Phase Separation of Ternary Self-Assembled Monolayers into

X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups ...
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Langmuir 2005, 21, 10581-10586

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Phase Separation of Ternary Self-Assembled Monolayers into Hydrophobic 1-Dodecanethiol Domains and Electrostatically Stabilized Hydrophilic Domains Composed of 2-Aminoethanethiol and 2-Mercaptoethanesulfonic Acid on Au(111) Pham Hong Phong, Yosuke Ooi, Daisuke Hobara, Naoya Nishi, Masahiro Yamamoto, and Takashi Kakiuchi* Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto, 615-8510 Japan Received February 19, 2005. In Final Form: August 11, 2005 Ternary self-assembled monolayers (SAM) composed of 2-aminoethanethiol (AET), 2-mercaptoethanesulfonic acid (MES), and 1-dodecanethiol (DDeT) form two types of domains as if it were a two-component SAM: DDeT-rich hydrophobic domains and electrostatically stabilized hydrophilic domains composed of MES and AET on Au(111). MES and AET behave virtually as a single surface-active species. Two distinct reductive desorption peaks in cyclic voltammograms (CV) and binarized images of scanning tunneling microscopy clearly show nanometer scale, yet macroscopically distinguishable, phase separation over a wide range of the mixing ratio of DDeT and MES-AET in the bathing solution. X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups are in the charged states, that is, the sulfonate group and the ammonium group. With decreasing the total concentration of the thiols, the mole fraction of DDeT in the bathing solution at which the surface coverage of MES-AET domains is equal to that of DDeT domains dramatically decreases. This suggests that the adsorption kinetics plays a crucial role in the formation of the domains structure.

Introduction The structure and the surface properties of a multicomponent self-assembled monolayers (SAM) of thiol derivatives on metal surface strongly depend on the intermolecular interaction between the adsorbed thiols. Binary SAMs composed of alkanethiol derivatives with different terminal functional groups and/or alkyl chain lengths can exhibit two-dimensional phase separation on Au(111),1-16 depending on the relative magnitude of the * To whom correspondence should be addressed. Tel: 75-3832489. Fax: 75-383-2490. E-mail: [email protected]. (1) Stranick, S. J.; Parikh, A. N.; Tao, Y. Y.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (2) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 38823893. (3) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558-1566. (4) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511-2518. (5) Sato, Y.; Yamada, R.; Mitzutani, F.; Uosaki, K. Chem. Lett. 1997, 26, 987-988. (6) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. (7) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (8) Hobara, D.; Ueda, K.; Imabayashi, S.; Yamamoto, M.; Kakiuchi, T. Electrochemistry 1999, 67, 1218-1220. (9) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078. (10) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. Langmuir 2000, 16, 9287-9293. (11) Hobara, D.; Kakiuchi, T. Electrochem. Commun. 2001, 3, 154157. (12) Smith, R. K.; Reed, M. S.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119-1122. (13) Lewis, P. A.; Smith, R. K.; Kelly, K. F.; Bumm, L. A.; Reed, M. S.; Clegg, R. S.; Gunderson, J. D.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 10630-10636. (14) Munataka, H.; Kuwabata, S.; Ohko, Y.; Yoneyama, H. J. Electroanal. Chem. 2001, 496, 29-36. (15) Sawaguchi, T.; Sata, Y.; Mizutani, F. J. Electroanal. Chem. 2001, 496, 50-60.

interaction between like neighboring adsorbed thiolates to that between unlike thiolates.17 The phase separation takes place when the attractive interaction between the like molecules is stronger than that between the unlike molecules. Phase-separated binary SAMs have been employed for different purposes: to examine the relationship between the nanometer-scale phase separation and wetting,18 to measure the rate of lateral diffusion of adsorbed alkanethiolates,19 to form nanopores in the SAM,20,21 to selectively deposit metals on specific domains,22,23 to selectively attach oligonucleotides,24 to harbor enzymes on the species domains of the SAM surface,25-27 and to release biological cells from the surface.28 Nanometer scale domains having hydrophilic surface are particularly suitable to accommodate proteins and (16) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109-4115. (17) Kakiuchi, T.; Sato, K.; Iida, M.; Hobara, D.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 7238-7244. (18) Imabayashi, S.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348-2351. (19) Imabayashi, S.; Hobara, D.; Kakiuchi, T. Langmuir 2001, 17, 2560-2563. (20) Nishizawa, N.; Sunagawa, T.; Yoneyama, H. J. Electroanal. Chem. 1997, 436, 213. (21) Oyamatsu, D.; Kanemoto, H.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 2001, 497, 97-105. (22) Kuwabata, S.; Kanemoto, H.; Oyamatsu, D.; Yoneyama, H. Electrochemistry 1999, 67, 1254. (23) Kongkanand, A.; Kuwabata, S. Electrochemistry 2004, 72, 412414. (24) Satjapipat, M.; Sanedrin, R.; Zhou, F. Langmuir 2001, 17, 76377644. (25) Hobara, D.; Uno, Y.; Kakiuchi, T. Phys. Chem. Chem. Phys. 2001, 3, 3437-3441. (26) Hobara, D.; Imabayashi, S.; Kakiuchi, T. Nano Lett. 2002, 2, 1021-1025. (27) D, H.; Uno, Y.; Kakiuchi, T. Bunseki Kagaku 2002, 51, 455460. (28) Jiang, X.; Ferrigno, R.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Chem. 2003, 125, 2366-2367.

10.1021/la050444e CCC: $30.25 © 2005 American Chemical Society Published on Web 09/20/2005

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enzymes without significantly altering their native structures.29 Hydrophilic SAMs so far used are singlecomponent SAMs composed either one of carboxyl-, amino-, sulfo-, hydroxyl-, zwitterionic-, or oligo(oxyethylene)group-terminated at the ω-terminal of alkanethiols.25,26,30-37 In addition, some other aromatic thiols38,39 and cysteine40 are also found to be useful to interface proteins with electrode surface. There is yet another hydrophilic SAMs, that is, the mixture of acid-terminated and ammoniumterminated thiols.35,41 We found recently that 2-mercaptoethanesulfonic acid (MES) and 2-aminoethanethiol (AET) form homogeneously mixed binary SAMs, where MES and AET are negatively and positively charged, respectively. Adsorbed MES and AET are electrostatically stabilized with each other in the SAM over the wide range of the molar ratio of MES and AET in the bathing ethanolic solution for preparing the SAM.42 In phase-separated binary SAMs of alkanethiolate derivatives, the mutual solubility of the two thiolates is usually high.8 For example, the solubility of octadecanethiol in the domain of MES is about 10% molar ratio.8 In other words, the hydrophilic domains contain a considerable amount of hydrophobic molecules. The mutual solubility becomes higher with decreasing the difference between the hydrophilicity of the two thiolate species.11 For certain purposes, hydrophilic domains with little perturbation with hydrophobic thiolate molecules are desirable, e.g., for the immobilization of proteins on the SAM in a particular orientation. In this respect, hydrophilic SAMs composed of MES and AET are promising for preventing the insertion of hydrophobic thiols into the MES-AET domains because of the strong lateral interaction between MES and AET in the SAM. Another interesting point about the MESAET domains is the electrical neutrality of the surface formed.42 Although a negatively charged surface made of carboxylate- or sulfonate-terminals can efficiently immobilize positively charged proteins such as horse-heart cytochrome c,31,43 the strong electrostatic interaction between the SAM surface and the protein orients cyt c not being optimum for facile electron transfer between the protein and the electrode.26,41,43-45 In the present paper, we report the phase separation of ternary SAMs composed (29) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 3464-3473. (30) Hill, H. A. O.; Page, D. J.; Walton, N. J.; Whitford, D. J. Electroanal. Chem. 1985, 187, 315-324. (31) Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 18471849. (32) Wong, L. S.; Vilker, V. L.; Yap, W. T.; Reipa, V. Langmuir 1995, 11, 4818-4822. (33) Cotton, C.; Glidle, A.; Beamson, G.; Cooper, J. M. Langmuir 1998, 14, 5139-5146. (34) Kasmi, A. E.; Wallace, J. M.; Bowden, E. F.; Binel, S. M.; Linderman, R. J. J. Am. Chem. Soc. 1998, 120, 225-226. (35) Holmlin, R. E.; Chen, X.; Chapman, R. G.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 2841-2850. (36) Leopold, M. C.; Bowden, E. F. Langmuir 2002, 18, 2239-2245. (37) Leopold, M. C.; Bownden, E. F. Langmuir 2002, 18, 978-980. (38) Taniguchi, I.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Chem. Soc. Chem. Commun. 1982, 1032-1033. (39) Haladjian, J.; Bianco, P.; Pilard, R. Electrochim. Acta 1983, 28, 1823-1828. (40) Gleria, K. D.; Hill, H. A. O.; Lowe, V. J.; Page, D. J. J. Electroanal. Chem. 1986, 213, 333-338. (41) Chen, X.; R. Ferrigno, J. Y.; Whitesides, G. M. Langmuir 2002, 18, 7009-71015. (42) Ooi, Y.; Hobara, D.; Yamamoto, M.; Kakiuchi, T.; Submitted for publication to Langmuir. (43) Imabayashi, S.; Mita, T.; Kakiuchi, T. Langmuir 2005, 21, 14701474. (44) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91-97. (45) Avila, A.; Gragory, B. W.; Niki, K.; Cotton, T. M. J. Phys. Chem. B 2000, 104, 2759-2766.

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of MES, AET, and dodecanethiol (DDeT) into two distinctive types of domains, that is, hydrophilic domains composed of MES and AET and hydrophobic domains mainly composed of DDeT. DDeT was chosen as an alkanethiol whose alkyl chain is long enough to make a solid phase in the SAM through the lateral van der Waals interaction.46 We will show that MES and AET behave as if it were a single chemical species and the MES-AET pair exhibits unique surface properties that singlecomponent SAMs do not have. Experimental Section Sodium 2-mercaptoethanesulfonate (NaMES) (Aldrich), 2-aminoethanethiol hydrochloride (AETHCl) (Aldrich), and DDeT (Wako Chem. Ind. Co.) were used without further purification. Water was purified with a Milli-Q system (Millipore Co.). All other chemicals were of reagent grade and used without further purification. A mica sheet (Nilaco, Japan) was baked at 580 °C prior to the vapor deposition and maintained at 580 °C during the deposition of Au (99.99%). The substrates were then annealed at 530 °C for 8 h in ambient atmosphere immediately before use. Three-component thiol SAMs were formed in ethanolic solution of NaMES, AETHCl, and DDeT. The total thiol concentration in ethanol for preparing a SAM, ctotal, was usually 1 × 10-3 mol dm-3. For comparative studies, ctotal at 1 × 10-5 mol dm-3 was also employed. The mixing ratio of the DDeT in the solution was sol , keeping changed by varying the molar ratio of the DDeT, χDDeT sol sol ctotal constant. Here, χDDeT is defined by χDDeT ) c DDeT/ctotal, and ctotal ) (cDDeT + cNaMES + cAETHCl), where ci is the molar concentration of i (i ) DDeT, NaMES, or AETHCl) and cNaMES sol ) cAETHCl at any value of χDDeT . Ternary SAMs composed of MES, AET and DDeT were prepared by immersing a gold substrate in the ethanolic solution of three thiols for 24 h. For cyclic voltammetry of the reductive desorption of SAMs,47 a gold deposited mica coated with the SAM was mounted at the bottom of a cone-shaped cell by using an elastic O-ring.47 The surface area of the electrode was estimated to be 0.126 cm2 from the diameter of the O-ring. A solution of 0.5 mol dm-3 KOH in the cell was deaerated with Ar bubbling for 20 min. The potential was referred to a Ag/AgCl(saturated KCl) electrode. All voltammetry measurements were made at the scan rate of 20 mV s-1 at 25 °C. In X-ray photoelectron spectroscopy (XPS) measurements, the monochromated Mg KR X-ray was used. Peak areas were calibrated with respect to the area for the peaks of the Au 4f5/2 and 4f7/2 signals. The intensities of N 1s and S 2p signals were normalized by the peak areas of N 1s and S 2p of the sample sol ) 0, respectively. Scanning tunneling miprepared at χDDeT croscopy (STM) images were acquired with a NanoScope III (Digital Instruments). Pt80Ir20 tips were electrochemically etched and coated with Apiezon wax. In situ STM measurements were carried out in a 0.1 mol dm-3 NaClO4 solution in the constantcurrent mode.

Results and Discussion 1. Voltammetric Evidence for the Phase Separation into Two Types of Domains. Curve a in Figure 1 shows a cyclic voltammmogram (CV) for the reductive desorption47 of the binary MES-AET SAM formed from a 1 × 10-3 mol dm-3 ethanolic solution of MES and AET sol ) 0. A single peak at -0.6 at the 1:1 molar ratio, i.e., χDDeT V in the forward scan reflects the simultaneous desorption of MES and AET from a homogeneously mixed 1:1 composite monolayer of MES and AET on Au(111), as has been demonstrated to be formed over the wide range of the ratio of NaMES and AETHCl in the bathing ethanolic solution.42 A narrow full width at half-maximum (fwhm) (46) Kakiuchi, T.; Usui, H.; Hobara, D.; Yamamoto, M. Langmuir 2002, 18, 5231-5238. (47) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.

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Figure 1. Cyclic voltammograms for the reductive desorption sol of SAMs prepared at ctotal ) 1 × 10-3 mol dm-3. (a) χDDeT ) 0, (b) 0.2, (c) 0.5, (d) 0.7, (e) 0.8 and (f) 1. CV measurements were recorded at scan rate of 20 mV s-1 at 25 ( 2 °C.

of about 20 mV results from strong lateral electrostatic interaction between MES and AET in the SAM.42 Curve f shows a CV for the reductive desorption of a DDeT single component SAM, having a single peak at -1.1 V. The fwhm value of 20 mV is in this case due to the van der Waals interaction between the alkyl chains.46 Curves b-e show CVs for the reductive desorption of sol ) 0.2, 0.5, 0.7, and the ternary SAMs prepared at χDDeT 0.8, respectively. The appearance of two distinct peaks with peak separation of about 350 mV clearly indicates the presence of the phase separation of the SAM into two types of domains.7 The location of the two peaks suggests that the positive peak at about -0.7 V (peak I) and the negative peak at about -1.1 V (peak II) in each voltammmogram in the forward scan correspond to the desorption of the domains that are mainly composed of MES and AET and of DDeT, respectively. The domains whose desorption gives peaks I and II both constitute the quasi-two-dimensional bulk phases having the area greater than ca. 15 nm2,7 which we hereafter call phases I and II, respectively. 2. Degree of the Mutual Solubility. The peak potential (Ep) is plotted in Figure 2a for peaks I and II as a function of the surface coverage of DDeT with respect surf , which was estito the total thiolates adsorbed, χDDeT mated from the ratio of the peak area for peak II to the sum of those for peak I and peak II, assuming that the contribution of the charging current to the peak area46 is proportional to the surface coverage of the corresponding domain. It is interesting to see that Ep for peak I varies surf surf when χDDeT < 0.25. This is very much little with χDDeT different from phase-separated binary SAMs so far reported. For example, hexadecanethiol (HDT) dissolves into 3-mercaptopropionic acid (MPA) domains to cause a substantial shift of the peak of the MPA desorption to the negative direction.7 Such a shift is more pronounced in the case of binary SAMs composed of tetradecanethiol and 3-mercapto-1-propanol.11 surf No significant dependence of Ep on χDDeT in peak I was surf observed when χDDeT < 0.25. The shift in Ep can be taken as a rough measure of the mutual solubility.8,11 The observed invariance of Ep therefore means that the solubility of DDeT in phase I is very limited. The shift in surf > 0.5. The low Ep becomes discernible only when χDDeT

Figure 2. (a) Dependence of the reductive desorption peak potentials, Ep of peaks I (O) and II (4) of the ternary SAM surf prepared at ctotal ) 1 × 10-3 mol dm-3 on χDDeT . Error bars indicate the standard deviation for at least triplicate measurements. The broken lines are the guide for eyes. (b) Dependence of full width at half-maximum (fwhm) of peaks I (O) and II (4) surf on χDDeT . Error bars indicate the standard deviation for at least triplicate measurements.

dissolution of DDeT in MES-AET domains is also supported by the variation of fwhm for peak I in Figure surf 2b; the fwhm remains to be 20 mV when χDDeT < 0.5. The solubility of DDeT in phase I was estimated to be 12% at sol χDDeT ) 0.75. In contrast, peak II shifted to the positive direction by surf ) 0.75 (Figure 2a). This is again in 50 mV when χDDeT marked contrast with the desorption of hydrophobic HDT domains in the MPA-HDT binary SAM, where Ep for the desorption of HDT domains varies only slightly with 7 χsurf HDT. The variation of Ep for peak II in Figure 2a suggests that phase II contains MES and AET, to the extent that the effect of the existence of MES and AET is not macroscopically recognized on the voltammogram. To examine the possibility of ion-pair formation in the solution phase in preparing SAMs, we measured the conductivity of ethanolic solutions containing NaMES and sol . No evidence of the AETHCl at different values of χDDeT formation of the ion-pair between MES anions and AET cations was obtained at the concentration employed for preparing the SAM. As it is difficult to envisage that the MES and AET can penetrate from the solution phase to the DDeT domains simultaneously, we speculate that the MES-AET clusters in phase II are probably left over from the early stage of the SAM formation, that is, kinetically trapped48 in phase II. Correspondingly, the fwhm of peak II that was initially 20 mV gradually broadened to 60 mV surf decreased to 0.1. The size of each MES-AET when χDDeT cluster should not then be large enough to be an independent AET-MES phase giving peak I.7 3. Relative Strength of the Surface Activities of MES-AET Pair and DDeT. The results in Figures 1 (48) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Y. J. Phys. Chem. B 2001, 105, 2975-2980.

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Figure 3. (a) Dependence of peak areas for peaks I (O) and II (4) and the surface coverage of brighter region in STM images sol in Figure 7a (1) on χDDeT of the ternary SAM prepared at ctotal ) 1 × 10-3 mol dm-3. (b) Dependence of peak areas for reductive desorption of AET-rich domains (O) and DDeT domains (4) on sol χDDeT of the AET-DDeT binary SAM prepared at ctotal ) 1 × 10-3 mol dm-3. (c) Dependence of peak areas of MES-AET domains (O) and DDeT domains (4) of the ternary SAM prepared sol at ctotal ) 1 × 10-5 mol dm-3 on χDDeT . The surface coverage of brighter region calculated from the STM image in Figure 7c is also plotted (1).

and 2a,b consistently show that MES and AET tend to retain the electrostatically stabilized structure even in the presence of DDeT. Furthermore, it is intriguing that the solubility of MES and AET in the DDeT domains is high. It appears as if MES and AET form a single component that has a strong mutual interaction with each other. This is remarkable and counterintuitive, because DDeT is much more surface active than the MES-AET pair, judging from the locations of peaks I and II. The sol ) 0 and at 1 was 0.45 V. difference in Ep values at χDDeT This corresponds roughly to the difference in the adsorption Gibbs energy of more than 40 kJ mol-1.46 Another unusual feature is displayed in Figure 3a, in which the sol . A rough area under the peak, Q, is plotted against χDDeT surf measure of χDDeT calculated from Q assuming that Q ) 120 µC cm-2 at the full coverage, taking account of the contribution of the charging current to the peak area,46 is also shown on the right-hand-side ordinates in Figure sol 3. The two curves cross over when χDDeT ) 0.7. This means surf that to prepare a surface with χDDeT ) 0.5 the concen-

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tration of DDeT in the bathing ethanolic solution should be at least twice more concentrated than that of the MESAET pair. In the case of the AET-DDeT two-component SAM on Au(111) prepared from the 1 × 10-3 mol dm-3 ethanolic solution of AET and DDeT, the 1:1 surface ratio of AET sol ) 0.18 (Figure 3b). and DDeT was achieved when χDDeT This is similar to the two-component SAMs composed of MPA and HDT, where the 1:1 surface ratio was found at 7 χsol HDT ) 0.07. In the case of binary SAMs of MES and sol < 0.05 DDeT, MES domains appear only when χDDeT (data not shown). Both of these binary cases may reasonably be interpreted in terms of the stronger surface activity of hydrophobic alkanethiols than hydrophilic thiols having a hydrophilic headgroup at the ω position. The present results in Figure 3a, however, clearly show the opposite. The difference between the binary SAMs of AET-DDeT and MES-DDeT and the ternary MES-AET-DDeT SAMs highlights the uniqueness of the MES-AET pair in forming the SAM. Interestingly, the decrease in ctotal to 1 × 10-5 mol dm-3 sol surf leads to the value of 0.3 for χDDeT giving χDDeT ) 0.5 (Figure 3c), which is much lower than 0.7 in Figure 3a. This contrasts with the concentration effect on the phase separation of binary SAMs. In the MPOH-TDT binary SAM on Au(111), the surface composition approaches the solution composition by lowering the total thiol concentration of the bathing solution.11 This appears to be ascribed to slower exchange between adsorbed and dissolved thiols in a dilute thiol solution. However, this reasoning does not apply to the present ternary SAM. A possible explanation is the involvement of the adsorption kinetics. Immediately after the immersion of a gold substrate into an ethanolic solution of DDeT, MES, sol ) 0.5, the probability of finding an and AET at χDDeT adsorbate, DDeT or one of MES and AET, is obviously equal. In the adsorption of thiols from a solution phase, the domains grow through the replacement reaction and the surface diffusion of adsorbed molecules, the latter of which is usually much slower.19 In the course of the domain formation to obtain the surface ratio of DDeT and MESAET on the SAM, MES-AET must therefore be replaced by DDeT having stronger surface activity. To achieve the replacement reaction, a DDeT molecule in the solution phase needs to approach first to a hydrophilic MES-AET pair or its cluster adsorbed on the surface and then attack either one of the electrostatically stabilized MES and AET, which is presumably a slow process. Such a slow replacement reaction has been reported in the replacement of hydrogen-bond-stabilized 11-mercaptododecanoic acid in the SAM with HDT in the ethanolic solution.17 However, this model needs a modification to explain sol surf ) 0.7 for χDDeT ) 0.5, because this the fact that χDDeT sol greater than 0.5. It model does not account for the χDDeT is well established that alkanethiols first adsorb lying flat on a Au(111) surface and at higher coverages the alkylchains stand up to form a densely packed SAM with the tilt angle of about 30° with respect to the surface normal.49-53 In the present ternary systems, this transition in the orientation may allow MES and/or AET to creep into the surface to occupy the adsorbed sites adjacent to the adsorbed DDeT. In other words, the change in the orientation of DDeT is induced not only by DDeT (49) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (50) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (51) Poirier, G. E. Langmuir 1999, 15, 1167-1175.

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Figure 4. Dependence of reductive desorption peak potentials surf of MES-AET (O) and DDeT (4) on χDDeT of the ternary SAM prepared at ctotal ) 1 × 10-5 mol dm-3. The broken lines are the guide for eyes.

transported from the solution phase but also by MES and AET that approach to the adsorption sites from the solution phase. When ctotal is 1 × 10-5 mol dm-3, the probablility of forming a MES-AET pair on the surface is much smaller than that at ctotal ) 1 × 10-3 mol dm-3. We thus surmise that isolated MES and AET molecules sporadically adsorbed on the surface are readily replaced with DDeT, leading to the formation of DDeT domains at a smaller sol . A further study is in progress in our group value of χDDeT to elucidate the details of the mechanism of forming phaseseparated SAMs by following the time course of the formation of binary and ternary SAMs.54 It is notable that, when ctotal ) 1 × 10-5 mol dm-3, the solubility of DDeT into MES-AET domains is lower, as shown in Figure 4. The position of Ep starts to shifts to surf > 0.5. It is known the negative direction only when χDDeT in the case of binary SAMs that mutual dissolution is reduced by lowering the concentration of thiols in a bathing solution.11 This concentration dependence of the mutual solubility indicates, first, that the mutual solubility in SAMs is not determined by the equilibrium properties of the system and, second, that at this initial stage of the adsorption a domain has enough time to rearrange itself on the surface before a new adsorbate reaches the neighboring sites from the solution phase. 4. XPS Studies of the Surface Composition of the Ternary SAM. To examine the surface composition and the charged state of MES and AET in hydrophilic domains of the ternary SAM, XPS spectra for the N 1s and S 2p sol from 0 to regions were recorded over the range of χDDeT 1 as shown in Figure 5, panels a and b. The peak at 401 eV is attributable to the protonated amino group.55 We can also observe a small peak at 398 eV, which is attributable to the unprotonated amino-terminated group.55 Most of AET molecules in the SAM are thus protonated. There appear two peaks in the S 2p region, at 162 eV and at 168 eV, corresponding to S 2p in the S-Au bond and S 2p in the sulfonate-terminal groups, respectively.55 The intensity of the signal at 162 eV did not vary with sol χDDeT , whereas the intensity at 168 eV decreased with (52) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 1561-1566. (53) Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Chem. Phys. 2003, 118, 1904-1911. (54) Phong, P.-H.; Sokolov, V. V.; Nishi, N.; Yamamoto, M.; Kakiuchi, T.; in preparation. (55) Briggs, D., Seah, M. P., Eds.; Practical Surface Analysis; Wiley: Chichester, U.K., 1995; Chapter 9; p 445.

Figure 5. XPS spectra of N 1s and S 2p regions for ternary sol SAMs prepared at ctotal ) 1 × 10-3 mol dm-3. χDDeT ) 0 (1), 0.2 (2), 0.5 (3), 0.7 (4), 0.9 (5), and 1.0 (6).

Figure 6. Dependence of normalized intensity of N 1s at 401 sol eV(0), S 2p at 168 eV(O), Na 1s (4), and Cl 2p (3) on χDDeT . sol increasing χDDeT , corresponding to the decrease in the area of MES-AET domains. Figure 6 shows the dependence of normalized intensity of N 1s signal at 401 eV (0) and S 2p signal at 168 eV (O). The intensity of these signals sol in continuously decreased with the same slope with χDDeT parallel with each other. First, this suggests that the composition of MES and AET on the surface is 1:1 over sol . Second, this unity ratio and the the range of χDDeT protonation of the amino terminal evidences the electrostatic stabilization of the positive and negative terminals of the adsorbed thiolates, either in the MES-AET domains or embedded in the DDeT domains. The Na 1s and Cl 2p signals were both very weak (Figure 6). The absence of the appreciable adsorption of Na+ and Cl- ions is another indication that the electroneutrality of the hydrophilic domains is maintained by the lateral neutralization of the amino-terminated AET and the sulfoterminated MES on the surface.

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Langmuir, Vol. 21, No. 23, 2005

Phong et al.

In Figure 7a, there are few small bright spots that do not belong to macroscopically phase-separated DDeT domains.7 This observation is in harmony with the appearance of a very small hump between the two major peaks in the CVs in curves c and d in Figure 1b. It can be said therefore that the degree of the phase separation of the MES-AET-DDeT ternary SAM into two types of the domains is very high. The surface coverage of the DDeT domain was estimated from the STM images after thresholding the image using an image processing software. The results are plotted in Figure 3a (1), assuming that the full coverage corresonds to 120 µC cm-2. The sol in paralell with the change in coverage varied with χDDeT Q estimated from CVs. Figure 7c is an in situ STM image sol ) 0.5. The prepared at ctotal ) 1 × 10-5 mol dm-3 and χDDeT shape of DDeT domains is similar to that in Figure 7a, but the coverage of the DDeT domain becomes greater. surf values calculated from STM images are plotted Two χDDeT in Figure 3c (1), showing a nice agreement with those estimated from Q. A similar growth of the domain size by lowering the thiol concentration has been reported in binary SAMs composed of 3-mercaptopropanol and tetradecanethiols.11 Such a dependence of the domains size of binary and ternary SAMs can be understood in analogy with the dependence of the crystal size in the rate of recrystallization from a solution. Conclusion Figure 7. In situ STM images of the ternary SAM prepared at ctotal ) 1 × 10-3 mol dm-3 (a) and ctotal ) 1 × 10-5 mol dm-3 (c). (b) Height profile along the line drawn in (a). Images size: 100 × 100 nm. Images were recorded at -0.15 V and 200 pA.

5. STM Images of the Phase-Separated Ternary SAM. Figures 7a shows an in situ STM image in 0.1 mol dm-3 NaClO4 solution of the ternary SAM prepared when sol ) 0.7. This 100 × 100 ctotal ) 1 × 10-3 mol dm-3 at χDDeT nm image shows three distinct regions having different heights as shown in the height profile in Figure 7b: the dark dots, the bright regions forming islands, and the gray region constituting the bed for the islands. This image clearly shows the phase separation of the ternary SAM to DDeT-rich and MES-AET-rich domains, aside from depessions or pits commonly seen in thiol SAMs on Au(111). From the variation of the total area of the brighter sol , we assigned that the bright regions domains with χDDeT in the STM images correspond to hydrophobic DDeT domains and the gray regions to hydrophilic MES-AET domains.

In the MES-AET-DDeT ternary SAMs, two different types of domains of nanometer scale are formed: one, the MES-AET domain, and the other, the DDeT-rich domain. MES and AET behave as if they were a single chemical species and show unique surface activities in that the combination of MES and AET appears to enhance the surface activity of themselves to form a hydrophilic and electrically neutral phase with minimal perturbation by hydrophobic DDeT molecules. The apparent large solubility of the MES-AET pair kinetically trapped in the DDeT domain focuses the importance of the early stage of the formation of SAMs in determining the structure of multicomponents SAMs. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 14205 120) from the Ministry of Education, Culture, Sport, Science and Technology, Japan. LA050444E