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Langmuir 2000, 16, 2394-2397
Dynamic Contact Angle Measurement of Au(111)-Thiol Self-Assembled Monolayers by the Wilhelmy Plate Method Koji Abe,*,† Hiroshi Takiguchi,‡ and Kaoru Tamada† National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305-8565, Japan, and Hiroshima University, Higashi-Hiroshima 739-8528, Japan Received May 21, 1999. In Final Form: October 27, 1999
Introduction Self-assembled monolayers (SAMs) composed of thiol derivatives on gold have been an issue attracting animated research in advanced surface sciences for the past decade.1 Surface characterization of SAMs has been made from the molecular level (e.g., molecular lattice, chain orientation)2,3 to the macroscopic level (e.g., composition of functional groups on surface, surface roughness).4,5 Dynamic contact angle measurement gives not only practical information on wetting but also valid indications for chemical composition, morphology, and stability of molecules at the surface.6-9 Most Au-thiol SAMs are stable and have extremely small hysteresis in dynamic contact angle (∆θ ) θa - θr ∼ 10°, θa, advancing angle; θr, receding angle);10-12 i.e., original ordered structures in SAMs are maintained during wetting and dewetting due to the strong interaction between sulfur and gold atoms (chemisorption, Au-S chemical bonding energy ∼40 kcal/mol).13 This striking characteristic has not been achieved in other monomolecular films such as deposited LangmuirBlodgett (LB) films14 or organosilane SAMs.15,16 Au-thiol SAMs have the potential to control the wetting properties of metal surfaces, whether hydrophobic or hydrophilic, simply by applying a functional group such as -OH, -NH2, -COOH, -OCH3, -CH3, or -(CF2)nCF3 to molecular * To whom correspondence should be addressed: E-mail:
[email protected]. Tel: +81-298-61-6314. Fax: +81-298-61-6232. † National Institute of Materials and Chemical Research. ‡ Hiroshima University. (1) Ulman, A. In An Introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: Boston, MA, 1991. (2) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (3) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (4) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1994, 98, 563. (5) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J.Langmuir 1994, 10, 611. (6) Abe, K.; Ohnishi, S. Jpn. J. Appl. Phys. 1997, 36, 6511. (7) Christenson, H. K.; Yaminsky, V. V. Colloid Surf. 1977, 129-130, 67. (8) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface 1996, 179, 37. (9) Wettability; Berg, J. C., Eds.; Marcel Dekker: New York, 1993. Johnson, R. E., Jr.; Dettre, R. H. In Surface and Colloid Science; Matijevic E., Eds.; Wiley-Interscience: New York, 1969; Vol. 2, p 85. Neumann, A. W. Adv. Colloid Interface Sci. 1974, 4, 105. (10) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (11) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 150, 327. (12) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (13) Dubois, L. H.; Nuzzo, R. G. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4739. (14) Christenson, H.; Claesson, P. M. Science 1988, 239, 390. (15) Rabinovich, Y. I.; Yoon, R. H. Langmuir 1994, 10, 1903. (16) Wood, J.; Sharma, R. J. Adv. Sci. Technol. 1995, 9, 1075.
Figure 1. Contact angle measurement with an asymmetric Au-thiol SAM plate.
tails.11,17,18 Au-thiol SAMs are potentially an ideal model surface for evaluating the reliability of theoretical predictions for wetting.19 A large number of publications have concerned contact angles of Au-thiol SAMs, usually employing the sessile drop method,18,20 known to be convenient but less reproducible and less reliable than the Wilhelmy plate method. This is because gold used in SAM formation is usually deposited only on one side (both side deposition of Au(111) layer onto substrates is difficult, since temperature control of substrates is required when deposition is in progress),10,11,20 and this asymmetric feature of the SAM plate allows only the sessile drop method be used.21 In this paper, we propose efficient use of the Wilhelmy plate method to measure dynamic contact angles with asymmetric Au-thiol SAM plates. The Wilhelmy plate method is superior to the sessile drop method for the following reasons:6,22-24 (1) precision for contact angle estimation due to detection with a sensitive microbalance, which is also free from operator error arising from eye determination; (2) high reproducibility due to large scanning area of substrates (centimeters across) and bulk liquid;25 (3) reliable characterization of dynamic effect during wetting and dewetting by accurate control of advancing and receding speeds with dc motor. Experimental Section Gold was thermally deposited on freshly cleaved mica (1.0 × 2.0 cm) in a vacuum chamber (Veetech Japan Co. Ltd., Ibaraki, (17) Miura, Y. F.; Takenaga, M.; Koini, T.; Graupe, M.; Garg, N.; Graham, Jr., R. L.; Lee, T. R. Langmuir 1998, 14, 5821. (18) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (19) Drelich, J.; Wilbur, J. L.; Miller, J. D.; Whitesides, G. M. Langmuir 1996, 12, 1913. (20) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (21) We found only one paper (Sondag-Huethorst, J. A.; Fokkink, L. G. J. Langmuir 1992, 8, 2560), in which the wetting property of Authiol SAMs was characterized by the Wilhelmy technique. But, they used polycrystalline gold electrodes (bulk gold mechanically cut and polished) whose surfaces were not adequate for the study on intrinsic wettability of SAMs. (22) Uyama, Y.; Inoue, H.; Ito, K.; Kishida, A.; Ikada, Y. J. Colloid Interface Sci. 1991, 141, 275. (23) Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237. (24) Seebergh, J. E.; Berg, J. C. Chem. Eng. Sci. 1992, 47, 4468. (25) Hato, M.; Minamikawa, H.; Okamoto, K. Chem. Lett. 1991, 1049.
10.1021/la990624m CCC: $19.00 © 2000 American Chemical Society Published on Web 01/17/2000
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Figure 2. Force balance and plate tilt induced by the horizontal force arising from the asymmetric plate. Here, F′ is the sum of buoyancy force and gravitational force due to the weight of the sample and clip: (a) forces in dynamic contact angle measurement; (b) forces normal to the sample plate. The balance between the two forces dominates tilt angle φ: (c) forces parallel to the sample plate and wire of the stirrup and resulting force Ft acting on the electrobalance. Systematic error was estimated from the resulting force. Japan) as described elsewhere.10,26 Mica plates are commonly used as a substrate for Au(111) deposition because their atomically flat surfaces are considered to play a role in the epitaxial growth of Au(111) layers. The usability of mica is also the key to success in this contact angle measurement. After SAM formation on a Au/mica plate (immersing Au/mica plates in 1 mM thiol solution for ∼24 h and rinsing with absolute solvent, then drying by dry N2 blow),10,20,26 the back of the mica plate (bare mica, probably with contamination by thiol) was cleaved just before contact angle measurements to obtain a clean surface. The freshly cleaved mica surface is not only ideally flat but also chemically homogeneous and inert, which shows constant wettability for most solvents. Advantageously, mica exhibits perfect wetting against pure water (θa ) θr ) 0°),29 which makes the equation for estimating contact angles quite simple. Since a thick sample plate causes large experimental error by the edge effect,30 we used relatively thin mica plates with a thickness of ca. 50 µm. Dynamic contact angles were measured by use of DCA 322 (Cahn, USA) with pure water (200 mL in a glass beaker) as a liquid phase in a clean room at 22 ( 1 °C. Water was filtered through a Milli-Q Labo (Millipore Co., Germany) and distilled before use. Force F acting on the Au-thiol SAM plate was monitored by a Cahn balance at immersion (advancing) and withdrawal (receding) at 20 µm/s. F is described by the following equation, with contact angles of SAM surfaces (θ) and bare mica (θm)
F ) LγLV(cos θ + cos θm)
(1)
where F is force arising from the vertical component of surface tension acting on the sample plate, γLV is the surface tension of liquid, and L is the width of the sample plate. When water is used as a liquid phase, eq 1 is written as follows, since the contact angle of water on cleaved mica is assumed to be 0° (cos θm ) 1) (Figure 1)
F ) Lγw(1 + cos θ)
(2)
where γw is the surface tension of water (72.5 mN/m). With this equation, contact angles of SAMs can be estimated easily from F. (26) ODS samples were purified carefully before use, since thiol and disulfide impurities in monosulfide are well-known to affect SAM properties drastically through competitive adsorption.27,28 Takiguchi, H.; Sato, K.; Ishida, T.; Abe, K.: Yase, K.; Tamada, K. Langmuir, in press. (27) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (28) Jung, Ch.; Dannenberger, O.; Xu, Y.; Buck, M. Langmuir 1998, 14, 1103.
Figure 3. Expected tilt angle and systematic error in contact angle due to the tilt under our experimental conditions: dotted line, tilt angle φ; solid line, the error in contact angle. The major problem with this technique is the horizontal component of surface tension (asymmetrical force F| ) Lγw sin θ, for water),31 which may cause sample plate tilting or bending. If the sample plate is rigid enough (mica plate with 50 µm thickness is classified into this category), it tilts but does not bend (Figure 2). Under our experimental conditions, the systematic error in contact angle due to tilt is comparable to tilt angle φ, estimated to be +3° in the worse case, i.e., the contact angle of SAM is 90° (Figure 3).32 This error can be reduced by modification of experimental setup, e.g., by attaching more weight at the upper or lower edges of the sample plate, when higher accuracy is required. The AFM system used in this study was a commercially available NanoScope IIIa (Digital Instruments, Inc., Santa Barbara, CA). The measurements were performed in the contact mode (30 µm scanner) in air at room temperature. A Si3N4 cantilever with a spring constant of 0.12 N/m was used (scanning (29) Schultz, J.; Tsutsumi, K.; Donnet, J.-B. J. Colloid Interface Sci. 1977, 59, 277. (30) Errors arose from estimation of the plate perimeter and also from different surface conditions at the section. (31) Della Volpe, C. J. Adhes. Sci. Technol. 1994, 10, 1458. (32) Tilt angle φ and systematic error due to the asymmetric force were estimated as shown in parts b and c of Figure 2. In the estimation, a set of sample and sample clip was assumed to be one rigid body weighing 1.4 g and hanging by a stirrup from the electrobalance. The tilt angle φ depressed with an increase in the weight. The estimated error is larger than the tilt angle φ and the difference between them is depressed with an increase in the weight. Under our experimental conditions, the error was slightly (
