In Situ AFM Studies on Self-Assembled Monolayers of Adsorbed

trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium ..... by half or more.65,66 Thus, it is probable that adsorbed C18TAC...
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Langmuir 2007, 23, 12966-12972

In Situ AFM Studies on Self-Assembled Monolayers of Adsorbed Surfactant Molecules on Well-Defined H-Terminated Si(111) Surfaces in Aqueous Solutions Akihito Imanishi,*,†,‡ Masato Suzuki,† and Yoshihiro Nakato†,‡ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, and Core Research for EVolutional Science and Technology (CREST), Japan Science and Technology Agency, 4-1-8 Honmachi, Kawaguchi, Saitama 332-0012, Japan ReceiVed May 24, 2007. In Final Form: August 15, 2007 The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0 × 10-4 M C18TAC, whereas a slightly roughened layer was formed for 1.0 × 10-4 M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0 × 10-4 M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.

Introduction The adsorption of surfactant molecules on solid surfaces in aqueous solutions has attracted much attention in the field of colloid and surface chemistry because of its interesting physical and chemical properties. Especially, the formation of selfassembled monolayers (SAMs) of adsorbed molecules is of much interest because they have a dense and stable structure on an atomic level and have potential applications to corrosion prevention, wear protection, etc. In addition, a biomimetic or biocompatible nature of the SAMs will allow us to apply them to chemical and biochemical sensors. Adsorbed surfactants at solid/liquid interfaces have been widely studied. However, most of them focused on the formation of the cylindrical or spherical micelle structure. In addition, in most studies, cleaved surfaces of layered materials such as mica1-8 * Author to whom correspondence should be addressed. Fax: +81-66850-6237, e-mail: [email protected]. † Osaka University. ‡ Japan Science and Technology Agency (JST). (3) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (4) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892-898. (5) Li, B.; Fujii, M.; Fukada, K.; Kato, T.; Seimiya, T. Thin Solid Films 1998, 312, 20-23. (6) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (7) Zou, B.; Qiu, D.; Hou, X.; Wu, L.; Zhang, X.; Chi, L.; Fuchs, H. Langmuir 2002, 18, 8006-8009. (8) Wall, J. F.; Zukoski, C. F. Langmuir 1999, 15, 7432-7437. (9) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506-6512. (10) Fujii, M.; Li, B.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 2001, 17, 1138-1142.

and graphite2,9-13 were used as substrates, in addition to silica surfaces.1,2,5,13 Recently, it has been established that successive etching with 5% hydrogen fluoride (HF) and 40% ammonium fluoride (NH4F) produces a nearly atomically flat and Hterminated Si(111) [hereafter abbreviated as H-Si(111)] surface with a well-defined step and terrace structure.14-29 The H-Si(11) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 42234228. (12) Fleming, B. D.; Wanless, E. J. Microsc. Microanal. 2000, 6, 104-112. (13) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (14) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (15) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (16) Takahagi, T.; Ishitani, A.; Kuroda, H.; Nagasawa, Y. J. Appl. Phys. 1991, 69, 803-807. (17) Yablonovitch, E.; Allara, D. L.; Chang, C. C.; Gimitter, T.; Bright, T. B. Phys. ReV. Lett. 1986, 57, 249-252. (18) Grundner, M.; Schulz, R. AIP Conf. Proc. 1988, 167, 329. (19) Graef, D.; Grundner, M.; Schulz, R. J. Vac. Sci. Technol. 1989, A7, 808813. (20) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656-658. (21) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897-2909. (22) Kim, Y.; Lieber, C. M. J. Am. Chem. Soc. 1991, 113, 2333-2335. (23) Hessel, H. E.; Feltz, A.; Reiter, M.; Memmert, U.; Behm, R. J. Chem. Phys. Lett. 1991, 186, 275-280. (24) Itaya, K.; Sugawara, R.; Morita, Y.; Tokumoto, H. Appl. Phys. Lett. 1992, 60, 2534-2536. (25) Fukidome, H.; Matsumura, M. Surf. Sci. 2000, 463, L649-L653. (26) Bensliman, F.; Aggour, M.; Ennaoui, A.; Matsumura, M. Jpn. J. Appl. Phys. 2000, 39, L1206-L1208. (27) Bensliman, F.; Fukuda, A.; Mizuta, N.; Matsumura, M. J. Electrochem. Soc. 2003, 150, G527-G531. (28) Allongue, P.; Henry de Villeneuve, C.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochim. Acta 2000, 45, 4591-4598. (29) Watanabe, S.; Sugita, Y. Surf. Sci. 1995, 327, 1-8.

10.1021/la7015275 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007

In Situ AFM Studies on Self-Assembled Monolayers

(111) surface is stable in aqueous solutions and has a hydrophobic property. Thus, this surface is quite appropriate as a substrate for the study of adsorption behavior and structures. In addition, this surface has a merit in that it can be modified with chemically attached alkyl chains with or without functional groups to control the morphological and hydrophobic/hydrophilic properties. In previous work, we investigated the adsorption of cationic surfactants on atomically flat H-Si(111) surfaces in aqueous solutions by using in situ atomic force microscopic (AFM) inspection.30 All surfactants used formed high-density adsorption monolayers on the H-Si(111) surface, most probably with the alkyl chains assembled nearly normal to the Si surface. On the other hand, the in situ FTIR observation revealed that the monolayer adsorption was nearly completed about 60 min after the start of the H-Si(111) immersion in the solution, and after then the adsorbed molecules changed their arrangement into a more ordered structure.31 Interestingly, the Si-H peak in the FTIR spectrum remained unchanged with time in an aqueous surfactant solution, in contrast to the case of immersion in pure water, indicating that the adsorbed surfactant can protect the H-Si(111) surface from oxidation. The above results indicated that the H-Si(111) surface covered with the surfactant monolayer is quite an interesting approach to realize the third generation electrode on which not only the surface structure but also its function is flexible. We can point out the following merits. First, the Si substrate is electrically conductive and allows us to measure the electrical conductive properties of the surface adsorption layer. Second, the adsorption monolayer of surfactants is flexible in packing structure because of the absence of direct covalent bonding of the surfactant molecules to the H-Si(111) surface and thus allows us to insert appropriate functional molecules such as enzymes into the layer in order to improve the functionality of it. This will offer a good model of bimolecular lipid membranes in living bodies. Many researchers reported that surface alkylation through formation of Si-C covalent bonds32-38 is effective to protect the Si surface from the surface oxidation, but in this case, the Si-H bonds are changed into Si-C bonds and alkyl groups are fixed at certain surface sites. In various physicochemical properties of the adsorption layer of surfactant molecules, the order of the arrangement and packing of the molecules plays the key role because it determines the mobility of electrons, ions, and molecules in the layer. Thus, it is very important to investigate the molecular arrangement in the surfactant layer and factors affecting it. Unfortunately, there is a difficulty in investigating strict structures of the surfactant molecules adsorbed at the solid/liquid interface, compared with that at the air or vacuum/solid interfaces. We adopted the in situ FTIR and AFM to investigate the adsorption structures of (30) Garcia, S. P.; Bao, H.; Manimaran, M.; Hines, M. A. J. Phys. Chem. B 2002, 106, 8258-8264. (31) Houbertz, R.; Memmert, U.; Behm, R. J. Surf. Sci. 1998, 396, 198-211. (32) Imanishi, A.; Suzuki, M.; Nakato, Y. Trans. Mater. Res. Soc. Jpn. 2004, 29, 3223-3225. (33) Imanishi, A.; Omoda, R.; Nakato, Y. Langmuir 2006, 22, 1706-1710. (34) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404-5412. (35) Nakato, K.; Takabayashi, S.; Imanishi, A.; Murakoshi, K.; Nakato, Y. Sol. Energy Mater. Sol. Cells 2004, 83, 323-330. (36) Wallart, X.; Henry de Villeneuve, C.; Allongue, P. J. Am. Chem. Soc. 2005, 127, 7871-7878. (37) Faucheux, A.; Gouget-Laemmel, A. C.; Henry de Villeneuve, C.; Boukherroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J. Langmuir 2006, 22, 153-162. (38) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305-307. (39) Cheng, J.; Robinson, D. B.; Cicero, R. L.; Eberspacher, T.; Barrelet, C. J.; Chidsey, C. E. D. J. Phys. Chem. B 2001, 105, 10900-10904. (40) Barrelet, C. J.; Robinson, D. B.; Cheng, J.; Hunt, T. P.; Quate, C. F.; Chidsey, C. E. D. Langmuir 2001, 17, 3460-3465.

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Figure 1. Structures of surfactant molecules used in this study. Cationic surfactants: octyltrimethylammonium chloride (CH3(CH2)8N(CH3)3Cl), abbreviated as C8TAC (a), dodecyltrimethylammonium chloride (CH3(CH2)11N(CH3)3Cl), abbreviated as C12TAC (b), and octadecyltrimethylammonium chloride (CH3(CH2)17N(CH3)3Cl), abbreviated as C18TAC (c). Anionic surfactants; sodium dodecyl sulfate (CH3(CH2)11OSO3Na), abbreviated as SDS (d), and sodium tetradecyl sulfate (CH3(CH2)13OSO3Na), abbreviated as STS (e).

surfactant layers in previous reports30,31 and showed that both methods were useful for investigating the surfactant structures in aqueous solutions. In particular, the in situ AFM measurement was a powerful tool to observe the local structure and thickness of the surfactant layer at the liquid/solid interfaces.30 Thus, in the present paper, we report the structure of adsorption layers of cationic and anionic surfactants with various alkyl chains, formed on the atomically flat H-Si(111) surfaces by using the in situ AFM. The effects of addition of alcohols and salts to the solution were also investigated. Experimental Section Single-crystal wafers of n-type Si(111) of a resistivity of 10-15 Ω·cm with a vicinal surface tilting in the 〈112h〉 direction at about 0.36 ( 0.1° were obtained from Osaka Tokushu Gokin Co. Ltd., Japan. They were cut into a square shape with a dimension of 10 × 10 mm2 and then cleaned by the conventional RCA cleaning method,39 consisting of successive immersion in 98% H2SO4 + 30% H2O2 (4:1 in volume) for 10 min, 5% HF for 5 min, 25% NH4OH + 30% H2O2 + H2O (1:1:5 in volume) for 10 min, and 36% HCl + 30% H2O2 + H2O (1:1:6 in volume) for 10 min. The wafers were then etched with 5% HF for 5 min and 40% NH4F for 15 min to obtain atomically flat, H-terminated Si(111) surfaces.40-44 Cationic surfactants used were octyltrimethylammonium chloride (CH3(CH2)8N(CH3)3Cl), hereafter abbreviated as C8TAC, dodecyltrimethylammonium chloride (CH3(CH2)11N(CH3)3Cl), hereafter abbreviated as C12TAC, and octadecyltrimethylammonium chloride (CH3(CH2)17N(CH3)3Cl), abbreviated as C18TAC. Anionic surfactant used were sodium dodecyl sulfate (CH3(CH2)11OSO3Na), abbreviated as SDS, and sodium tetradecyl sulfate (CH3(CH2)13OSO3Na), abbreviated as STS. They were obtained from Kao Co. Ltd. The structures of them are shown in Figure 1. In situ AFM observation of surface morphology of the adsorption layer was made with an atomic force microscope (NanoScope IIIa, Digital Instruments) at room temperature. A sharpened silicon nitride tip (Digital Instruments) and a cantilever having a spring constant 0.06 N/m, coupled with a 10 µm scanner, were used. All AFM images were taken in a tapping mode with a tapping frequency of (41) Kern, W.; Puotinen, D. A. RCA ReV. 1970, 31, 187-206. (42) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656-658. (43) Zhou, X. W.; Ishida, M.; Imanishi, A.; Nakato, Y. Electrochim. Acta 2000, 45, 4655-4662. (44) Zhou, X. W.; Ishida, M.; Imanishi, A.; Nakato, Y. J. Phys. Chem. B 2001, 105, 156-163. (45) Imanishi, A.; Ishida, M.; Zhou, X.; Nakato, Y. Jpn. J. Appl. Phys. 2000, 39, 4355-4358. (46) Imanishi, A.; Hayashi, T.; Nakato, Y. Langmuir 2004, 20, 4604-4608.

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Figure 2. In situ AFM image of a vicinal H-Si(111) surface tilting in the 〈11-2〉 direction at an angle of 0.36 ( 0.1°, in contact with pure water. about 8 kHz at a scan rate of 1-2 Hz. The force between the tip and sample was chosen to be 0.026 nN, small enough to observe adsorbed surfactant layers without disturbing their structures.30 We confirmed that for adsorbed surfactant layers with short alkyl chains (C8- and C12TAC), the tip-sample force larger than 0.25 nN caused a morphological change in the adsorption layer in every sweep of the tip, whereas the force of 0.026 nN or lower did not induce any change in the order and the morphology of the layer even after 10 time sweeps. An in situ AFM cell used is schematically shown in Figure S1 of Supporting Information. Adsorbed layers of surfactants were formed on the Si surface in the AFM cell, by introducing a surfactant solution into the cell through in- and out-ports. Before the surfactant solution was introduced, pure water was first injected into the cell, and an AFM image was obtained for the H-Si(111) surface with no adsorption layer. When a surfactant solution was introduced, it takes a few hours to obtain a stable, well-packed adsorption monolayer, as mentioned in the previous section. Thus, all the results shown here were taken after the change of the morphology (or arrangement) was finished (actually 120 min after the start of flow of a surfactant solution in all cases). Upon investigations of the effect of additives (such as salts and alcohols), they were added to the surfactant solution before it was introduced into the in situ AFM cell. In the present experiments, we did not remove dissolved oxygen in a surfactant solution, in the same way as in our previous work.30,31 This was partly because the oxidation of the Si surface by dissolved oxygen was not so serious, as explained later, and partly because it is known27 that an oxygen-free aqueous solution even in the absence of F- ions causes effective Si-surface etching, which may strongly affect the morphology of adsorption layers. In all experiments, special-grade chemicals were used without further purification. Pure water was obtained by purifying deionized water with a Milli-Q water purification system.

Results Figure 2 shows an in situ AFM image of an HF- and NH4Fetched Si(111) surface in pure water after immersion in it for 60 min. Parallel lines seen in the image represent a step structure of the Si(111) surface, indicating that a nearly atomically flat and H-terminated Si(111) surface was produced by the etching. Bright spots seen in terraces of Figure 2 are most probably due to contaminations. The formation of atomically flat H-Si(111) surfaces by the HF and NH4F etching was also confirmed by observation of a much clearer step and terrace structure in air with AFM30,31 and observation of a sharp strong FTIR peak at 2083.6 cm-1, assigned to the stretching mode of Si-H bonds at the (111) terrace, in air, together with some weak peaks assigned to vibration modes of Si-H and SiH2 bonds at steps.41,44

Figure 3. In situ AFM images of an H-Si(111) surface in contact with an aqueous solution of 1.0 × 10-6 M C18TAC (a), 1.0 × 10-4 M C18TAC (b), 1.0 × 10-4 M C8TAC (c), and C12TAC (d) for 120 min.

Figure 3 (a) and (b) shows in situ AFM images of the H-Si(111) surfaces in contact with aqueous solutions of (a) 1.0 × 10-6 M and (b) 1.0 × 10-4 M C18TAC for 120 min. For 1.0 × 10-6 M C18TAC (a), the surfactant adsorption was not uniform. The average thickness of the adsorption layer, estimated from the depth of pits accidentally formed, was 2.7-3.1 nm and comparable to the alkyl chain length of C18TAC, suggesting that the alkyl chains of the adsorbed surfactant assembled normal to the Si surface, with the cationic headgroup pointed away from the surface. For 1.0 × 10-4 M C18TAC (b), on the contrary, a very even adsorption layer was obtained. We can see that the topography of the surfactant covered surface in b resembles that of the naked H-Si(111) surface in Figure 2. It is evident that vertical straight stripes observed in b originate from the step structure of the underlying Si surface. In addition, the thickness of the surfactant layer, also estimated from the depth of pits accidentally formed, was 3.0 nm, nearly the same as the alkyl chain length of C18TAC. These results lead to a conclusion that a fairly ideal SAM was formed on the Si surface in this case. It may be noted that the above-mentioned “accidentally formed pits”, used to estimate the thickness of the adsorption layer, are not artificially produced with an AFM tip. The pits were observed on all adsorption layers inspected, though their density was small, and kept unchanged with time, contrary to the case of artificially formed tips, which disappeared about 120 min after the formation by the readsorption of surfactant molecules. It is thus expected that the “pits” were formed in areas of oxidized Si surface, which no longer had a hydrophobic property to induce surfactant adsorption. Inversely speaking, the formation of a very even adsorption layer in a wide area of the Si surface such as shown in Figure 3b indicates that almost no oxidation proceeds in the major part of the Si surface. This conclusion is in agreement with a reported result31 that the Si-H peak in the FTIR spectra remained unchanged with time in an aqueous surfactant solution. Figure 3c and 3d shows in situ AFM images of the H-Si(111) surfaces in contact with aqueous solutions of (c) C8TAC and (d) C12TAC of 1.0 × 10-4 M for 120 min. The arrangement of

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Figure 4. In situ AFM images of an H-Si(111) surface in contact with an aqueous solution containing a surfactant and an alcohol for 120 min. As indicated within the figure, the images with the same surfactant molecule are displayed in a column, whereas those with the same alcohol molecule are exhibited in a row (except the lowest one).

Figure 5. In situ AFM images of the H-Si(111) surface in contact with 1.0 × 10-4 M SDS in the absence (a) and the presence of 4.0 × 10-4 M 2-propanol (b) and 2.3 × 10-4 M 1-octanol (c), for 120 min.

adsorbed surfactant molecules in these cases was worse than that of the case of C18TAC of the same concentration (Figure 3b). This result indicates that the order of the arrangement increases with increasing of the length of the alkyl chain. The effect of addition of alcohols with alkyl chains to the surfactant solutions is an interesting subject. Figure 4 shows the in situ AFM images of the H-Si(111) surfaces in contact with 1.0 × 10-4 M C8TAC, C12TAC, and C18TAC for 120 min, to which 1.0 × 10-4 M 2-propanol, 1.0 × 10-4 M 1-propanol, 7.7 × 10-3 M, and 2.5 × 10-3 M 1-octanol were added. As indicated within the figure, the images with the same surfactant molecule are displayed in a column, whereas those with the same alcohol molecule are exhibited in a row (except the lowest one). In the case of the addition of 1.0 × 10-4 M 2-propanol (upper row), very even adsorption layers were obtained for C8 and C12TAC, whereas the rough surface was observed for C18TAC. In other words, the slightly roughened layers of C8TAC and C12TAC in

the absence of alcohol, shown in Figure 3c and 3d, were changed into a well-ordered arrangement mode by the addition of 2-propanol, but a very even layer of C18TAC in the absence of alcohol was roughened and granulated by the addition of 2-propanol. The similar results were obtained in the case of the addition of 1-propnol (d and e) and 1-octanol (f and g). Figure 5 shows the in situ AFM images of the H-Si(111) surface in contact with 1.0 × 10-4 M SDS in the absence (a) and the presence of (b) 4.0 × 10-4 M 2-propanol and (c) 2.3 × 10-4 M 1-octanol, for 120 min. Note that SDS is an anionic surfactant which differs from the cationic surfactants thus far mentioned. In the case of an anionic surfactant, the adsorbed molecules also changed their arrangement into a well-ordered mode by the addition of alcohols, regardless of the chain length of them. The effect of addition of salts to the surfactant solutions is also an interesting subject because it changes the ionic strength of the solution and thus critical micelle concentration. Figure 6 shows

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Figure 6. In situ AFM images of the H-Si(111) surface in contact with 1.0 × 10-4 M C12TAC in the absence (a) and the presence of 0.3 M KCl (b) and 0.5 M KCl (c), for 120 min.

Figure 7. In situ AFM images of the H-Si(111) surface in contact with solutions of anionic surfactants with or without KCl for 120 min: (a) 1.0 × 10-4 M SDS, (b) 1.0 × 10-4 M SDS + 0.3 M KCl, (c) 1.0 × 10-4 M STS, and (d) 1.0 × 10-4 M STS + 0.3 M KCl.

Figure 8. (a) In situ AFM images of the H-Si(111) surface in contact with 1 × 10-4 M SDS + 1 × 10-4 M C12TAC for 120 min. (b) A schematic illustration of adsorbed surfactant molecules.

the in situ AFM images of the H-Si(111) surface in contact with 1.0 × 10-4 M C12TAC in the absence (a) and the presence of (b) 0.27 M KCl and (c) 0.5 M KCl, for 120 min. We can see that a very even surfactant layer was formed by the addition of the KCl. In Figure 7, the in situ AFM images of the H-Si(111) surface in contact with the solutions of anionic surfactant molecules, SDS (a) and STS (c), for 120 min are shown, together with those c and d after the addition of 0.3 M KCl, respectively. The ideally flat layers were formed in the solutions of anionic surfactants containing KCl, though only the rough layers were obtained in the absence of KCl. These results indicate that the addition of KCl to the surfactant solution makes the order of the arrangement of the surfactant molecules much better. Figure 8a shows the in situ AFM images of the H-Si(111) surface in contact with an aqueous solution containing both the cationic and anionic surfactants with the same alkyl chain length, 1 × 10-4 M SDS + 1 × 10-4 M C12TAC, for 120 min. The flat, ideal surfactant layer was formed on the H-Si(111) surface, though such an ordered surfactant layer was not observed for 1 × 10-4 M SDS alone or 1 × 10-4 M C12TAC alone (see Figure 5a and Figure 3d, respectively). This indicates that the ordered adsorption layer was formed by the stabilization through electrostatic interaction between cationic and anionic surfactant molecules.

The experimental results in the present work (Figure 3) have shown that the self-assembled monolayer (SAM) of adsorbed surfactant molecules forms a well-arranged structure of the adsorbed molecules when the layer is in equilibrium with an aqueous solution of a surfactant with a sufficiently high concentration (1.0 × 10-4 M). On the other hand, the SAM has a randomly oriented structure when it is in equilibrium with an aqueous solution of a surfactant with a low concentration (1.0 × 10-6 M). We previously reported the same behavior for the adsorption layer of sodium di-2-ethylhexyl sulfosuccinate on the H-Si(111) surface, which was in equilibrium with an aqueous solution of the surfactant.31 A similar behavior was also reported for the SAMs of alkanethiols, chemisorbed on Au-metal substrates.45-53 Many

Discussion

(47) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (48) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4305. (49) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514. (50) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 10321037. (51) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 13851387. (52) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128-1132. (53) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307-2312.

In Situ AFM Studies on Self-Assembled Monolayers

researchers reported that the S-S average distance, in other words, the density of adsorbed alkanethiol molecules, had a close relation with the tilt angle of the alkyl chains. The alkyl chains of alkanethiols were nearly parallel to the substrate surface when the coverage (density) was low, whereas the chains became normal to the substrate surface when the coverage got high (or when the adsorbed molecules became densely packed), owing to mutual interaction among the alkyl chains.54-56 The similarity in the behavior between the present work and reported work on alkanethiols gives strong support to the above-mentioned structure for the adsorbed surfactant layers in the present work. It is well-known that the molecular arrangement of selfassembled monolayers (SAM) on the Au substrate becomes better with increasing of the alkyl chain length, and this is explained by the fact that the intermolecular interaction between alkyl chains increases with the increasing of the chain length. In the present work, the result of Figure 3b-d shows the same tendency as the above-reported work and is explained by the same reasoning. Considering the fact that the adsorbed surfactant molecules are not anchored to the Si substrate with chemical bonds in the present work, unlike the case of the SAM on the Au substrate, the surfactant molecules are expected to move more freely, resulting in that the structure of the adsorption layer is more sensitive to the intermolecular interaction. The structural change of the surfactant micelle on the solid surface, induced by the addition of alcohols, has been studied by some researchers.57,58 Wanless et al. investigated the adsorption of SDS on a graphite surface in aqueous solutions. They reported that in the absence of alcohols, the surfactants were associated into the form of long hemicylindrical aggregates arranged in parallel to each other, resulting in a hemimicelle on the graphite surface. On the other hand, when 0.05-0.5 mM dodecanol was added to the solution, the curvature of the hemimicelles decreased and the area of flat surfaces increased. Wall et al. investigated the alcohol-induced structural transformation of cetyltrimethylammonium bromide (CTAB) aggregates adsorbed on mica. They reported that added alcohol molecules were inserted into the surface layer of the micelle and induced the structural transition of the micelle. Experiments in the present work also show a clear effect of the addition of alcohols to the surfactant solution on the arrangement structure in the adsorption surfactant layer (Figures 4 and 5), but a mechanism for the effect is not sufficiently clear at present. In the case of surfactants with short alkyl chains (C8TAC and C12TAC), the order of the arrangement of the surfactant molecules was improved by the addition of alcohols (compare Figures 3 and 4). This can tentatively be explained by the increase in the hydrophobic interaction between alkyl chains of surfactant molecules by the insertion of alkyl chains of alcohol molecules, which might result in better-packed alkyl layers.58-60 In addition, electrostatic repulsion between ionic head groups of surfactants might also be decreased owing to the enlargement of the distance between surfactant molecules by the insertion of (54) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21-25. (55) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (56) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147-1152. (57) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (58) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultans Bureau: New York, 1959; pp 177-217. (59) Wall, J. F.; Zukoski, C. F. Langmuir 1999, 15, 7432-7437. (60) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 42234228. (61) Kim, E.; Shah, D. O. J. Phys. Chem. B 2003, 107, 8689-8693. (62) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208-223.

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alcohol molecules, leading to the stabilization of the well ordered adsorption layer. In the present study, a difference in the alkyl chain length of alcohols did not cause any significant difference in the structure of the surfactant adsorption layer (i.e., both 1-propanol and 1-octanol changed the C12TAC adsorption layer into the well-ordered mode), though it was reported that the shape of the micelle was strongly affected by the alkyl chain length of added alcohols.58 Besides, some researchers reported that alcohols with short chain lengths such as 1-propanol were unable to affect the structure of the adsorbed surfactant layer because a poor hydrophobic property of such alcohols made it difficult for them to be inserted in the surfactant layer,59,60 contrary to the present result in which 1-propanol really affected the structure of the C12TAC adsorption layer. Even 2-propanol with a branched alkyl chain contributed to the formation of a wellordered C12TAC layer in the present work. In the case of a surfactant with a long alkyl chain (C18TAC), the well-ordered adsorption layer in the absence of alcohols was roughened and granulated by the addition of them (Figures 3 and 4). There may be two possible reasons to explain this phenomenon. First, it is well-known that the critical micelle concentration (CMC) of a surfactant aqueous solution was decreased by the addition of alcohols.60,61 In the present case, the concentration of C18TAC was 1.0 × 10-4 M, which is close to the CMC of C18TAC, lying in the order of magnitude of 10-4 M.62-64 In addition, the surface CMC is usually lower than the bulk CMC by half or more.65,66 Thus, it is probable that adsorbed C18TAC surfactant molecules did not form a layer but a micelle in the presence of alcohols. In fact, the diameter of particular aggregates of adsorbed surfactant molecules in Figure 4c,e,g was about 3 nm in height and 5-10 nm in width, which were nearly the same as the size expected for the (hemi)micelle of C18TAC, 3 nm in height and 7 nm in width. Incidentally, the CMC of C8 and C12TAC is of the order of 10-1 and 10-2 M, respectively, which are much higher than the surfactant concentration used in the present work even under the consideration of the above-mentioned decrease in the CMC at the surface. Second, when a high concentration of alcohol is added, it penetrates deeply into the surfactant layer, resulting in the dissolution of the hydrophobic chain of surfactants with alcohols, thus finally leading to the decomposition of the well-ordered C18TAC adsorption layer into smaller aggregates so that the entropy of the adsorbed molecules may increase.57 Wall et al. reported that the transition from the cylindrical to spherical micelle by the addition of alcohols could be explained by the same reasoning.57 The well ordered layer of an anionic surfactant with a short alkyl chain, SDS (C12-surfactant), was also formed by the addition of alcohols, similarly to the case of cationic surfactant layers (C8TAC and C12TAC). This result will also be explained by the same mechanism as argued above for the cationic surfactants. It is well-known that the addition of counterions to the ionic surfactant solution induces the decrease in the CMC,64,67 as is expressed by Corrin-Harkins equation. This is generally explained as follows: The electrostatic repulsion between the head groups was decreased by the addition of the counterions (63) Shirahama, K.; Kashiwabara, T. J. Colloid Interface Sci. 1971, 36, 6570. (64) Ozeki, S.; Ikeda, S. Bull. Chem. Soc. Jpn. 1981, 54, 552-555. (65) Herrman, K. W. J. Phys. Chem. 1962, 66, 295-300. (66) Senoo, M. Colloidal Chemistry; Tokyo Kagaku Dojin: Tokyo, 1995; vol. 2, pp 87-136. (67) Zou, B.; Qiu, D.; Hou, Z.; Wu, L.; Zhang, X.; Chi, L.; Fuchs, H. Langumuir 2002, 18, 8006-8009. (68) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558-8567. (69) Corrin, M. L.; Harkins, W. D. J. Am. Chem. Soc. 1947, 69, 683-688.

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due to the increase of the concentration of counterions around the ionic head groups of surfactant molecules. This enhanced the hydrophobic interaction between the alkyl chains, resulting in the decrease in the CMC. On the other hand, Tanida et al. investigated the density of the Br- ions at the interface between air and an aqueous solution of dodecyltrimethylammonium bromide.68 They reported that the concentration of the Br- ions at the interface strongly influenced the structure of the surfactant layer formed at the interface. In the preset work, the slightly roughened C12TAC layer was changed to the well-ordered layer by the addition of KCl. This can be explained by the stabilization effect of the couterions (Cl- ions), similarly to the explanation in the previous report.68 The local concentration of Cl- ions around the cationic head group of the surfactant is expected to be increased by the addition of KCl, which resulted in the decrease in the electrostatic repulsion between the cationic head groups of C12TAC. This in turn induces the increase of the hydrophobic interaction between the alkyl chains, causing the formation of a well-ordered adsorption layer. The similar effect was observed in the case of adsorption layers of anionic surfactancts (STS and SDS). Although the common counterions to those of the surfactant molecules (Na+ ions) were not added in this case, the added K+ ions can probably stabilize the anionic surfactant layer, in a similar way to Na+ ions. We observed that the well-ordered adsorption layer was formed in a solution of mixed cationic and anionic surfactants, C12TAC and SDS. The micelle formation in such a mixed solution has been studied by many researchers.69-73 They reported that the structure of the micelle was stabilized by the alternative arrangement of the cationic and anionic surfactant molecules. (70) Takiue, T.; Kawagoe, Y.; Muroi, S.; Murakami, R.; Ikeda, N.; Aramoto, M. Langmuir 2003, 19, 10803-10807. (71) Talhout, R.; Engberts, J. B. F. N. Langmuir 1997, 13, 5001-5006. (72) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792-13802. (73) Kameoka, N.; Chorro, M.; Talmon, Y.; Zana, R. Colloids Surf. 1992, 67, 213-222. (74) Scheuing, D. R.; Weers, J. G. Langmuir 1990, 6, 665-671. (75) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380-2384.

Imanishi et al.

Thus, the reason for the formation of the ordered adsorption layer is explained by a similar mechanism to the case of the addition of KCl. Assuming that the cationic and anionic surfactant molecules are arranged alternately on the Si surface (see Figure 8b), the electrostatic repulsion between the head groups were decreased, compared with that of the homogeneous surfactant molecules adjoined with each other. This resulted in the formation of the well-ordered surfactant layer.

Conclusion The formation of the SAMs of adsorbed cationic (C8TAC, C12TAC, and C18TAC) and anionic (STS and SDS) surfactant molecules on atomically flat H-Si(111) surfaces with a hydrophobic property in aqueous solutions was investigated by in situ AFM measurements. The molecular arrangement in the adsorbed surfactant layers was increased with increasing the surfactant concentration in the solution as well as the length of the alkyl chains of surfactants. The addition of alcohols, which are incorporated into the adsorption layer, to the solution, as well as the addition of a salt, KCl, which exerts the counterion effect, to the solution improved the molecular arrangement of the adsorbed surfactant layers. In addition, the well-ordered molecular arrangement in the adsorption layer was formed in a solution of mixed cationic and anionic surfactants, though each surfactant alone did not form the well-ordered arrangement. These results indicated that the control of the electrostatic interaction between the head groups of surfactant molecules and hydrophobic interaction between the alkyl groups of them is of key importance to obtain the well-ordered molecular arrangement in the adsorbed surfactant layers on the H-Si(111) surface. Further detailed studies using high-resolution photoelectron and FTIR spectroscopy are necessary to obtain definite conclusions on the structures of adsorbed surfactant molecules. Supporting Information Available: An in situ AFM cell used is schematically shown in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org. LA7015275