Langmuir 2007, 23, 1253-1257
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To Adjust Wetting Properties of Organic Surface by In Situ Photoreaction of Aromatic Azide Feng Shi,† Jia Niu,† Zan Liu,† Zhiqiang Wang,*,† Mario Smet,‡ Wim Dehaen,‡ Yong Qiu,† and Xi Zhang*,† Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, Peoples’ Republic of China, and Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan B-200F, 3001 HeVerlee, LeuVen, Belgium ReceiVed August 13, 2006. In Final Form: October 20, 2006 This article describes development of a simple and convenient method to provide stable low-surface-energy coatings on organic surfaces, by designing and synthesizing a surface-reactive molecule 4-azido-N-dodecylbenzamide, which bears an azide group as the reactive surface anchor and an alkyl chain as the hydrophobic tail. After the hydrophobic modification, rough organic surfaces with contact angle of about 0° can change their surface wetting properties from superhydrophilicity to superhydrophobicity, whose contact angles are above 152° and tilt angles lower than 5°. Moreover, by changing the alkyl chain to a PEO segment, a similar concept can be used to adjust the surface wetting properties from hydrophobic (contact angle ∼130°) to superhydrophilic (contact angle ∼0°).
Introduction The study of surface wetting properties and their control are cutting-edge topics with not only fundamental interest but also promising application.1-5 Particularly, since the self-cleaning property of lotus leaves6-8 has been demonstrated to result from a cooperative effect of a low-surface-energy coating and high surface roughness, artificial lotus-like coatings have aroused great interest for their potential applications in microfluidics, lab-ona-chip devices, drag reduction, self-cleaning coatings, and so forth. In general, two main approaches have been developed to mimic such lotus-like surfaces. One is to increase directly the surface roughness of low-surface-energy materials, such as resolidification of melted paraffin,9 reconformation of polymers,10,11 plasma fluorination,12 chemical vapor deposition,13,14 phase separation of block copolymer,15 surface sol-gel process of organosilicon,16-17 and others.18 The other is to first fabricate a suitable surface roughness with certain materials and then modify * E-mail:
[email protected]. † Tsinghua University. ‡ University of Leuven. (1) Wang, X.; Kharitonov, A. B.; Katz, E.; Willner, I. Chem. Commun. 2003, 1542. (2) Prins, M. W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 277. (3) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (4) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921 (5) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (6) Johnson, R. E.; Dettre, R. H. AdV. Chem. Ser. 1964, 43, 12. (7) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (8) Sun, T. L.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (9) Shibuida, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (10) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (11) Zhao, N.; Xie, Q. D.; Weng, L. H.; Wang, S. Q.; Zhang, X. Y.; Xu, J. Macromolecules 2005, 38, 8996. (12) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (13) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (14) Ma, M. L.; Yu, M.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742. (15) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662. (16) Gao, L. C.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052. (17) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966. (18) Tadanaga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590.
the as-prepared surface with low-surface-energy materials, such as self-assembled monolayers of organic thiols on a surface of gold or silver aggregates,19-23 siliceous microstructures crosslinked with organic silanes,24,25 lauric acid coated on brucitetype cobalt hydroxide,26 copper surfaces modified with fatty acids,27 and spin-coating perfluorononane on a surface of aluminum.28 The latter method can extend the formation of selfcleaning surfaces to many microfabrication systems and is no longer limited to low-surface-energy materials. Thus, the formation of low-surface-energy coatings is either based on fabrication of certain kinds of special low-surface-energy materials or on the special interactions between the low-surface-energy molecules and the rough surface, and it is still difficult to apply on broad varieties of organic surfaces. So, there remains a challenge to develop novel methods for surface modification, leading to adjusting the wetting properties on organic surfaces. Until now, only a few methods have been developed to introduce a low-surface-energy coating on a rough organic surface for the fabrication of self-cleaning surfaces. Rubner and coworkers created a low-surface-energy coating on a honeycomblike polyelectrolyte multilayer surface by overcoating the asprepared rough surface with silica nanoparticles, followed by modification of the silica nanoparticles with a semifluorinated silane.29 Jiang et al. modified the rough surface of nanotubes by oxidizing the nanotubes and immersing them in a methanol (19) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (20) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 4713. (21) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (22) Shi, F.; Wang, Z. Q.; Zhang, X. AdV. Mater. 2005, 17, 1005. (23) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289. (24) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 896. (25) Shi, F.; Chen, X. X.; Wang, L. Y.; Niu, J.; Yu, J. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 6177. (26) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458. (27) Wang, S. T.; Feng, L.; Liu, H.; Sun, T. L.; Zhang, X.; Jiang, L.; Zhu, D. B. Chemphyschem 2005, 6, 1475. (28) Guo, Z. G.; Zhou, F.; Hao, J. C.; Liu, W. M. J. Am. Chem. Soc. 2005, 127, 15670. (29) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349.
10.1021/la062391m CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2006
1254 Langmuir, Vol. 23, No. 3, 2007 Scheme 1. Surface Modification by Surface-Reactive Azido Compoundsa
(a) Chemical structure of ADBA for hydrophobic modification; (b) chemical structure of ATGBA for hydrophilic modification; (c) the experimental procedure of the surface modification. the rough surface with a superhydrophilicity was fabricated by hydrothermal synthesis and that of hydrophobicity was prepared by electrohydrodynamics of polystyrene.
solution of a hydrolyzed fluoroalkylsilane to form a self-assembled monolayer.30 These two methods are based on physical adsorption and need a two-step modifying process. Therefore, it is very important to develop a facile and universal method to modify the rough organic surface chemically, which can provide a stable low-surface-energy coating and then lead to a self-cleaning surface. The azide group acts as a highly active and functional surface anchor, which can easily form chemical bonds with the organic surface under UV irradiation.31,32 We wondered whether we could design and synthesize surface-reactive molecules, which bear azide groups as surface anchors, to introduce low-surface-energy coatings on organic surfaces. For hydrophobic or superhydrophobic modification, we designed the surface-reactive molecule 4-azido-N-dodecylbenzamide (ADBA) (Scheme 1a) bearing an azide group as the reactive surface anchor and an alkyl chain as the hydrophobic tail. Similarly, for hydrophilic or superhydrophilic modification, we also designed and synthesized 4-azidotetraethylene-glycolbenzamide (ATGBA) (Scheme 1b) bearing an azide anchor and a hydrophilic tail of polyglycol. After in situ photoreaction, the ADBA or ATGBA can be chemically anchored to organic surfaces, providing stable coatings with hydrophobic or hydrophilic properties, respectively, as shown in Scheme 1c. The surface modification cannot only be applied to a smooth organic surface, but also to a rough one. In the latter case, there exists an amplified effect allowing the fabrication of superhydrophobic or superhydrophilic surfaces. Experimental Section Synthesis of the Aromatic Azide ADBA. 4-Azidobenzoic acid33 (1.1 g, 6.7 mmol) was suspended in thionyl chloride (2 mL), and 1 drop of DMF was added. The mixture was heated at reflux for 3 h and allowed to cool to room temperature. The solvent was (30) Li, H. J.; Wang, X. B.; Song, Y. L.; Liu, Y. Q.; Li, Q. S.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 1743. (31) Devaraj, N. K.; Miller, G. P.; Ebina, W.; Kakaradov, B.; Collman, J. P.; Kool, E. T.; Chidsey, C. E. D. J. Am. Chem. Soc. 2005, 127, 8600. (32) Scriven, E. F. V.; Turbull, K. Chem. ReV. 1988, 88, 351. (33) Liu, Q.; Tor, Y. Org. Lett. 2003, 5, 2571.
Shi et al. evaporated under reduced pressure, and the obtained crude 4-azidobenzoyl chloride was dissolved in CH2Cl2 (15 mL). The obtained solution was added in a dropwise manner to a stirred solution of 1-aminododecane (1.25 g, 6.7 mmol) and Et3N (0.68 g, 17 mmol) in CH2Cl2 (20 mL) at 0 °C. After complete addition, the mixture was stirred for 1 h at room temperature, and the precipitate was removed by filtration. The filtrate was concentrated under reduced pressure and subjected to column chromatography (SiO2, CH2Cl2). ADBA was obtained as a white crystalline solid (2.0 g, 91%). 1H NMR (300 MHz, CDCl3, TMS): δ ) 0.89 (t, 3J (H-H) ) 6.6 Hz, 3H), 1.201.40 (m, 18H), 1.55-1.65 (m, 2H), 3.43 (m, 2H), 6.18 (br. m, 1H), 7.04 (d, 3J (H-H) ) 8.0 Hz, 2H), 7.77 ppm (d, 3J (H-H) ) 8.0 Hz, 2H). 13C NMR (75 MHz, CDCl3, TMS): δ ) 14.5, 23.1, 23.2, 27.4, 29.7, 29.95, 29.98, 30.0, 32.3, 40.6, 119.3, 129.0, 131.7 143.5, 166.8 ppm. FT-IR: ν ) 1286, 1466, 1498, 1533, 1628, 2124, 2848, 2921, 3334 cm-1. CI MS: m/z 331 [MH+]. Synthesis of the Aromatic Azide ATGBA. ATGBA was obtained analogously using aminotetraethylene glycol monomethyl ether34 after column chromatography (SiO2, ethyl acetate) as a slightly yellow oil (62%). 1H NMR (300 MHz, CDCl3, TMS): δ ) 3.43 (s, 3H), 3.50-3.53 (m, 2H), 3.59-3.66 (m, 14H), 6.93-7.00 (br. m, 1H), 7.05 (d, 3J (H-H) ) 8.0 Hz, 2H), 7.85 ppm (d, 3J (H-H) ) 8.0 Hz, 2H). 13C NMR (75 MHz, CDCl3, TMS): δ ) 40.2, 59.4, 69.7, 70.3, 70.6, 70.9, 71.1, 71.2, 72.2, 119.3, 128.6, 131.5, 143.5, 166.5 ppm. CI MS: m/z 353 [MH+]. Formation of Superhydrophilic Surfaces. Glucose (4-8 g, analytical purity, Beijing Chemical Reagent Factory) was dissolved in water (40 mL) to form a clear solution, which was placed in a 40-mL Teflon-lined stainless steel autoclave. A polytetrafluoroethylene (PTFE) substrate was put into this stainless steel autoclave and immersed in the aqueous solution of glucose. Then, the autoclave was sealed and kept at 160 °C in an oven for 8 h. Then, it was taken from the oven and allowed to cool to room temperature naturally, after which the PTFE substrate with special microstructures was drawn out from the autoclave, rinsed with methanol and pure water, and carefully oven-dried at 80 °C for at least 4 h. Formation of Superhydrophobic Surfaces.35,36- Polystyrene (homopolymer, Mw ) 220 000, Beijing Chemical Reagent Factory) was dissolved in DMF (Beijing Chemical Reagent Factory) by stirring for 5 h to form a 7 wt % transparent solution. About 2 mL of the precursor solution was placed in a 5-mL syringe equipped with a blunt metal needle of 0.6-mm inner diameter. The syringe was placed in a syringe pump that maintained a solution feed rate of 0.5 mL h-1. A glass substrate was employed as the collector. The distance between the needle tip and the collector was 14 cm, and the voltage was set at 14 kV. Formation of Smooth Hydrophilic Surfaces for UV, XPS, and Contact Angle Measurements. A quartz substrate was cleaned by immersing in piranha solution (H2SO4:H2O2 ) 3:7) for 30 min and then rinsed with distilled water and oven-dried at 100 °C for 3 h. After cooling down, the quartz substrate was immersed in a 1 mmol/L toluene solution of APTS for 24 h, under a water-free atmosphere, rinsed with dichloromethane, acetone, and pure water successively, and dried with nitrogen. Modification Process by ADBA. ADBA was dissolved in dichloromethane at a concentration of 5 mg/mL and spin-coated on the substrate with a rotation speed of 1500 rpm for 15 s. The above substrates were irradiated under a 130-W ultraviolet lamp for 600 s at a distance of 10 cm. The substrates were rinsed successively and thoroughly with dichloromethane, acetone, and ethanol, in order to fully remove the physically adsorbed species, and then dried with nitrogen. Modification Process of ATGBA. ATGBA was dissolved in ethanol at the concentration of 1 mg/mL and spin-coated on the substrate with a rotation speed of 1500 rpm for 15 s. The above substrates were irradiated under a 130-W ultraviolet lamp for 600 (34) Voicu, R.; Boukherroub, R.; Bartzoka, V.; Ward, T.; Wojtyk, J. T. C.; Wayner, D. D. M. Langmuir 2004, 20, 11713. (35) Zhao, Y. Y.; Yang, Q. B.; Lu, X. F.; Wang, C.; Wei, Y. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 2190. (36) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338.
Wetting Properties of Organic Surfaces
Figure 1. (a) FE-SEM image of the rough surface prepared under hydrothermal condition; (b) magnified image of (a). The contact angle measurement (c) on the unmodified rough substrate; (d) on the same substrate after modification by photo-cross-linking with ADBA (drop volume is 4 µL).
s at a distance of 10 cm. The substrates were rinsed thoroughly with dichloromethane, acetone, and ethanol, successively, to fully remove the physically adsorbed species, and then dried with nitrogen. General Techniques. The measurement of static contact angles was carried out by commercial instruments (OCA 20, DataPhysics Instruments GmbH, Filderstadt). A distilled water droplet of 4 µL was used as the indicator. FT-IR spectra were collected with an IFS-66v/S FT-IR spectrometer (Bruker) with a MCT detector cooled with liquid nitrogen. UV-vis spectra were recorded with a Hitachi UV-vis 3010 spectrophotometer. The field-emission scanning electron microscope (FE-SEM) images were obtained with a JEOL JSM-6700F scanning electron microscope at 10.0 kV. The XPS was performed on a VG ESCALAB MK II spectrometer with an Mg KR X-ray source (1253.6 eV). The base pressure in the analysis chamber during spectral acquisition was 3 × 10-7 Pa.
Results and Discussion The rough surface with superhydrophilicity was prepared by polymerization and carbonization of glucose.37 The morphology of the rough surface formed on the PTFE by hydrothermal synthesis was investigated carefully with a field-emission scanning electron microscope (FE-SEM). From Figure 1a,b, it can be seen that many three-dimensional structures were formed on the surface. These three-dimensional structures are composed of carbon nanospheres with diameters around 500 nm. The wetting properties of the as-prepared rough surface were studied by contact angle measurements. When a water droplet (drop volume is 4 µL) was placed on the surface, it was adsorbed into the rough surface within 40 ms, as shown in Figure 1c. Hence, it can be concluded that the rough surface of carbon microstructures, formed by polymerization and carbonization of glucose, resulted in the superhydrophilicity. Since both surface roughness and a low-surface-energy coating are prerequisites for inducing superhydrophobicity, we modified the as-prepared rough surface with ADBA bearing an azide group as the surface-reactive anchor and a hydrophobic alkyl chain. After modification, we observed that the contact angle was about 152°, as shown in Figure 1d. This result indicates that the surface wetting properties changed from superhydrophilicity to superhydrophobicity. Moreover, the tilt angle was lower than 5°, (37) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597.
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indicating that the water droplet can roll off the surface easily. The tilt angle is the threshold angle between the surface and the horizontal plane when a water droplet starts to roll on a tilting superhydrophobic surface. It reflects (but is not equal to) the contact angle hysteresis of the surface. If the tilt angle is very low (
