3024
Langmuir 2004, 20, 3024-3027
Control of Wettability by Anion Exchange on Si/SiO2 Surfaces Young Shik Chi,† Jae Kyun Lee,‡ Sang-gi Lee,*,‡ and Insung S. Choi*,† Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea, Life Sciences Division, Korea Institute of Science and Technology (KIST), P.O. Box 131, Cheongryang, Seoul 130-650, Korea Received December 11, 2003. In Final Form: February 21, 2004 Water wettability of Si/SiO2 surfaces was controlled by the formation of SAMs terminating in 1-alkyl3-(3-silylpropyl)imidazolium ions and the anion exchange on the surfaces (“direct anion exchange”). The exchange was confirmed by X-ray photoelectron spectroscopy, and the water wettability was measured as a water contact angle by contact angle goniometry. We found that anions played a great role in determining water wettability of Si/SiO2 surfaces. For example, water contact angles of Si/SiO2 surfaces presenting 1-methyl-3-(3-silylpropyl)imidazolium ions changed from 28 to 42° when the counteranion Cl- was exchanged with PF6-. In addition to the anions, the N-alkyl groups of imidazolium cations were also found to be important in determining water wettability: we did not observe any significant changes in the contact angles of Si/SiO2 surfaces presenting 1-butyl-3-(3-silylpropyl)imidazolium ions by the anion exchange. We also demonstrated that the reaction rate of the direct anion exchange was affected by a choice of solvents: the anion exchange from Cl- to PF6- was the fastest in an aqueous solution.
Introduction Controlling wettability of solid surfaces by surface modification has intensively been studied because of the many technologically important applications including superhydrophobic surfaces,1-4 control over the orientations of liquid crystals,5 drug delivery and biomimetic materials,6,7 and microfabrication.8-10 Formation of stimuliresponsive surfaces via control of wettability is also important in developing nanoelectromechanical,11,12 bioanalytical,13,14 and microfluidic devices15 with tunable properties. Among the surface modification methods, the formation of self-assembled monolayers (SAMs) proved to be a simple and practical technique for controlling wettability,16-22 corrosion,23-25 and (bio)adhesion26-31 of * To whom correspondence should be addressed. E-mail: ischoi@ kaist.ac.kr (I.S.C.);
[email protected] (S.-g.L.). † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Science and Technology. (1) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (2) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (3) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. J. Phys. Chem. B 2000, 104, 8836. (4) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457. (5) Luk, Y.-Y.; Abbott, N. L. Science 2003, 301, 623. (6) Galaev, I. Y.; Mattiasson, B. Trends Biotechnol. 1999, 17, 335. (7) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (8) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452. (9) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (10) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519. (11) Craighead, H. G. Science 2000, 290, 1532. (12) Jacobs, H. O.; Tao, A. R.; Schwartz, A.; Gracias, D. H.; Whitesides, G. M. Science 2002, 296, 323. (13) Ista, L. K.; Perez-Luna, V. H.; Lopez, G. P. Appl. Environ. Microbiol. 1999, 65, 1603. (14) Nath, N.; Chilkoti, A. Adv. Mater. 2002, 14, 1243. (15) Rohr, T.; Ogletree, D. F.; Svec, F.; Fre´chet, J. M. J. Adv. Funct. Mater. 2003, 13, 264. (16) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (17) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (18) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539. (19) Colorado, R., Jr.; Lee, T. R. Langmuir 2003, 19, 3288. (20) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923. (21) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624.
solid surfaces. Diverse strategies have been developed for controlling wettability of solid surfaces on the basis of SAMs and polymeric films in response to environmental changes (i.e., solvents,32-34 pH,35 temperature,13,14,36 and surface pressure)37 and external stimuli (i.e., light,20,21 charge,22 and oxidation-reduction).38 The reported methods for controlling wettability are mainly based on reorganization of the internal or surface structures of SAM-forming molecules on surfaces. Herein, we report a counteranion-directed control of water wettability of Si/SiO2 surfaces coated with SAMs terminating in imidazolium ions. 1,3-Dialkylimidazolium salts, known as one of the ionic liquids, have widely been used as environmentally benign (22) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (23) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596. (24) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (25) Sinapi, F.; Forget, L.; Delhalle, J.; Mekhalif, Z. Appl. Surf. Sci. 2003, 212, 464. (26) Lo´pez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. 1993, 115, 5877. (27) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (28) Mrksich, M. Cell. Mol. Life Sci. 1998, 54, 653. (29) Kingshott, P.; Griesser, H. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 403. (30) Mrksich, M. Curr. Opin. Chem. Biol. 2002, 6, 794. (31) Schaeferling, M.; Schiller, S.; Paul, H.; Kruschina, M.; Pavlickova, P.; Meerkamp, M.; Giammasi, C.; Kambhampati, D. Electrophoresis 2002, 23, 3097. (32) Minko, S.; Mu¨ller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (33) Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2003, 19, 7832. (34) Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912. (35) Chatelier, R. C.; Drummond, C. J.; Chan, D. Y. C.; Vasic, Z. R.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 4122. (36) de Crevoisier, G.; Fabre, P.; Corpart, J.-M.; Leibler, L. Science 1999, 285, 1246. (37) Ohnishi, S.; Ishida, T.; Yaminsky, V. V.; Christenson, H. K. Langmuir 2000, 16, 2722. (38) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57.
10.1021/la036340q CCC: $27.50 © 2004 American Chemical Society Published on Web 03/18/2004
Letters
Langmuir, Vol. 20, No. 8, 2004 3025 Scheme 1. Synthesis of (Cntespim)Cl
solvents in various organic reactions.39-43 The hydrophobicity of dialkylimidazolium salts is modulated by changing the length of the alkyl groups or counteranions, and, consequently, water miscibility can be varied.44-48 We reasoned that the tunable water miscibility of dialkylimidazolium salts in solution could be translated to water wettability on surfaces. Previously, we demonstrated that water wettability of SAMs presenting imidazolium ions at the tail ends on gold surfaces could be related to water miscibility of ionic liquids.49 While SAMs of alkanethiols on gold have intensively been utilized as a model system for the fundamental investigation of interfacial phenomena and as a scaffold for various technological applications because of the well-ordered structure of SAMs of alkanethiols, SAMs of siloxanes on Si/SiO2 surfaces have independently been studied as a result of their characteristic properties.50 For example, SAMs of siloxanes on Si/SiO2 are covalently linked onto the surfaces and, therefore, thermally more stable than those on gold (the desorption of alkanethiols occurs around 60 °C). The thermal stability of the SAMs of siloxanes on Si/SiO2 would yield more feasibility for manipulating the surfaces, and considering the wide use of Si/SiO2 in the micro- and nanofabrication, the SAMs on Si/SiO2 would have easier applicability to the fabrication of the possible devices just mentioned. In this report, we formed SAMs presenting the imidazolium moiety on Si/SiO2 surfaces51 and investigated the effects of alkyl-chain lengths and counteranions on wettability. The kinetics of the direct anion exchange on Si/SiO2 surfaces was also studied. Results and Discussion 1-Alkyl-3-(3-triethoxysilylpropyl)imidazolium chloride [(C1tespim)Cl or (C4tespim)Cl; alkyl ) methyl or butyl] was synthesized by reacting methyl (or n-butyl)imidazole with (3-chloropropyl)triethoxysilane (Scheme 1). (Cntespim)Cl (n ) 1 or 4) was treated with either NaBF4 or NaPF6 in acetonitrile to yield (Cntespim)BF4 or (Cn(39) Welton, T. Chem. Rev. 1999, 99, 2071. (40) Keim, W.; Wasserscheid, P. Angew. Chem., Int. Ed. 2000, 39, 3772. (41) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667. (42) Ionic Liquids: Industrial Applications for Green Chemistry; Roger, R. D., Seddon, K. R., Eds.; ACS Symposium Series 818; American Chemical Society: Washington, D.C., 2002. (43) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH Verlag: Weinheim, 2003. (44) Swatloski, R. P.; Visser, A. E.; Reichert, M. W.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2002, 4, 81. (45) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (46) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (47) Gammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (48) Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Chem. Commun. 2001, 2070. (49) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S.-g. J. Am. Chem. Soc. 2004, 126, 480. (50) Ulman, A. Chem. Rev. 1996, 96, 1533. (51) 1-Butyl-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazolium ions have been synthesized in a different method and have been attached on the surface of a silica gel for the formation of supported ionic liquid phases. See: Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932.
Figure 1. (a) Schematic representation of the direct anion exchange of Cl- with PF6- on Si/SiO2 surfaces. (b) Contact angles of a surface presenting (C1tespim)Cl and a surface presenting (C1tespim)PF6. The anion (Cl-) was exchanged with PF6directly on the surface.
tespim)PF6, respectively.51 The SAMs were formed by immersing freshly cleaned Si/SiO2 substrates in a 1% toluene (or 1,1,2,2-tetrachloroethane) solution of (Cntespim)X (X ) Cl, BF4, or PF6) at 100 °C for 24-48 h, and the quality of the SAMs was checked by ellipsometric measurement. The ellipsometric thicknesses were 6 Å for the SAMs of (C1tespim)X and 8 Å for those of (C4tespim)X, respectively. Water contact angles of the SAMs of (C1tespim)Cl, (C1tespim)BF4, and (C1tespim)PF6 on Si/ SiO2 were 24, 30, and 42°, respectively, which clearly shows that water wettability of the 1-methyl-imidazoliumterminated surfaces was determined by counteranions. In contrast, we observed relatively small (or no) changes in the contact angles of surfaces presenting (C4tespim)X: the water contact angles of (C4tespim)Cl, (C4tespim)BF4, and (C4tespim)PF6 were 51, 51, and 52°, respectively. In the case of (C4tespim)X, water wettability was mainly controlled by the butyl group and not affected by anions. Little change in the contact angles implies that anions are embedded in the butyl chains and in close contact with imidazolium cations. In an analogy to this result, spectroscopic characterizations of dialkylimidazolium salts showed that there was a close interaction (such as hydrogen bonding) between imidazolium rings and anions.45,46,52 We formed the SAM of (C1tespim)Cl on Si/SiO2 and changed the anions directly on the surface (“direct anion exchange”; Figure 1a). For the direct exchange of anions, the substrate coated with the SAM of (C1tespim)Cl was immersed in an aqueous 10 mM solution of NaBF4 or NaPF6 at room temperature for 12 h, and the resulting Si/SiO2 substrate was thoroughly washed with water and dried with a stream of argon. Upon the anion exchange of Cl- to BF4- or PF6-, we observed changes in the contact angles that were expected from the contact angles of the independently formed SAMs of (C1tespim)BF4 and (C1tespim)PF6: the contact angles were changed from 24° to 30° (BF4-) or 42° (PF6-), respectively. Figure 1b shows contact angles of Si/SiO2 surfaces before and after the exchange of Cl- with PF6-. The changes in the contact angles clearly show that water wettability could be controlled by changes in counteranions of imidazolium (52) Elaiwi, A.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y.-M.; Welton, T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 3467.
3026
Langmuir, Vol. 20, No. 8, 2004
Letters
Figure 3. (a) Graph of contact angles versus incubation time. The Si/SiO2 substrate presenting (C1tespim)Cl was incubated in a 10 mM solution of NaPF6 for a designated period of time. (b) Graph of the natural logarithm of the contact angles (CAs) versus incubation time. Using ln[18/(42 - CA)] as a Y axis, the estimated values of the slopes were 1.94 (for water), 0.36 (for ethanol), and 0.20 (for acetone) h-1. (2) Water; (0) ethanol; (b) acetone.
Figure 2. XPS data of surfaces presenting (C1tespim)Cl, (C1tespim)BF4, and (C1tespim)PF6. (a) Survey spectra between 0 and 800 eV. Peaks from Cl(2p) and F(1s) were observed in the survey spectra. (b and c) High-resolution spectra between (b) 190 and 205 eV and (c) 130 and 142 eV. Peaks from B(1s) and P(2p) were observed in the high-resolution spectra.
ions. The direct exchange of anions on the surface was confirmed by X-ray photoelectron spectroscopy (XPS). After the exchange, the peak of Cl(2p) at 198.7 eV disappeared and peaks from BF4- [B(1s) at 193.1 eV and F(1s) at 686.5 eV] or PF6- [P(2p) at 136.9 eV and F(1s) at 687.1 eV] were observed (Figure 2). We also studied the effect of solvent systems on anion exchange rates on surfaces (Figure 3). We chose the anion exchange from Cl- to PF6- because the change in contact angles (18°) was easier to follow than that for the change to BF4- (6°) and screened three solvent systems: water, ethanol (protic solvent), and acetone (aprotic dipolar solvent). Figure 3a shows that the exchange rate was
greatly affected by the solvent systems. In water, the exchange from Cl- to PF6- was fastest and the exchange was completed in 2 h. In contrast, the exchange was slowest in acetone and completed after 10 h. The ratio of the amount of PF6- (in solution) to that of Cl- (on the surface) was very large, and we presume that the exchange reaction could be considered to be pseudo-first order. Figure 3b shows a plot of the natural logarithm of the contact angles versus time: the ratio of the observed rate constants was 10:2:1 (water:ethanol:acetone). The main difference between protic solvents (water and ethanol) and aprotic dipolar solvents (acetone) is the ability to solvate anions.53 A small anion (Cl-) with a high charge density is more strongly solvated through hydrogen bonding than a large anion (PF6-) in protic solvents. In aprotic dipolar solvents, the solvation of anions is achieved by polarizability interactions, which is greatest for large, polarizable anions and least for small, weakly polarizable anions. The observed solvent dependency of the exchange rates could, therefore, be explained by the relative solubility of Cland PF6- or the solubility products of NaCl and NaPF6. Stimuli-responsive organic and polymeric surfaces have a great potential in various technological applications as (53) Reichardt, C. Solvents and Solvent Effect in Organic Chemistry; Wiley-VCH: Weinheim, 2003; pp 57-328.
Letters
Langmuir, Vol. 20, No. 8, 2004 3027
well as fundamental understanding of interfacial phenomena. We designed and synthesized imidazoliumterminated triethoxysilanes with the aim of controlling water wettability of Si/SiO2 surfaces by anion exchange. We studied the anion effect on water wettability of Si/SiO2 surfaces and the solvent effect on the kinetics of anion exchange by contact angle measurements. A detailed understanding of the mechanism on anion exchange at interfaces is under investigation. Materials and Methods Prior to the formation of SAMs, Si/SiO2 substrates were treated for 1 h in hot piranha solution (3:7 by volume of 30% H2O2 and H2SO4; caution, piranha solution reacts violently with most organic materials and must be handled with extreme care) to generate OH groups as well as to clean the surfaces, rinsed with water, and dried under a stream of argon. The reaction condition for forming the SAMs of triethoxysilanes was optimized to generate the SAMs reproducibly: the SAMs were formed by immersing freshly cleaned Si/SiO2 substrates in a 1% toluene (or 1,1,2,2-tetrachloroethane) solution of triethoxysilanes at 100 °C for 24-48 h. After the formation of SAMs, the substrates were rinsed with dimethyl sulfoxide, ethanol, and chloroform several times and then dried under a stream of argon. The thicknesses of the monolayer films were measured with a Gaertner L116s ellipsometer (Gaertner Scientific Corporation, IL) equipped with a He-Ne laser (632.8 nm) at a 70° angle of incidence. A refractive index of 1.46 was used for all films. Contact angle measurements were performed using a Phoenix 300 goniometer (Surface Electro Optics Co., Ltd., Korea). For the direct exchange of anions, the substrate coated with the SAM of (C1tespim)Cl was immersed
in an aqueous 10 mM solution of NaBF4 or NaPF6 at room temperature for 12 h, and the resulting Si/SiO2 substrate was thoroughly washed with water and ethanol and dried with a stream of argon. For the study on the effect of solvent systems on anion exchange rates, the substrate coated with the SAM of (C1tespim)Cl was immersed in a 10 mM (water, ethanol, or acetone) solution of NaPF6 at room temperature. The substrate was taken out at the designated time, thoroughly washed with water and ethanol, and dried with a stream of argon. Contact angles were measured immediately after the drying. The XPS study was performed with a VG-Scientific ESCALAB 250 spectrometer (U.K.) with a monochromatized Al KR X-ray source. Emitted photoelectrons were detected by a multichannel detector at a take-off angle of 90° relative to the surface. During the measurements, the base pressure was 10-9-10-10 Torr. Survey spectra were obtained at a resolution of 1 eV from three scans and high-resolution spectra were acquired at a resolution of 0.05 eV from 5 to 20 scans.
Acknowledgment. This work was supported by the National R&D Project for Nano Science and Technology (KAIST), National Research Laboratory (KIST), and the Future Core Technology Program (KIST). We thank Mi Ra Kim and Dr. Mi-Sook Won in Korea Basic Science Institute (KBSI) for the XPS analysis. Supporting Information Available: Synthetic procedures and analysis data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA036340Q
