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Langmuir 2006, 22, 2706-2711
Submicrometer Structure of Surface-Chemical Gradients Prepared by a Two-Step Immersion Method Sara M. Morgenthaler, Seunghwan Lee, and Nicholas D. Spencer* Laboratory for Surface Science and Technology, Department of Materials, Swiss Federal Institute of Technology, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zu¨rich, Switzerland ReceiVed October 3, 2005. In Final Form: December 24, 2005 Lateral force microscopy and microdroplet density measurements have been used to examine the microstructure of surface-chemical gradients of thiols on gold, prepared by a two-step immersion method. A single-component coverage gradient, generated by gradual immersion of a gold surface into a solution of a single thiol, yielded islands of ∼25 nm in diameter at the end that had only been briefly immersed, whereas an increasingly continuous film was formed along the gradient. After saturation with a second thiol with a different end group, the structure generated during the initial immersion step was found to persist.
1. Introduction Self-assembled monolayers (SAMs) are often used as model systems to investigate surface interactions and are applied in such diverse fields as biological sensing, molecular-device fabrication, diagnostics, and nanotribology.1-7 SAMs have attracted a lot of attention because of their spontaneous formation from solution, high degree of order due to van der Waals interchain interactions, stability, and well-defined surface properties. The most widely investigated SAMs have been prepared from alkanethiols or disulfides, adsorbed on gold surfaces.2,8,9 A full monolayer of chemisorbed, simple alkanethiol chains is known to display a (x3 × x3) R30° super-lattice structure on a Au(111) substrate.10 The evolution of such phases during SAM growth has been studied in detail by STM. Differently packed, “lying-down” phases have been found in the initial growth stage before the final well-packed, “standing-up” phase is formed.10-12 Submonolayer-coverage islands have also been employed as a model system to study the structure of alkanethiol SAMs under varying applied loads.13,14 The islands were found to ripen over time with an increase in stability. Mixed monolayers are, however, more interesting for applications since their properties can be readily tailored by choosing appropriate mixing * To whom correspondence should be addressed.
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(1) Ulman, A. Introduction to Ultrathin Organic Films: from LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169. (3) Mrksich, M. Chem. Soc. ReV. 2000, 29, 267-273. (4) Flink, S.; van Veggel, F.; Reinhoudt, D. N. AdV. Mater. 2000, 12, 13151328. (5) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133-135. (6) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071-2074. (7) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. ReV. Mater. Sci. 1997, 27, 381-421. (8) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483. (9) 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. (10) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (11) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G. Y.; Jennings, G. K.; Yong, T. H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002-5012. (12) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855-861. (13) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 1999, 111, 97979802. (14) Barrena, E.; Palacios-Lidon, E.; Munuera, C.; Torrelles, X.; Ferrer, S.; Jonas, U.; Salmeron, M.; Ocal, C. J. Am. Chem. Soc. 2004, 126, 385-395.
ratios of two different thiols. The structure of mixed SAMs prepared by coadsorption of two components depends on the mixing ratio, the solvent used for preparation, the nature of the molecular backbones and terminal groups, as well as the difference in length.15 It is generally known, and has been shown by molecular dynamics simulations, that if alkanethiols with a chainlength difference of more than three CH2 groups are coadsorbed, phase separation occurs.16 This phase separation is due to increased van der Waals interaction between the longer chains and consequent preferential aggregation between them16-21 and can be inhibited by performing the coadsorption at higher solution temperatures.22 Differences in the chemical nature of backbone chains and end groups can also lead to phase separation,23-25 usually due to increased hydrogen bonding between one of the two components. Most authors report that no phase separation has been observed for mixed SAMs composed of two components with different end groups, such as -CH3, -OH, -COOH, or -CN,9,26 as long as the backbone chain length remains the same, although Brewer et al. claimed phase separation in mixed OHand CH3-terminated SAMs of similar length by means of atomic force microscopy (AFM) pull-off force measurements.27 A mixed monolayer whose surface composition steadily changes along a sample constitutes a surface-chemical gradient. Several gradient preparation methods for alkanethiols have been (15) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563-571. (16) Shevade, A. V.; Zhou, J.; Zin, M. T.; Jiang, S. Y. Langmuir 2001, 17, 7566-7572. (17) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882-3893. (18) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 13301341. (19) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (20) Shon, Y. S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278-1281. (21) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 15581566. (22) Chen, S. F.; Li, L. Y.; Boozer, C. L.; Jiang, S. Y. J. Phys. Chem. B 2001, 105, 2975-2980. (23) Nyquist, R. M.; Eberhardt, A. S.; Silks, L. A.; Li, Z.; Yang, X.; Swanson, B. I. Langmuir 2000, 16, 1793-1800. (24) Smith, R. K.; Reed, S. M.; 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. (25) Stranick, S. J.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (26) Stranick, S. J.; Atre, S. V.; Parikh, A. N.; Wood, M. C.; Allara, D. L.; Winograd, N.; Weiss, P. S. Nanotechnology 1996, 7, 438-442. (27) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109-4115.
10.1021/la0526840 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/07/2006
Submicrometer Structure of Surface-Chemical Gradients
developed and gradients with compositional changes over a few micrometers to centimeters have been prepared.28-33 The macroscopic properties of these gradients have been well characterized by X-ray photoelectron spectroscopy (XPS),29,33 contact-angle measurements,29,33 ellipsometry,29 infrared spectroscopy,29 scanning tunneling microscopy (STM),32 and fluorescence microscopy.31 To our knowledge, information on the microstructure of such gradients has not been reported. However, microstructure is of great importance when such gradients are applied in protein-adsorption studies or in biological sensing; Huang et al. have shown that the microstructure of mixed ω-functionalized monolayers strongly affects the adsorption of proteins.34 Atomic force microscopy, especially lateral force microscopy (LFM), has proven to be useful when mixed or patterned SAMs with different end functionalities need to be characterized.35 Interactions between the AFM tip and the SAM strongly depend on the nature of the end group. Given that other structural parameters are identical, the more hydrophilic the end group of the SAM, the higher the friction force measured against a regular Si3N4 AFM tip under ambient conditions. This friction contrast may be enhanced by the use of a chemically modified tip,6,7,36 although tip modifications may result in the broadening of the tip geometry, which itself leads to a degradation of resolution. In this study, we present LFM measurements with a regular Si3N4 tip on surface-chemical gradients prepared from methyland hydroxyl- or carboxyl-terminated alkanethiols by a two-step immersion procedure.33 During the first step, one type of alkanethiol is adsorbed with a gradient in surface concentration onto a gold surface by a gradual immersion process. This singlecomponent gradient is saturated with a component of different end functionality in a second, rapid immersion process to create a densely packed, two-component monolayer. In the first section, we describe the island structure of single-component gradients and later show that this structure persists after saturation with the second component. 2. Experimental Section Chemicals. 1-Dodecanethiol (98+%, Aldrich Chemicals, St. Louis, MO), 11-mercapto-1-undecanol (97%, Aldrich Chemicals, St. Louis, MO), 1-mercapto-undecanoic acid (95%, Aldrich Chemicals, St. Louis, MO), H2SO4 (95-97%, Merck, Germany), and H2O2 (30%, Merck, Germany) were used as received. Ethanol (analytical grade, Scharlau Chemicals SA, Spain) was used as a solvent for all experiments. Water was purified using a Milli-Q water system (Millipore, Billerica, MA). Preparation of Gold Surfaces. Template-stripped gold surfaces were used for AFM measurements. They were prepared according to the procedure reported by Wagner et al.37 A freshly cleaved mica sheet (50 mm × 80 mm) was mounted on a heating stage in a Bal-Tec BAE-370 vacuum coating system (Bal-Tec, Lichtenstein). The mica sheet was preheated for 12 h at 300 °C to outgas adsorbed (28) Ruardy, T. G.; Schakenraad, J. M.; van der Mei, H. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 3-30. (29) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821-3827. (30) Terrill, R. H.; Balss, K. M.; Zhang, Y. M.; Bohn, P. W. J. Am. Chem. Soc. 2000, 122, 988-989. (31) Jeon, N. L.; Dertinger, S. K. W.; Chiu, D. T.; Choi, I. S.; Stroock, A. D.; Whitesides, G. M. Langmuir 2000, 16, 8311-8316. (32) Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. AdV. Mater. 2002, 14, 154-157. (33) Morgenthaler, S.; Lee, S.; Zu¨rcher, S.; Spencer, N. D. Langmuir 2003, 19, 10459-10462. (34) Huang, Y. W.; Gupta, V. K. J. Chem. Phys. 2004, 121, 2264-2271. (35) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825-831. (36) Headrick, J. E.; Berrie, C. L. Langmuir 2004, 20, 4124-4131. (37) Wagner, P.; Hegner, M.; Gu¨ntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867-3875.
Langmuir, Vol. 22, No. 6, 2006 2707 water and other volatile contaminants. In a first step, a 5 nm layer of gold (99.99%, Umicore Materials AG, Lichtenstein) was deposited at a rate of 1 Å/s and a pressure of below 1 × 10-6 mbar. After the sample was annealed for 6 h at 300 °C, another 195 nm gold layer was deposited. The thin film was cooled at a rate of 10 °C/min and cut into 10 mm × 40 mm and 10 mm × 10 mm samples. Si wafers (POWATEC GmbH, Switzerland) were cut and precleaned twice by ultrasonication in toluene and ethanol and dried with pure nitrogen. The gold-coated mica sheets were then glued, gold face down, onto the Si slides with an epoxy resin (Epo-tek 377, Polyscience AG, Switzerland) and cured at 150 °C for more than 2 h. Immediately before use, the mica was detached mechanically from the gold by tweezers. Microdroplet density measurements (µDD) were carried out on polycrystalline gold films. Si wafers were precleaned as described above. Additionally, they were cleaned for 10 min in piranha solution (7:3 concentrated H2SO4/30% H2O2), rinsed with plenty of water and dried with nitrogen. Attention: Piranha solution reacts Violently with all organics and should be handled with care! The Si wafers were then coated with a 6 nm chromium adhesion layer followed by an 80 nm gold film in an evaporation chamber (MED 020 coating system, Bal-Tec, Liechtenstein) at a pressure of about 2 × 10-5 mbar. Before use, the polycrystalline substrates were cleaned by ultrasonication in ethanol followed by 30 s of air plasma treatment (PDC-001, Harrick Scientific Corporation, NY) and another 10 min immersion in ethanol to remove gold oxides that were produced during plasma treatment.38 Gradient Preparation and SAM Formation. Gradients were prepared on 10 mm × 40 mm samples according to the previously reported procedure.33 The gold-coated substrates were mounted onto a computer-controlled, linear-motion drive (OWIS GmbH, Germany), which allowed them to be slowly immersed into a dilute ethanolic alkanethiol solution. The first gradual adsorption step was carried out in 0.005 mM dodecanethiol solutions. The substrate was immersed at a speed of 75 µm/s. Following total immersion, it was removed quickly from the solution, rinsed abundantly with ethanol, and dried with nitrogen. The substrate was then immersed overnight in a second 0.01 mM solution of either 11-mercapto-1-undecanol or 1-mercaptoundecanoic acid. Both single- (after the first step) and two-component (after the second step) gradients were then analyzed with AFM. As a control, mixed monolayers were prepared in two different ways on 10 mm × 10 mm samples; either they were coadsorbed from a mixed 0.01 mM solution, or they were prepared in two subsequent immersion steps by mimicking the gradient preparation method described above. In the coadsorption case, the composition of the monolayers was controlled by varying the mixing ratio of the two components in solution. For the two-step case, the substrate was immersed completely into a 0.005 mM dodecanethiol solution for a given time, and then it was saturated overnight in a 0.01 mM solution of a different alkanethiol. As another control, a microcontact printed pattern composed of two alkanethiols was generated with a poly(dimethylsiloxane) (PDMS) stamp as described by Xia et al.39 Contact-Angle Measurements. Static contact angles were measured at room temperature and ambient humidity along the gradient samples. Single measurements of sessile drops were performed every 5 mm along the substrate with a contact-angle goniometer (Rame´ Hart model 100, Rame´ Hart, Inc., Mountain Lakes, NJ). One drop of Milli-Q water was 6 µL. All contact-angle measurements were averaged over several samples. Lateral Force Microscopy. LFM measurements were performed with a commercial AFM (Dimension 3000, Veeco Instruments, Plainview, NY) equipped with a standard scanner. The images were obtained under ambient conditions using a regular V-shaped Si3N4 cantilever and tip (Veeco Instruments Inc., Plainview, NY) with a spring constant of 0.12 N/m. Images were recorded every 5 mm along the sample length, and at least three measurements were taken by changing the position across the sample width at a given length. Height and friction images were recorded simultaneously for each (38) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566-4573 (39) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551-575.
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Morgenthaler et al. point. The images were flattened using installed software (Veeco Instruments ver. 5.12, Plainview, NY) before further analysis was performed. Friction histograms were plotted for each image and were fitted with two peaks. Best fits were obtained with a 100% Lorentzian curve and a fixed full width at half-maximum (fwhm). Peak areas were used to estimate low- and high-friction areas. Microdroplet Density. µDD measurements were performed as described by Hofer et al.40 Briefly, the sample was placed onto a metal stage in a transparent humidity chamber. While the metal stage was cooled by iced water, the nucleation and growth of waterdroplets was recorded using a CCD camera (model WV-BP 310/6, Matsushita Communication Deutschland GmbH, Germany) coupled to a microscope (Carl Zeiss (Schweiz) AG, Switzerland). Images at the point of droplet nucleation were saved and the number of droplets determined by means of digital image analysis.
3. Results and Discussion
Figure 1. Topography (left) and friction (right) images (1 µm × 1 µm) from contact-mode AFM along a single-component (dodecanethiol) gradient from the briefly to the lengthily immersed end. Island structures are visible both in topography and friction images. The island coverage increases toward the end that was immersed for a longer time (bottom). Note that, while the same dynamic range is employed in each topographical or friction image, the offset has been automatically adjusted by the SPM software to preserve the same mean value. Thus, absolute colors cannot be compared from one end of the gradient to the other.
Structure of Single-Component Gradients (after First Step). Single-component gradients were prepared as described in the Experimental Section. A single-component gradient consists of an incomplete methyl-terminated molecular film whose surface concentration changes along the sample. Dodecanethiol was selected as the first component to be adsorbed in order to minimize replacement occurring during the second step; dodecanethiol replaces mercapto-undecanol more effectively than the other way around. Figure 1 shows topographic and friction contrast images along a single-component gradient obtained by LFM. Islands can be distinguished from the underlying substrate on both the topography and the friction images. The triangular structures of the underlying Au(111) terraces appearing at 5 and 25 mm from the briefly immersed end can be detected through the organic layer. Thus, the friction images were employed for quantitative analysis. Individual islands with clear boundaries can be found at the low-concentration end, whereas these islands coalesce with increasing surface density of dodecanethiol. The low-friction (darker) areas are attributed to densely packed islands of alkanethiols, which display a much lower friction force than the underlying gold or a disordered film of similar thiols.41 We assume that the regions around the islands are not bare gold but are covered with a layer of lying-down alkanethiol molecules, as has been reported by Barrena et al.14 The islands showed an average height of around 10 Å above the surrounding region, which corresponds well with this picture. The fraction of surface covered by islands was estimated from an analysis of friction-force histograms. The friction-force histograms were analyzed for their lower- and their higher-friction components, assuming that the underlying gold is completely covered with a layer of lying-down molecules. The peak areas for the lower friction contribution were normalized with the total peak intensity and are plotted in Figure 2. The inset in Figure 2 shows a fit of a histogram taken at 20 mm from the briefly immersed end. Two peaks can clearly be distinguished, best fits being obtained by keeping the peak shape (Lorentzian) and the fwhm constant. The amount of the lower-friction component increases toward the end that was immersed for longer, which agrees with an increased coverage at longer immersion times. A Langmuir-type adsorption isotherm (solid line) fits the curve well, and thus we can conclude that island growth follows Langmuir-type adsorption behavior. However, this growth behavior does not correspond directly to the overall adsorption kinetics of dodecanethiol from a 5 µM solution. Structure of Two-Component Gradients (after Second Step). The single-component gradients are saturated with the (40) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4123-4125. (41) Carpick, R. W.; Salmeron, M. Chem. ReV. 1997, 97, 1163-1194.
Submicrometer Structure of Surface-Chemical Gradients
Figure 2. Fraction of lower friction component along singlecomponent gradients determined by histogram analysis. The lowerfriction contribution increases from the left to the right end showing an increasing coverage. The inset shows the peak fitting of the friction histogram at 20 mm from the end. A Langmuir-type isotherm fits the data well.
Figure 3. Images from lateral force microscopy displaying the structure of the gradients at 20 mm from the ends (a, single-component gradient; b, two-component gradient (OH-terminated); c, twocomponent gradient (COOH-terminated)). Overview images (top, 1 µm × 1 µm) and zoomed-in images (below, 0.1 µm × 0.1 µm) are presented. An island-like structure is found in all three images.
second component during a second adsorption step. Frictionforce images from three gradients are compared in Figure 3. Images at 20 mm from the ends of a single-component gradient (a), a two-component gradient saturated with OH-terminated
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Figure 5. Microdroplet density measurements on gradients (squares) and mixed monolayers prepared by coadsorption (circles). The microdroplet density changes little with contact angle for homogeneously mixed monolayers; however, it changes significantly with contact angle for a gradient.
alkanethiols (b), and another two-component gradient saturated with COOH-terminated alkanethiols (c) are shown. A similarly inhomogeneous structure can be observed in all three images. Islands with clear boundaries are visible on a single-component gradient. These clear boundaries blur out when the gradient is saturated with a second component. However, the shape and size of the islands remain (Figure 3, zoomed-in images). Comparable behavior is observed in both hydroxyl- and carboxyl-terminated, two-component gradients. In Figure 4a, a microcontact-printed pattern of dodecanethiol backfilled with mercapto-undecanol on polycrystalline gold is displayed to show the feasibility of using lateral-force images to distinguish between spatially segregated methyl- and hydroxylterminated regions. The hydroxyl-terminated region displays higher friction than the methyl-terminated region, as has been previously reported. 39 In Figure 4b, a two-component gradient is compared with mixed monolayers (Figure 4c-d). Mixed monolayers can be prepared either by two immersion steps (c), similar to the gradient preparation technique, or by coadsorption of the two components from a mixed solution (d). Contact angles were measured on all three samples (51° on average) and differ by only (3°, which implies that the overall composition of all three monolayers is very similar. The inhomogeneity of a gradient sample (b) is comparable with the inhomogeneity of a mixed monolayer prepared by two steps (c). However, a mixed
Figure 4. Comparison of friction force images of gradients and mixed monolayers prepared by different techniques. Image (a) shows a microcontact-printed pattern of dodecanethiol backfilled with mercapto-undecanol. Image (b) displays the friction force image for a twocomponent gradient (OH-terminated) at 20 mm from the end and images (c) and (d) show mixed monolayers (c, prepared by two successive adsorption steps; d, prepared by coadsorption of two thiols from a mixed solution). All images are 1 µm × 1 µm in size. Coadsorbed monolayers show more homogeneous friction images than monolayers prepared in two steps.
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Figure 6. Sketch of the proposed structure for both a single-component (A) and a two-component gradient (B). A 4 cm-long, singlecomponent gradient (a coverage gradient), transitions from a densely to a sparsely covered end (A, I). On a smaller scale, islands are observed (A, II). A layer of disordered alkanethiols surrounds the islands, sometimes leaving free gold crevices at grain boundaries (see clear gray spots in top view). When the incomplete gradient is saturated, a well-ordered SAM with a gradually changing end-functionality is generated (B, I). The alkanethiol islands remain mainly intact, while the flat-lying molecules arrange with the second component to form a well-ordered mixed monolayer (B, II).
monolayer prepared by coadsorption definitely shows a more homogeneous microstructure. From Figure 3, we conclude that the island-type structure observed after the first immersion step persists during the saturation of the gradient, as the island size and shape remain unchanged. We have confirmed that a clear difference between coadsorbed methyl- and hydroxyl-terminated alkanethiols and mixed monolayers prepared by two steps or gradients exists (Figure 4b-d). A higher friction force contrast was observed on microcontact printed dodecanethiol monolayers, backfilled with mercapto-undecanol. We attribute the reduced friction force contrast in two-step or gradient monolayers to the fact that after the first adsorption step a phase of lying-down molecules is present between the islands. These molecules arrange themselves into a mixed, well-packed monolayer with the second component during the second adsorption step. Thus, a mixed monolayer of methyl- and hydroxyl-terminated alkanethiols forms the background to the methyl-terminated islands generated during the first immersion step. Hofer et al. used microdroplet density measurements as a simple and powerful tool to determine the homogeneity of hydrocarbonbased SAMs.40 They investigated mixed alkyl phosphate monolayers and found that the microdroplet density does not depend on the hydrophobicity of the substrate but on the microstructure. Incomplete monolayers showed a high microdroplet density, whereas for mixed monolayers prepared by coadsorption, no change in microdroplet density was found when varying the mixing ratios. We used microdroplet density to determine the microstructure of gradients and mixed monolayers with different contact angles prepared by coadsorption (Figure 5). Similarly to the studies on alkyl phosphate SAMs, mixed alkanethiol SAMs prepared from coadsorption revealed no significant change in the microdroplet density as a function of water contact angle. However, on a gradient, the microdroplet density increases toward the more hydrophilic end. We have shown elsewhere by polarization-modulation infrared spectroscopy (PM-IRRAS) that gradient samples prepared by the two-step method exhibit a highly ordered structure along the gradient and do not show noticeable differences when compared
with single or mixed, fully covered monolayers.42 We thus conclude that the change in microdroplet density of the gradients must originate from inhomogeneities in the composition of topmost terminal groups. The microdroplet density at the more hydrophobic end is similar to that of a mixed monolayer of the same wettability, suggesting that the microstructure of a gradient at this position is similar to that of a mixed monolayer prepared by coadsorption. After the first immersion step into dodecanethiol solution, the gradient microstructure is almost homogeneous at the high concentration end (Figure 1, 35 mm). Thus, the second component only fills up defect sites and is distributed homogeneously over the substrate, the resulting surface resembling a homogeneously mixed monolayer of the same composition. However, islands of tens of nanometers in size are found toward the briefly immersed end (Figure 1, 10 mm), which remain when the sample is saturated with a second component. This structure clearly differs from a homogeneously mixed monolayer of the same composition since, for a coadsorbed monolayer, the lower friction component would be homogeneously distributed in the monolayer. The microdroplet density is thus much higher for the more inhomogeneous monolayer on the gradient sample. The difference between the structure of a coadsorbed monolayer and a gradient increases toward the more hydrophilic end, and thus the difference in microdroplet density also increases. A sketch of the proposed structure for both the singlecomponent (A) and the two-component gradient (B) is given in Figure 6. Side and top views are sketched on the molecular scale.
4. Conclusions Lateral force microscopy and microdroplet density measurements have revealed that the structure of chemical gradients prepared by a two-step immersion procedure33 is inhomogeneous on the
