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Phase Separation of Alkanethiol Self-Assembled Monolayers during the Replacement of Adsorbed Thiolates on Au(111) with Thiols in Solution Takashi Kakiuchi,*,† Kimiharu Sato,‡,§ Minehiko Iida,‡,| Daisuke Hobara,† Shin-ichiro Imabayashi,‡ and Katsumi Niki⊥ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan, and Department of Physical Chemistry, Yokohama National University, Yokohama 240-8501, Japan Received December 6, 1999. In Final Form: June 2, 2000 Replacement of self-assembled monolayers (SAM) of hexadecanethiol (HDT) on the Au(111) surface with 12-mercaptododecanoic acid (MDDA) in ethanol solution has been studied using voltammetry of the reductive desorption and scanning tunneling microscopy (STM). The exchange of adsorbed HDT molecules with MDDA dissolved in ethanol proceeds domainwise, that is, not in a random fashion. Two well-separated peaks, corresponding to the desorption of MDDA-rich domains and HDT-rich domains, appear in a voltammogram over the replacement process. The peak potential associated with the desorption of HDTrich domains remains unchanged in the course of the replacement, indicating that the solubility of MDDA in HDT-rich domains is very small. STM imaging of the substrate shows that domains whose sizes are greater than 15 nm2 are predominant and occupy 85% of the area of HDT-rich domains. The entire exchange process is pseudo-first-order with the rate constant being 9.1 × 10-3 h-1 in 1 mmol dm-3 MDDA in ethanol at 31 °C. The reverse process, i.e., the replacement of adsorbed MDDA with dissolved HDT in ethanol, is much slower, suggesting the stabilization of MDDA monolayers by lateral hydrogen bonding. A significant shift in the peak potential of MDDA-rich domains during the replacement indicates the considerable dissolution of HDT in MDDA domains.
Introduction When a thiol self-assembled monolayer (SAM) on a metal substrate is exposed to an organic solution containing thiols or alkyl disulfides, adsorbed alkanethiolate molecules on a metal surface are gradually replaced with thiols dissolved in the adjacent solution.1-3 The details of this replacement reaction have been studied in view of its importance in studying the mechanism of the formation of multicomponent SAMs,4-8 in determining the stability of SAMs on a metal surface,9,10 in demonstrating the cleavage of the disulfide bond of alkyl disulfides on adsorption,11,12 in studying surface diffusion,13,14 in pre* Corresponding author. Tel: (81)-75-753-5528. Fax: (81)-75753-3360. E-mail:
[email protected]. † Kyoto University. ‡ Yokohama National University. § Present address: Dainihon Insasu. Inc., Japan. | Present address: Honen Unilever, Japan. ⊥ Present address: Department of Chemistry, Illinois State University, Normal, IL. (1) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 36653666. (2) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155- 7164. (3) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167- 3173. (4) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301- 4306. (5) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192- 1197. (6) Tsao, M. W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 43174322. (7) Kolega, R. R.; Schlenoff, J. B. Langmuir 1998, 14, 5469- 5478. (8) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440- 5443. (9) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252812536. (10) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973- 2979. (11) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770.
paring a molecular wire on SAMs,15,16 in blocking electrochemical reactions through defect sites of SAMs,4,10 and in engineering metal surfaces using stepwise procedures,17,18 photopatterning,19,20 or electrochemical replacement techniques.21,22 One of the interesting points in the exchange reaction is whether the replacement accompanies the formation of sizable domains. In an earlier work of the exchange reaction, no evidence was available for the formation of discrete, separate phases on the surface (i.e., islands).3 On the other hand, Chidsey et al. found at least two distinctive steps in the replacement of ferrocene-terminated alkanethiols on gold with a long-chain alkanethiol,4 i.e., the fast replacement of ferrocene-terminated thiols in the domain boundaries and subsequent slow replacement of ferrocene-terminated thiols within domains where the molecules are tightly packed. Collard and Fox (12) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825- 1831. (13) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H. J.; Bamberg, E.; Allison, H.; Evans, S. D. Langmuir 1996, 12, 3898- 3904. (14) Jaschke, M.; Scho¨nherr, H.; Wolf, H.; Ringsdorf, H.; Besocke, M. K.; Bamberg, E.; Butt, H.-J. J. Phys. Chem. 1996, 100, 2290- 2301. (15) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705- 1707. (16) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721- 2732. (17) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122- 8127. (18) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017- 8021. (19) Hutt, D. A.; Cooper, E.; Parker, L.; Leggett, G. J.; Parker, T. L. Langmuir 1996, 12, 5494- 5497. (20) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024- 1032. (21) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502- 4504. (22) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073- 5078.
10.1021/la991590l CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000
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confirmed the fast replacement in defect area and the slow replacement in crystalline domain site for the replacement of ferrocene-terminated alkanethiols.5 As pointed out by Chidsey et al., the rate of replacement depends on the manner of the packing of molecules on the surface, which complicates the interpretation of the replacement kinetics. Although the way of packing of ferrocene-terminated thiols is different from that of alkanethiols, apparently similar replacement processes showing two distinctive processes have been found also in the self-exchange reaction of adsorbed alkanethiolates with alkanethiols in the solution phase by Schlenoff et al. using radiolabeled alkanethiols; the replacement was described by pseudo-first-order kinetics with rate constants in the range of 10-5 s-1.9 Recent FT-IR studies of the replacement of adsorbed n-eicosanethiol with partially deuterated eicosanethiol confirmed the importance of the compactness of the monolayer in determining the kinetics of the exchange reaction.23 Lin and Guyot-Sionnest recently reported that the replacement of bis(phenylethynyl)benzenethiol on Au(111) with decanethiol starts at the domain boundaries and proceeds slowly, leaving the arenethiol islands.24 It is interesting to see if adsorbed alkanethiolates are replaced by other alkanethiols with similar island formation. The rate of the exchange is also important for preparing mixed self-assembled monolayers by using multistep adsorption techniques.15,16,18-20 In the preparation of artificially phase-separated two-component SAMs using electrochemical selective-desorption followed by readsorption of thiols from a solution,21,22 the possible replacement of thiol molecules remaining on a metal surface with another thiol species in a solution can proceed in parallel with the readsorption on the free surface. If the rate of this replacement reaction were not negligible, the domains would become less homogeneous, leading to the change in the surface properties. This constitutes another motivation for the present work. In this paper, we show from electrochemical and scanning tunneling microscopy (STM) studies that the replacement of adsorbed thiol molecules with dissolved thiol species takes place by forming domains; the replacement is not a random process. We will report the domainwise replacement reactions of hexadecanethiol (HDT) monolayers with 12-mercaptododecanoic acid (MDDA) and also the significantly slower replacement of MDDA monolayers with HDT, the latter of which suggests the importance of stabilization of MDDA monolayers by lateral hydrogen bonding. Experimental Section Materials. 11-Mercaptododecanoic acid (MDDA) was synthesized from the corresponding ω-bromoalkanoic acids.25 All other chemicals were of reagent grade. Au films were prepared on freshly cleaved mica surface by vapor deposition at less than 1 × 10-6 Torr. Au-deposited mica sheets were annealed at 550 °C for 6 h before use. The formation of Au(111) terraces was confirmed by STM. Other details of the preparation were described previously.26 Thiol-adsorbed Au electrodes were prepared by immersing a Au substrate in a 1 mmol dm-3 ethanol solution of a thiol overnight. Methods. Linear scan voltammograms of reductive desorption27,28 were recorded in 0.5 mol dm-3 KOH using a cone-shaped (23) Chung, C.; Lee, M. J. Electroanal. Chem. 1999, 468, 91- 97. (24) Lin, P.-H.; Guyot-Sionnest, P. Langmuir 1999, 15, 6825- 6828. (25) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (26) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33- 38. (27) Widrig, C.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335- 359.
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Figure 1. Linear-scan voltammograms for reductive desorption of HDA (curve 1) and MDDA (curve 2) self-assembled monolayers on Au(111) in 0.5 mol dm-3 KOH. Scan rate: 20 mV s-1. cell.27 A gold substrate was mounted at the bottom of the cell using an O-ring and a cramp. Deaeration was made by bubbling Ar for 30 min. The potential was controlled with a three-electrode potentiostat using a Ag/AgCl/saturated KCl and a Pt wire. STM measurements were made in air using Nanoscope E equipped with a low-current amplifier. Usual conditions for STM measurements were 8 pA for setpoint and 1.5 V for bias voltage. Pt/Ir tips were employed. All measurements were made under ambient conditions.
Results and Discussion Figure 1 shows linear-scan voltammograms for the reductive desorption of HDT (curve 1) and MDDA (curve 2) separately recorded at a scan rate of 20 mV s-1. The peaks at -1.09 and -0.91 V correspond to the desorption of HDT and MDDA, respectively. We first examined the replacement of HDT SAMs with MDDA dissolved in ethanol. We prepared Au(111) substrates covered with HDT SAMs. Each substrate was kept in an ethanol solution of 1 mmol dm-3 MDDA at 31 °C. After a certain period of time, we took out a substrate from the ethanol solution, rinsed with ethanol and water, and recorded a linear-scan voltammogram in 0.5 mol dm-3 KOH. The change in the shape of the voltammogram with time is shown in Figure 2. The peak at -0.9 V (peak I) gradually grew with increasing time of the immersion, while, concomitantly, the height of the peak at -1.1 V (peak II) diminished. After 336 h, peak II completely disappeared, indicating that no appreciable amount of HDT molecules remained on the surface by the replacement with MDDA. This completion of the replacement makes a sharp contrast with the replacement of ferrocene-terminated thiol SAMs with alkanethiol; a certain portion of ferrocene-terminated thiols remains unexchanged on the time scale of 2 weeks or so.4,5 The peak width at half-maximum of curve 10 in Figure 2 is 48 mV, which is considerably broader than that of curve 2 in Figure 1, 32 mV. Since the narrower the peak width the stronger the intermolecular attraction between adsorbed molecules,29 the broadening of curve 2 in Figure 2 reflects the less ordered structure of MDDA monolayers after the replacement. (28) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687- 2693. (29) Kakiuchi, T.; Usui, H.; Hobara, D.; Yamamoto, M. Manuscript in preparation.
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Figure 3. Variation of peak potentials with immersion time for the reductive desorption of the HDT-rich domain (b) and MDDA-rich domain (O).
Figure 2. Linear-scan voltammograms for reductive desorption of HDT self-assembled monolayers after immersed in 1 mmol dm-3 MDDA ethanol solution at 31 °C. Time of immersion: 0 (curve 1), 24 (curve 2), 48 (curve 3), 72 (curve 4), 96 (curve 5), 124 (curve 6), 144 (curve 7), 168 (curve 8), 219 (curve 9), and 336 (curve 10) h.
The appearance of two peaks during the course of the replacement reaction indicates that there must be phase separation in the SAM into sizable domains. Previously, we found that the appearance of two separate peaks for the reductive desorption of two-component mixed SAMs reflects the phase separation of the SAMs into two different types of domains; one thiol species is enriched over the other.30 In the case of two-component SAMs composed of HDT and 3-mercaptopropionic acid (MPA), we estimated to be 15 nm2 as the minimum size of the domain, so that the domain behaves as a two-dimensional bulk phase. If two thiol species mix with each other or the size of the domains is smaller than 15 nm2, the reductive desorption would give a single peak at the potential in the middle of the two peak potentials corresponding to the reduction of HDT and MDDA. In fact, in the case of HDT-MPA mixed monolayers, a small hump that appeared between the two peak potentials corresponding to HDT-rich and MPArich domains was ascribed to the desorption of small domains which are not entitled to belong to either of the two domains.30 In the present case, however, no hump was discernible between the two peaks in Figure 2. We believe therefore that the total area occupied by small domains whose area is too small to be grouped into either of the two domains is negligibly small. With increasing degree of replacement, the peak potentials for both peaks I and II shifted to the positive direction by 5 and 10 mV, respectively, after 200 h (Figure 3), but in the case of HDT-MPA binary SAMs formed by coadsorption, they were ca. 50 and 10 mV, respectively.30 The degree of the shift presumably reflects (30) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113- 119.
Figure 4. Variation of apparent adsorbed amounts of HDT (b), MDDA (O), and the sum (4) calculated from the area under reductive desorption peaks.
the degree of mutual solubility of the two thiols on SAMs.31 The results in Figure 2 indicates that the solubility of HDT in MDDA-rich domains is small in comparison with other two-component SAMs formed by coadsorption.31 The area under the reductive desorption peak was formally converted to the adsorbed amount of thiols, Γ, and is shown in Figure 4. After the completion of the replacement of HDT with MDDA, the total amount of thiols on the surface, i.e., the total area under the two peaks, decreased by 20%. When adsorbed thiols form the (x3 × x3)R 30° structure, the maximum adsorption is 7.6 × 10-10 mol cm-2. The initial value, 1.1 × 10-9 mol cm-2 for HDT SAMs, is greater than the theoretical prediction for the (x3 × x3)R 30° structure. This discrepancy is primarily attributable to the charging current contribution,32 which is further decomposed into static and dynamic charging currents.29 The narrower the peak, the greater the contribution of the charging current. The decrease in Γ after the replacement can therefore be associated, in part, with the decrease in the charging current. The possibility of the decrease in the effective surface area caused by a certain passivation of the surface cannot also be excluded. The degree of the replacement was also monitored with the decrease in CH2 antisymmetric stretching band at 2920 cm-1 and CH2 symmetric stretching band at 2850 cm-1 in the spectra of reflectiveabsorption IR spectroscopy. Curves similar to those in Figure 4 were obtained (data not shown). The domain formation during the process of replacement was studied by STM in air. Typical results are shown in Figure 5 when t ) 0 (1), 70 h (2), 136 h (3), and 229 h (4). (31) Hobara, D.; Ueda, K.; Imabayashi, S.; Yamamoto, M.; Kakiuchi, T. Electrochemistry 1999, 67, 1218-1220. (32) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391- 12397.
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Figure 5. STM images of self-assembled monolayers of HDT before the immersion of the substrate into 1 mmol dm-3 MDDA ethanol solution (1) and after the immersion for 70 h (2), 136 h (3), and 229 h (4). Corresponding cross sectional profiles along the line in each image were shown in panels on the right-hand side. Bias voltage: 1.5 V. Setpoint: 8 pA.
In Figure 5-1 for HDT SAMs, the surface was smooth except pits which are common in thiol SAMs. After 70 h of immersion, the STM image in Figure 5-2 clearly shows the inhomogeneity of the surface. This texture of the surface persisted in a STM image of the SAM at t ) 136 h. The decrease in the total area of the brighter portions with time indicates that the brighter and darker domains correspond to the HDT-rich and the MDDA-rich ones, respectively. In the STM image at t ) 229 h, no distinctive domains are observed, except smaller white dots which may be attributable to remaining small HDT islands. Thus the STM images confirm that adsorbed HDT is almost completely replaced with MDDA. The change in the height profile with immersion time in Figure 5 illustrates the
increase in surface roughness in the middle of the replacement process. The domain formation is also ascertained through the change in the shape of histograms of the height distribution (Figure 6) obtained from the STM images in Figure 5.33 The distribution of the height in the HDT SAM is very narrow, indicating uniformity in the height in the STM image, i.e., the presence of a tightly packed SAM. After the immersion of the substrate into an ethanol solution containing 1 mmol dm-3 MDDA, the distribution of the height became broader. Unlike HDT-MPA binary SAMs (33) Note the depth in Figure 6 is referred to the highest point in each STM image and, hence, what is relevent to the present analysis is the distribution of the depth in each image.
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Figure 8. Distribution of the size of HDT domains taken from Figure 5-2. The dashed line shows the cumulative area of the HDT domain.
Figure 6. Histograms of the height distribution in Figure 5, parts 1 (1), 2 (2), 3 (3), and 4 (4).
Figure 7. Variation of the total areas of higher and lower portions calculated from STM images in Figure 5.
where the height difference of the two domains is large enough to show two distinct peaks in the histogram,30 the present HDT-MDDA systems show only the broadening of the histogram, which reflects, however, the domain formation during the replacement reaction. Further progress in the replacement reaction resulted in the renarrowing of the histogram. This indicates that MDDA becomes the majority of adsorbed molecules on the surface, leading to the appearance of a smoother surface again. The change in the total area of the higher and the lower domains in STM images is plotted in Figure 7, which is similar to Figure 4 obtained electrochemically; there exists a one-to-one correspondence between the electrochemical behavior and STM imaging in the nanometer scale. Figure 8 shows the size distribution of the HDT domains obtained
after thresholding Figure 5-2. One can see in Figure 8 that domains whose size ranges from 15 to 100 nm2 are predominant and 85% of the total area of HDT domains, represented by the dashed line, is occupied by larger domains having the area greater than 15 nm2, which is a critical value for ensuring each domain to behave as a two-dimensional bulk phase.30 These results again unequivocally demonstrate that the replacement takes place domainwise and not in a random fashion. It means that, once a part of a HDT domain is gradually replaced with MDDA from the domain edge, the exchange proceeds solely in this domain, while other domains in the vicinity remain intact. In the case of ferrocene-terminated SAM with alkanethiol, Chidsey et al. suggested lattice-domain boundary models in which the exchange of thiols in the interior of the domains requires chains to diffuse from within the densely packed chain region to a domain-boundary region, which is quite slow.4 In the case of the replacement of bis(phenylethynyl)benzenethiol on Au(111) with decanethiol, the replacement starts at the domain boundaries and proceeds slowly leaving the arenethiol islands of 5-10 nm dimensions,24 which is very similar to what we found in the present study. The domainwise replacement is more likely in the former system, because the lateral interaction between a like pair would be significantly different from that between an unlike pair. If the manner of packing is different from domain to domain in starting HDT monolayers, the variation in the rate of lateral diffusion of adsorbed HDT molecules could cause the domainwise replacement. However, it is unlikely that HDT SAMs have such degree of inhomogeneity. An alternative explanation is that the penetration and occupation of MDDA in HDT domains is a cooperative process; HDT molecules in contact with a newly formed MDDA domains become energetically less stable and more easily replaced with MDDA, while adsorbed MDDA molecules are stabilized by lateral hydrogen bonding between carboxyl groups. The importance of this stabilization of carboxyl-group terminated thiols in SAMs has been suggested for explaining unusual double layer (34) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114- 5119. (35) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397-5401.
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Figure 9. Variation of adsorbed amount of HDT calculated from the peak area for reductive desorption. The solid line represents the theoretical curve of the pseudo-first-order reaction when kf ) 9.1 × 10-3 h-1.
interactions,34 large pK shift,35 and domainwise replacement.20 This point will further be discussed below in relation to the replacement of adsorbed MDDA with HDT. The replacement reaction of thiols in a SAM with thiols in the solution phase may be formally represented by
Au-S(CH2)15CH3 + HS(CH2)11COOH(W) ) HS(CH2)15CH3(W) + Au-S(CH2)11COOH (1) This reaction is possibly composed of several elementary steps, that is, the adsorption of HS(CH2)11COOH, cleavage of the H-S bond, surface diffusion of adsorbed species, and reduction of Au-S(CH2)15CH3 followed by desorption into the ethanol solution phase. If one of these steps is distinctively slow, the rate of the replacement may be expressed by
-dΓHDT ) k1ΓHDTcMDDA - k2cHDTΓMDDA dt
(2)
where Γi is the adsorbed amount of i (i ) HDT or MDDA) on Au and ci is the surface concentration of i. The time scale of the replacement shown in Figures 2 and 4 is much longer than that of the diffusion of the thiols in ethanol. The surface concentration of HDT is then vanishingly small, while the surface concentration of MDDA is 1 mmol dm-3. The second term on the rhs of the above equation is, therefore, negligible. Then, eq 2 is simplified to
-dΓHDT ) k′1ΓHDT dt
(3)
that is, the replacement reaction is pseudo-first-order with respect to ΓHDT. Figure 9 shows that this model is well fitted to experimental points. The obtained value for the rate constant k1′ was 9.1 × 10-3 h-1. This type of pseudofirst-order reaction has been found for several systems: the replacements of propanethiolate SAM with HDT,12 ferrocene-terminated SAMs with HDT in hexane,5 and self-exchange of octadecanethiol having the rate constant of 10-5 s-1 in tetrahydrofuran.9 However, the rate constant obtained in the present study is much smaller than those previously reported. In many of those systems, two distinctive steps were observed. The initial fast replacement of ferrocene-terminated thiols with an alkanethiol has been ascribed to the replacement primarily occurring at defect sites.4,5 Such a fast process did not depend appreciably on the concentration of thiol in the adjacent solution phase.9 A recent FT-IR study by Chung and Lee showed that the compactness of the monolayer is critical
Figure 10. Linear-scan voltammograms for reductive desorption of MDDA self-assembled monolayers after immersed in 1 mmol dm-3 HDT ethanol solution. Time of immersion: 0 (curve 1), 44 (curve 2), 96 (curve 3), 139 (curve 4), and 264 (curve 5) h.
to the exchange kinetics of SAM for eicosanethiol SAM on Au.23 A slow-exchange process followed by initial fast exchange was ascribed to the replacement of compact eicosanethiol. Since there was no appreciable fast exchange in the present study, the substrates of HDT SAMs we employed seem to be mainly covered by tightly packed HDT domains. The applicability of pseudo-first-order kinetics to the replacement of HDT with MDDA may seem to be at odds with the domanwise replacement proposed above. If we have a means of monitoring the replacement reaction in nanometer scale, we would be able to observe a nucleation and growth process. However, the domain size, typically ranging from 15 to 100 nm2 (Figure 8), is small enough to allow the replacement kinetics to be treated phenomenologically as a pseudo-first-order reaction. The ratedetermining step is probably the rate of the lateral growth of MDDA domain which accompanies proton transfer on Au surface.7 In the preparation of artificially phaseseparated two-component SAMs by inserting foreign molecules into an alkanethiol SAM by adsorption, the immersion time employed for the readsorption of thiols is typically several hours.15,21 It is therefore likely that the replacement of adsorbed thiolates in a tightly packed domains with organic thiols in the solution phase is negligibly small in the time scale of several hours, provided that one uses tightly packed SAMs as the HDT SAMs employed in the present study. The reverse process, that is, the replacement of MDDA in a SAM with dissolved HDT, is much slower, as shown in Figure 10. This is puzzling, because the stronger lateral van der Waals interaction in HDT domains seems to make adsorbed HDT molecules more tightly packed than MDDA. Moreover, HDT has a greater value of the standard adsorption Gibbs energy, ∆Gad°, than MDDA, because of the difference in the alkyl chain length and ω-terminal. In fact, the peak potential of the reductive desorption of HDT is 0.2 V more negative to that of MDDA, suggesting that the difference in ∆Gad° is 19 kJ mol-1, if we neglect the effect of the intermolecular interaction on the peak
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Figure 11. Variation of apparent adsorbed amounts of HDT (b), MDDA (O), and the sum (4) calculated from the area under reductive desorption peaks when MDDA monolayer was in contact with 1 mmol dm-3 HDT ethanol solution.
potential.29 A natural expectation would therefore be opposite to what was found experimentally (Figures10 and 11). This asymmetry in the replacement kinetics suggests the presence in the MDDA domains of a certain stabilizing mechanism, aside from the adsorption Gibbs energy and attractive interaction between alkyl chains. One probable factor is the hydrogen bonding between adsorbed MDDA molecules. The stabilization of adsorbed ω-carboxylalkanethiolate molecules is considered to be an important factor for the large shift in the surface pK values.34,35 It is likely therefore that the considerable stabilization of MDDA domains is responsible to the resistance against the penetration of HDT molecules from the solution phase. A hydrated water layer on the top of the MDDA monolayer may prevent the approach of HDT from the ethanol solution side. The present finding is in harmony with the recently proposed explanation for the asymmetry in the replacement of ω-carboxylalkanethiols with alkanethiols and vice versa by Cooper and Leggett.20 On the other hand, the peak potential corresponding to the desorption of both MDDA-rich domains shifted to the negative direction with increasing degree of the replacement with HDT. The magnitude of the shift is greater
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than the shift in Figure 3 in the replacement of adsorbed HDT with MDDA; HDT is more soluble in the MDDA domains than MDDA in the HDT domains. This possible higher mutual solubility apparently contradicts with the much slower exchange reaction. Newly adsorbed HDT molecules may be weaker in cooperativity in comparison with the cooperativity of adsorbed MDDA. The possible slow initial replacement of adsorbed MDDA with HDT at domain boundaries can also account for the asymmetry of the replacement rate. The exchange is then primarily caused through random replacement, which would be slower than the replacement starting from domain boundaries but can lead to higher degree of HDT dissolution within the MDDA domains. Conclusion The manner that HDT molecules adsorbed on Au(111) forming a self-assembled monolayer are replaced with MDDA molecules in an ethanol solution is domainwise; the replacement is not a random event. The entire replacement process is described by a pseudo-first-order kinetics. The rate of replacement is much slower than those reported in some other systems, probably due to the tightness of the HDT domains. This slow exchange is good news for preparing desired surfaces using multiple adsorption/desorption procedures. The replacement is asymmetric; the replacement of adsorbed MDDA with HDT molecules in solution is much slower than the opposite, reflecting the difference in two-dimensional structuredness of HDT SAMs and MDDA SAMs, which is ascribed to the difference between intermolecular interactions of adsorbed HDA and MDDA molecules. Acknowledgment. This work was supported in part by Grant-in-Aid for Scientific Research No. 08640769, Grant-in-Aid for Exploratory Research No. 09875208, and Grant-in-Aid for Priority Research No. 11118235 from the Ministry of Education, Science, Sports, and Culture of Japan. LA991590L
