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Silver n-Octadecanethiolate Langmuir Monolayers Mimicking Self-Assembled Monolayers on Silver Weizhong Zhao† and Mahn Won Kim Exxon Research and Engineering Company, Annandale, New Jersey 08801
D. Brad Wurm, Scott T. Brittain,‡ and Yeon-Taik Kim* Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487 Received July 10, 1995. In Final Form: September 27, 1995X As a model for single layers of self-assembled monolayers, Langmuir monolayers of silver noctadecanethiolates were prepared to study the interfacial properties involving the sulfur and metal ions. The interfacial property of the thiol groups was compared with those of two hydrophilic functional groups, namely, a hydroxyl and a carboxyl group. In situ real-time ellipsometry together with surface pressurearea isotherm measurements were carried out to gain insight into the structures of the monolayers and their silver complexes. Furthermore, Brewster angle microscopy was used to supplement the structural information obtaining real surface images with nanometer thickness resolution. The results are discussed in terms of the formation constant of silver complexes between silver and the functional groups.
Introduction Self-assembled monolayers of alkanethiols on metals have been extensively studied not only to understand their fundamental properties1 but also to apply them to technological areas of lubrication, catalysis, corrosion, adhesion, and sensors.2 These organic monolayers on various substrates have been characterized by a variety of techniques, including infrared, Raman, He-diffraction and X-ray photoelectron spectroscopy, contact angle measurement, electrochemistry, and scanning probe microscopy.3 The monolayers are ordered and densely packed with a certain tilt angle depending on the charge density of the metals. Although the molecular structure of the organic alkyl group in self-assembled monolayers is well understood, the interfacial chemical and electronic properties involving the sulfur and metal atoms have been obscured due to the dominant metal property of the substrate during the characterization. Thus, it is desirable to isolate metal alkanethiolates from the bulk metal so that only the interfacial electronic and chemical properties can be investigated. An alternative method for producing monolayers is the Langmuir technique, which allows us to form monolayers at the air-water interface.4 Majda and co-workers reported the formation of n-octadecanethiol monolayers with * Corresponding author. Telephone: (205) 348-0610. FAX: (205) 348-9104. E-mail:
[email protected]. † Current address: Xerox Corporation, 0114-23D, Webster, NY 14580. ‡ Current address: Department of Chemistry, Harvard University, Cambridge, MA 02138. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (2) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, J. Nature 1988, 332, 426. (b) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (c) Dubois, L.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (d) Bard, A. J.; Abruna, H. D.; Chidsey, C. R.; Falukner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (e) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (f) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (g) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886.
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a collapse pressure of 19 mN/m at the air-water interface. The low collapse pressure was the result of the insufficient polarity of the thiol group.5 However, Sobotka and Rosenberg reported that alkanethiols did not form stable monolayers at the air-water interface.6 Itaya and coworkers showed the formation of octadecanethiol layers on a 0.5 mM CdCl2 subphase with a limiting area per molecule of 9 Å2 that appears to be the formation of bilayers.7 Despite these studies on the formation of alkanethiol Langmuir monolayers, it remains difficult to address the formation of monolayers because of the insufficient information from surface pressure measurements. An ellipsometer is an in situ nondestructive surface analysis tool, which probes optical properties of thin films. The sensitivity of ellipsometry at the air-water interface has been recognized on numerous occasions. The amplitude and phase changes of low-intensity light reflected from the sample are measured in terms of ellipsometric parameters, Ψ and ∆,8 which provide the optical property (3) (a) Camillone, N., III; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 99, 744. (b) Camillone, N., III; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (c) Camillone, N., III; Chidsey, C. E. D.; Liu, G.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (d) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 93, 3665. (e) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897. (f) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (g) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (h) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (i) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (j) Singh, R.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 1190. (k) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 558. (l) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (m) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 128, L23. (n) Haussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (o) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (p) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1993, 97, 211. (q) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (r) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (s) Chidsey, C. E. D.; Loiacano, D. N. Langmuir 1990, 6, 682. (t) Miller, C. J.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877. (u) Redepenning, J.; Tunison, H. M.; Finklea, H. O. Langmuir 1993, 9, 1404. (v) Sun, L.; Crooks, R. M. Langmuir 1993, 9, 1951. (w) Chailapakul, O.; Sun, L.; Xu, C. J.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (4) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, NY, 1990. (5) Bilewicz, R.; Majda, M. Langmuir 1991, 7, 2794. (6) Sobotka, H.; Rosenberg, S. Monomolecular Layers; Sobotka, H., Ed., Amer. Ass. Advan. Sci.: Washington, DC, 1954, p 175. (7) Itaya, A.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1989, 5, 1123.
© 1996 American Chemical Society
Silver n-Octadecanethiolate Langmuir Monolayers
of thin films. In particular, the phase change of the incident beam (∆) can determine the thickness change at the angstrom level. Thus, the combination of ellipsometry with pressure-area measurements can directly monitor the formation of various states of Langmuir monolayers during the compression. This allows us to determine the exact pressure corresponding to fully compressed monolayers (codensed phase) at the air-water interface. Brewster angle microscopy (BAM) provides a surface image by sensing changes in optical properties at an airwater interface.9 At the Brewster angle, the parallel component (p) of the incident light is not reflected. The formation of organic monolayers disturbs the Brewster angle allowing the p-component to be reflected. The intensity of the reflected p-component is dependent on the optical properties (refractive index) and the thickness of organic monolayers. It shows angstrom sensitivity in the perpendicular direction to the axis of organic monolayers but shows lateral resolution on the order of micrometers. Thus, it is an ideal technique to monitor the formation of organic monolayers at the air-water interface. The goal of this paper is to understand the formation of alkanethiol Langmuir monolayers and the subsequent monolayer formation of silver thiolates. The monolayers of silver thiolates mimic self-assembled monolayers and thus serve as a model system for the investigation of the interfacial properties involving the sulfur and silver atoms. Here we report the results of the formation of noctadecanethiol and silver n-octadecanethiolate Langmuir monolayers monitored by in situ real-time ellipsometry and Brewster angle microscopy (BAM). Furthermore, systematic comparisons were carried out to understand the interfacial properties involving silver and functional groups such as hydroxyl and carboxyl groups. Experimental Section Chemicals. n-Octadecanethiol, n-octadecanoic acid, and n-octadecanol were purchased from Aldrich. All of them were purified by recrystallization out of hexane. AgNO3 (99.9%) was purchased from Aldrich and used without purification. Chloroform (Aldrich, HPLC grade) was used as a spreading solvent for all the monolayers. The concentration of the spreading solutions was 8 mg/mL and 10 µL of the stock solution was spread using a microsyringe on to the water surface for all the three monolayers. The subphase water was in-house deionized water with the Millipore system. The resistivity of the water was greater than 12 MΩ. Characterization. Langmuir Monolayer Techniques. All experiments were performed at room temperature (23 ( 0.2 °C) on a Teflon-coated Lauda trough with an area of 750 cm2. The surface pressure was measured using a Langmuir film balance technique by monitoring the horizontal force of the monolayer against a Teflon barrier. The surface concentration was adjusted with a sliding Teflon barrier. The compression speed for all the monolayers was 0.2 cm/s. The trough was thoroughly cleaned with acetone and finally rinsed with deionized water before each experiment. The monolayers were spread by just contacting the surface of the water subphase with drops of a chloroform solution (8) (a) Collins, R. W.; Kim, Y.-T. Anal. Chem. 1990, 62, 887A. (b) Aspnes, D. E.; Studna, A. A. Appl. Opt. 1975, 14, 220. (c) Aspnes, D. E. J. Opt. Soc. Am. 1976, 64, 812. (9) (a) Hoenig, D.; Moebius, D. Thin Solid Films 1992, 210-11, 64. (b) Thibodeaux, A. F.; Radler, U.; Shashidhar, R.; Duran, R. S. Macromolecules 1994, 27, 784. (c) Mann, E. K.; Lee, L. T.; Henon, S.; Langvin, D.; Meunier, J. Macromolecules 1993, 26, 7037. (d) Zatisev, S. Y.; Moebius, D. Thin Solid Films 1994, 244, 890. (e) Zatisev, S. Y.; Belohradsky, M.; Zavada, J.; Moebius, D. Thin Solid Films 1994, 248, 78. (f) Foster, W. J.; Shih, M. C.; Pershan, P. S. Mater. Res. Soc. Symp. Proc. 1995, 375, 187. (g) Overbeck, G. A.; Hoenig, D.; Moebius, D. Biosens. Bioelectron. 1995, 10 (part 2), 99. (h) Charych, D. H.; Anvar, D. J.; Marcin, M. Thin Solid Films 1994, 242, 1. (i) Kotov, N. A.; Zaniquelli, M. E. D.; Meldrum, F. C.; Fendler, J. H. Langmuir 1993, 9, 3710.
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Figure 1. Surface pressure vs molecular area isotherms of octadecanethiol, octadecanoic acid, and octadecanol on a pure water subphase. formed at the end of the microsyringe needle. Care was taken not to touch the water subphase with the needle itself. Ellipsometry. A homemade single wavelength ellipsometer based on the photoelastic modulation principle was used.10 The experiments were performed in air at a fixed angle of incidence of 64 ( 0.2° measured relative to the axis normal to the surface. A 7 mW He-Ne laser (6328 Å) with 1 mm beam diameter was used as a light source. Background signals were subtracted in all the ellipsometry measurements during the compression and expansion of the Langmuir trough. The ellipsometric phase change (δ∆) is directly proportional to the film thickness when the thickness is significantly small compared to the wavelength of the light source.11 Brewster Angle Microscopy. A homemade Brewster angle microscope employing a 7 mW He laser light source with one polarizer before the reflection was used to examine the surface image of the monolayers. The images were directly recorded by a video tape recorder connected to a charge-coupled device (CCD) detector. The monolayers were spread in a homemade Teflon trough with an area of 165 cm2. Two Teflon bars were moved manually to change the surface concentration of the monolayers.
Results and Discussion The Formation of Langmuir Monolayers. Surface pressures (π) vs area (A) per molecules for octadecanol (C18OH), octadecanethiol (C18SH), and octadecanoic acid (C17COOH) on a pure water phase are shown in Figure 1. The limiting areas per molecule for the three amphiphilic compounds before the collapse are determined to be approximately 18 ( 1 Å2/molecule by extrapolation of the rising portion of the π-A diagrams in Figure 1. The values are close to the theoretical cross sectional area of an alkyl chain. This indicates the fully compressed monolayers for the three compounds. In addition, the possible formation of multilayers of n-octadecanethiols is noticed by the plateau pressure region with low surface area/molecule in the π-A diagram. We also found that when the barrier was stopped at the condensed-phase pressure (6 mN/m) of n-octadecanethiols, the surface pressure dropped slowly, which indicates that the monolayers are not stable. Furthermore, the pressure decays at the fixed pressure are not reproducible. These observations lead us to propose the following structures of condensed-phase monolayers. The proposed monolayer structure of n-octadecanethiols is seen in Figure 2, which consists of domains contacting each other without forming a single domain. This structure is being relaxed or reoriented as the barrier stops resulting in the pressure drop. The pressure decay is not necessarily reproducible (10) Kim, M. W.; Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G. Langmuir 1990, 6, 236. (11) (a) den Engelson, D.; de Koning, B. R. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1603. (b) Engelsen, D. J. Opt. Soc. Am. 1971, 61, 1460.
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Figure 2. A schematic of domains formed by octadecanethiol monolayers.
Figure 3. Ellipsometric phase changes vs molecular areas of octadecanethiol on a pure water subphase.
in each trial because of different domain structures. The imperfect monolayer structure of n-octadecanethiols at the air-water interface might be a major problem in forming a compact Blodgett film after the transfer to a solid substrate.5 This was further confirmed by the ellipsometry and BAM observations (see discussion below). In contrast, octadecanol and octadecanoic acid formed stable monolayers and exhibit no pressure drop when the barrier was stopped. This suggests the formation of a single domain at the air-water interface. Subsequently, the compressed monolayers were expanded. The octadecanol and octadecanoic acid did not show hysteresis on expansion, but octadecanethiol did. The large hysteresis of n-octadecanethiols suggests that the molecular interaction between the hydrocarbon chains with sulfur atoms is strong enough to hold the molecules together as domains during the compression and expansion process. In Situ Real-Time Ellipsometry Measurements. The phase changes (δ∆) in the ellipsometric parameters were measured during the compression of the three compounds. In this measurement, we only measured the δ∆ and did not record the amplitude changes (δΨ) because of the insensitive response of δψ at the He-Ne laser wavelength (632.8 nm). Figure 3 shows the in situ realtime δ∆ during the compression of octadecanethiol. The C18SH monolayers showed 0.017 rad δ∆. The calculated thickness corresponding to the measured δ∆ value for C18SH is 27 Å. The calculation was performed assuming the three-phase optical model (air atmosphere/alkane/ H2O substrate). The determined thicknesses are compared to the fully stretched theoretical length of the molecules (27.7 Å, obtained by the addition of 1.3 Å per
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methylene group and 5.1 Å for CH3SH12), which tell us that the molecules are oriented almost normal to the surface at the air-water interface. We found that the ellipsometric phase change (δ∆) of C18SH is abrupt during the compression while it is not for the other molecules. Furthermore, it fluctuates between 0 and 0.017 rad as seen in Figure 3. Note that the phase change (δ∆) of fully compressed monolayers is 0.017 rad, while that for the water surface is 0 rad: the above observation indicates that as soon as the molecules are spread at the air-water interface C18SH forms domains of condensed-phase monolayers independent of the barrier pressure. These domains form prior to compression, in spite of the fact that there is an insufficient amount of C18SH present to cover the entire trough area. During compression, the position of the domains shifts on the surface. As a result, the ellipsometry beam is alternately reflected from domains of condensed-phase monolayers and from the water surface between these domains. When the ellipsometry beam is reflected from one of the condensed-phase domains, a 0.017 rad phase change is produced. When the beam is reflected from an area of water surface between domains, a 0 rad phase change results. This observation was further confirmed by the BAM measurements (see discussion below). Thus, we propose that the intermolecular interaction of C18SH is very strong at the air-water interface, resulting in the coexistence of a two-dimensional condensed phase and a gas phase upon spreading at the air-water interface. As a consequence, the C18SH molecules experience the firstorder phase transition of a gas and solid phase while the monolayers are compressed. The formation of a silver complex was also monitored by in situ real-time ellipsometry. AgNO3 was introduced by the injection of 1.2 g/10 mL AgNO3 solution from the end of the trough when the monolayers reached the condensed phase. Before the addition of the AgNO3 solution, the same volume (10 mL) of pure water in the trough was removed to keep the total volume constant before and after the introduction of the AgNO3 solution. It was necessary to keep the trough volume constant to prevent the ellipsometry beam from being misaligned. We observed the pressure to decrease after Ag ions completed the complex formation with the three amphiphillic compounds, although the kinetics for the pressure drops and the final pressure were different. In the mean time, the ellipsometric phase changes increased until they reached constant values with all three monolayers of the silver complexes. This indicates the formation of Ag complexes caused a physical change in the monolayers, which resulted in the pressure drop. The ellipsometric phase changes as a function of time are plotted in Figure 4 for the three compounds. The addition of the AgNO3 solution is marked by arrows in Figure 4. A distinct change owing to the silver thiolate formation can be easily noticed. The complete formation of the complexes is confirmed by the constant ellipsometry signals at the end of the experiments. It shows that the C18S-Ag complex forms faster than the other two complexes. The ellipsometric signals for the C18S-Ag monolayers are stable and constant for at least 3 days although the surface pressure cannot directly indicate the formation of the complexes. After 3 days, the C18SAg monolayer was compressed and the ellipsometry signal was monitored. Figure 5 shows that the ellipsometry signals started to fluctuate as soon as the monolayers were compressed. The surface pressure remained zero (12) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960.
Silver n-Octadecanethiolate Langmuir Monolayers
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Figure 6. A schematic representation of the formation of silver complexes with octadecanethiol, octadecanoic acid, and octadecanol on a pure water subphase. Figure 4. Ellipsometric phase changes during the formation of silver complexes of octadecanethiol, octadecanoic acid, and octadecanol on a pure water subphase.
Figure 5. Surface pressure and ellipsometric phase changes during the compression of silver n-octadecanethiolates.
until the domains of the monolayers were in contact resulting in a pressure increase as seen in Figure 5. However, the ellipsometry signals never exceeded the constant value (0.07 rad) that was observed before the compression. This indicates that there exist domains of fully compressed silver thiolate monolayers in the zero pressure region, the size of which were larger than the ellipsometry beam size. In this case, the ellipsometry beam was reflected from the fully compressed monolayers. However, the barrier started to move the domains and disturb the positions of the monolayers. This caused the ellipsometry beam to be reflected from the partially covered monolayers producing the signal fluctuation as noticed in Figure 5. We found that C17COO-Ag complex reached equilibrium approximately 150 min after the introduction of Ag+ ions. The C18O-Ag monolayers were stabilized in approximately 400 min. However, the final δ∆ for the three silver complexes did not reach the same value. The ellipsometric signal changes for the three Ag complexes are different when they reach equilibria as noticed in Figure 4. The ellipsometric changes (δ∆) are approximately 0.055, 0.040, and 0.026 rad for C18S-Ag, C17COO-Ag, and C18O-Ag, respectively. It can be shown from the optical simulation (see following paragraph) that a higher Ag content causes larger changes in the ellipsometric signal. This simulation suggests that the monolayers of silver thiolates show higher metallic behavior than the other two. The higher metallic behavior can be interpreted in two different ways. First, the differences can be understood by the characteristics of the chemical bonding between silver and the functional groups (S-, O-, COO-) resulting in the marked differences in the electronic properties. Second, the equilibrium constant for complex formation may play a major role in the ellipsometric observation. Under this assumption,
Figure 7. An optical model for the ellipsometric simulation.
we can present schematics of the three monolayers at equilibrium as seen in Figure 6, which suggests that the population of the silver complexes decreases in the order S-Ag, COO-Ag, and O-Ag at the interface. This agrees well with the magnitude of the pressure drops after the formation of the silver complexes which is in the order S-Ag, COO-Ag, and O-Ag. A large change in the pressure drop means the formation of a large amount of silver complex. Subsequently, this allows us to predict that the relative strength of the formation constants is in the order S-Ag, COO-Ag, and O-Ag, which agrees with the formation constants reported in the literature.13 This observation leads us to propose that the second interpretation is the more reasonable explanation for the pressure drop and the ellipsometric observation during the formation of silver complexes. The ellipsometric phase change of C18S-Ag monolayers was simulated using bulk optical constants of silver and quartz to account for the silver ions and the alkyl chains in the silver octadecanethiolate. The ellipsometric simulations at 632.8 nm were performed according to the model in Figure 7. The simulation shows that the ellipsometric phase change before and after the introduction of an ionic diameter (2.52 Å)14 of Ag layers is 0.14 rad, which is far greater than the experimental observation. Note that the (13) (a) Kuehn, C. G.; Iseid, S. S. Prog. Inorg. Chem. 1980, 27, 153. (b) Sillen, L. G.; Martell, A. E. In Stability Constants of Metal-Ion Complexes; The Chemical Society: London, 1964. (14) In Lange’s Handbook of Chemistry, 12th ed.; Dean, J. A., Ed.; McGraw Hill Press: New York, NY, 1978.
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Figure 9. A Brewster angle microscopic image of octadecanoic acid monolayers on a pure water subphase at 30 Å2/molecule. The width of the image corresponds to 2 mm.
Figure 8. A Brewster angle microscopic image of (a) noctadecanethiol monolayers on a pure water subphase and (b) octadecanethiol monolayers on a pure water subphase at 30 Å2/molecule. The width of the images corresponds to 2 mm.
phase change owing to the silver complex formation with thiol is approximately 0.055 rad. This indicates that due to the large decrease in the metallic property of the metal ions the optical constant of self-assembled alkanethiol monolayers cannot be simply treated as that of quartz to find the thickness of the monolayers. Subsequently, the simple assumption of the optical property of the alkanethiol monolayers as quartz, as reported in literature, could lead to positive deviations of the thicknesses determined by ellipsometry.1b Thus, we suggest the use of a two-layer optical model to account for both the hydrocarbon layers and the metal ionic head group for understanding the structure of self-assembled monolayers on metals. In Situ Real-Time Brewster Angle Microscopy Measurements. The BAM images for the formation of C18SH monolayers are markedly different from those of C18OH and C17COOH. Figure 8 shows the BAM images for C18SH at 30 Å2/molecule. The images were obtained at the same surface pressure, but different locations. They are characterized by uniform monolayers (Figure 8a) and large empty spaces (Figure 8b) as proposed in Figure 2. Figure 8a indicates that the intermolecular interaction of C18SH is so strong that it forms domains of monolayers without being compressed. This observation agrees well with the ellipsometric observation. As noted above, the ellipsometry phase changes during the compression of C18SH molecules fluctuated from 0 to 0.017 rad and finally the signals were stabilized showing 0.017 rad after the formation of fully compressed monolayers. Thus, it is clear
Figure 10. A Brewster angle microscopic image of octadecanoic acid monolayers on a pure water subphase at 20 Å2/molecule. The width of the image corresponds to 2 mm.
Figure 11. A Brewster angle microscopic image of the formation of silver octadecanethiolate monolayers on a pure water subphase at 18 Å2/octadecanethiol molecule determined before addition of AgNO3. The width of the image corresponds to 2 mm.
that the ellipsometry phase change of 0.017 rad corresponds to the formation of the fully compressed monolayer. Note that the 0.017 rad ellipsometry change was also observed in the region of the gas phase as seen in Figure 3. This tells us that the optical property of the monolayers shown in Figure 8a should be very close to that of the fully
Silver n-Octadecanethiolate Langmuir Monolayers
Figure 12. A Brewster angle microscopic image of silver octadecanoate monolayers on a pure water subphase at 18 Å2/ octadecanoic acid molecule determined before addition of AgNO3. The width of the image corresponds to 2 mm.
compressed monolayers indicating the strong intermolecular interactions of C18SH molecules. The BAM images for the formation of C17COOH and C18OH resemble each other. Figure 9 shows the BAM image for C17COOH at 30 Å2/molecule. Unlike the case of C18SH, the monolayers are well connected to each other and circular empty spaces are formed resulting in a net shape. As the surface pressure was increased, the empty circular spaces were gradually filled up, finally reaching fully compressed monolayers as seen in Figure 10. This observation also agrees with the gradual increase in the ellipsometry phase change during the compression. A AgNO3 solution was injected from the end of the Langmuir trough to make Ag ions slowly diffuse to the fully compressed monolayers by keeping the position of the barrier fixed. Figure 11 shows the change in the BAM image of the C18SH monolayers. The domain is separated by the introduction of a sharp crack along the line caused by the decrease in the area of the monolayer after the
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formation of the C18S-Ag complex. This observation accounts for the abrupt decrease in the surface pressure as soon as C18S-Ag forms. However, the BAM images of the formation of C17COOAg are completely different from that of C18S-Ag. The complex formation was also noticed by the separation of grains, but the separation starts by the introduction of circular empty spaces as seen in Figure 12, which was obtained approximately 30 min after the introduction of AgNO3. However, the empty spaces were larger than expected, which might be increased owing to the solubility of the C17COO-Ag salt. The BAM images for the formation of the C18O-Ag do not show much change before and after the complex formation, which clearly indicates that the complex formation constant is relatively small resulting in a smaller C18O-Ag population at the monolayer. In conclusion, the ellipsometric phase changes and BAM images indicate that the formation constants of silver complexes with the amphiphilic compounds are responsible for the experimental observations involving silver ions and functional groups. Furthermore, it is understood why alkanethiols do not form homogeneous fully compressed monolayers at the air-water interface. The structure of alkanethiol monolayers at the air-water interface was proposed with the measurement of π-A diagrams and proved by in situ real-time ellipsometry and BAM. In addition, it is noted, because of different optical properties involving the metal and metal ion, that the optical property of n-octadecanethiols on metals cannot be simply treated as that of alkane layers when using an ellipsometer to obtain the correct thickness. Acknowledgment. Y.T.K. gratefully acknowledges financial support for this work from a start-up fund and the School of Mines and Energy Development grant by the University of Alabama. D.B.W. thanks those responsible for a fellowship from the Alabama Space Grant Consortium, NASA Training Grant NGT-40010. LA950568Z
