang , Nature 2004 , 432 , 36 ; c) Y. M. Zheng , X. F. Gao , L. Jiang , Soft Matter 2007 , 3 , 178 ; d) W. R. Hansen , K. Autumn , Proc. Natl. Acad. Sci. USA 2005 , 102 , 308 ; e) A. R. Parker , C. R. Lawrence , Nature 2001 , 414 , 33 .

underwater oil capture by a 3d network organosilane surface

Documents