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Langmuir 1998, 14, 2348-2351
Effect of Nanometer-Scale Phase Separation on Wetting of Binary Self-Assembled Thiol Monolayers on Au(111) Shin-ichiro Imabayashi, Narutoshi Gon, Takayuki Sasaki, Daisuke Hobara, and Takashi Kakiuchi*,† Department of Physical Chemistry, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Received December 15, 1997. In Final Form: February 11, 1998 The advancing contact angle of water has been measured on three types of the binary self-assembled monolayers (SAMs) on Au(111) with known surface structure at nanometer scale: a homogeneously mixed SAM of undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUDA) (1), a SAM of hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) phase-separated into nanometer-scale domains with different heights (2), and a SAM of UDT and MUDA artificially phase-separated into nanometer-scale domains having approximately equal heights (3). The homogeneously mixed SAM, 1, shows a linear variation of COOH , while the cosine of the contact angle, cos θ, with the surface mole fraction of COOH-terminated thiol, χsurf COOH COOH plot is strongly concave; the cos θ depends little on χsurf for in the case of 2 the cos θ versus χsurf COOH COOH COOH χsurf < 0.5 and abruptly increases with χsurf for χsurf > 0.5. In contrast, the phase-separated SAM, COOH , as is the case of 1, indicating that the contact angle does 3, shows a linear variation of cos θ with χsurf not reflect the phase separation in nanometer scale. The difference in the height of the domains of the COOH plot for 2. The Young-Laplace two thiols is the primary factor for the nonlinear cos θ versus χsurf equation explains the nonwetting of the lower domains of 2.
Introduction The wetting of solid surfaces by liquids is relevant to a wide spectrum of physicochemical phenomena and is eventually to be interpreted in terms of intermolecular forces. The molecular-level understanding of wettability is a challenge in contemporary physical chemistry of surfaces.1,2 On the other hand, many industrial applications require the spreading of liquids such as a paint, a lubricant, an ink, and a dye on solid surfaces or the water repellency of surfaces. The control of wetting properties of solid surfaces is therefore both scientifically interesting and technologically important. Mixed self-assembled monolayers (SAMs) formed by the coadsorption of two or more alkanethiols with varied tail groups and chain lengths offer the possibility of engineering metal surfaces with the wettability tailored at the molecular level.3-18 * To whom correspondence should be addressed. Fax: +81-75753-3360. E-mail:
[email protected]. † Present address: Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan. (1) Adamson, A. W. Physical Chemsitry of Surfaces, 3rd ed; Wiley: New York, 1976. (2) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827-863. (3) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (4) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995. (5) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (6) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7165-7175. (7) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (8) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378. (9) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (10) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167-3173. (11) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (12) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330-1341.
The wetting properties of binary SAM surfaces are determined by the tail groups of the components,4-18 the mixing ratio of the components on surfaces,4-18 and the difference in the chain length between the two components.6,9-18 While the degree of phase separation of two thiol components is considered to be one of the factors which affect the wetting of mixed SAMs,9,11,12,16 the effect of the nanometer-scale phase separation of thiols on the wetting of mixed SAMs has not been fully understood because of the lack of the detailed knowledge of the nanometer-scale phase properties of SAM surfaces. We recently found the binary SAMs showing different phase properties at the molecular level. In SAMs of hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) prepared by the coadsorption of the two thiols, HDT and MPA are phase-separated into two types of domains; a minor thiol component is phase-segregated into domains of 5-20 nm across.19 Homogeneously mixed SAMs are formed, on the contrary, by the coadsorption of undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUDA).20 We also proposed an electrochemical method to selectively replace one of two thiols in phase-separated binary SAMs.21 This new approach enables us to prepare a phase-separated binary SAM which would form a (13) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380-1382. (14) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156161. (15) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499-1506. (16) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E. Adv. Colloid Interface Sci. 1992, 39, 175-224. (17) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761-766. (18) Offord, D. A.; John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994, 10, 883-889. (19) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem., in press. (20) Kakiuchi, T.; Sato, K.; Iida, M.; Hobara, D.; Imabayashi, S.; Niki, K. Manuscript in preparation. (21) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504.
S0743-7463(97)01377-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/03/1998
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homogeneously mixed SAM at thermodynamic equilibrium (e.g., the binary SAM of UDT and MUDA). These binary SAM systems in which the phase properties are well-characterized at the molecular level are suitable to elucidate the wetting in terms of the surface microstructure. In the present work, we measured the advancing contact angle of water on these SAM surfaces as a function of the composition of the SAM. We will discuss the effect of the phase separation on the contact angle by comparing the wetting on the homogeneously mixed and the artificially phase-separated SAMs of UDT and MUDA. Experimental Section Materials. n-Alkanethiols (UDT and HDT) and MPA were purchased from Aldrich and Dojin Chemical Laboratory, respectively, and were used without further purification. MUDA was synthesized from 11-bromoundecanoic acid.22 Water was distilled and purified with a Milli-Q system (Millipore Co.). All other chemicals were of reagent grade and used without further purification. Gold substrates were prepared by vapor deposition of gold (99.99% purity) onto a freshly cleaved mica.23 Sample Preparation. The homogeneously mixed SAMs of UDT and MUDA (1) and the binary SAMs of HDT and MPA (2) were prepared by the coadsorption of two thiols by immersing a gold substrate for 25 ( 5 h in an ethanolic solution of the corresponding alkanethiols at the total thiol concentration being 1 mM. The substrate was then rinsed with ethanol and dried in air. The mixing ratio of the two thiols in the mixed SAMs was controlled by changing the composition of a mixed-thiol solution as reported elsewhere.19,20 The phase-separated mixed SAMs of UDT and MUDA (3) were prepared through the selective replacement of MPA with MUDA in the initially formed UDT/ MPA binary SAMs (4) by the coadsorption of the two thiols.24 The MPA domains were partially desorbed by holding the potential of the mixed SAM-adsorbed gold substrate at - 0.7 V for 45 min in 0.5 M KOH. MUDA was subsequently adsorbed where MPA was initially adsorbed by immersing the substrate in 1 mM ethanolic solution of MUDA for 3 h.24 Surface Composition and Phase Properties of Binary SAMs. The three types of phase-separated binary SAMs, 2, 3, and 4, exhibit two peaks for the reductive desorption of the binary SAM, both of which were close to the corresponding peak potentials for the reductive desorption, Ep, of single-component SAMs of the two thiols. The presence of two peaks in the cyclic voltammogram reflects the presence of two types of domains, each of which is predominantly composed of either thiol, and scanning tunneling microscopy (STM) images indicate that the domain size of the minor thiol is 5-20 nm across.19,24 The surface mole fraction of the COOH-terminated thiol, χCOOH surf , was estimated from the area of the two peaks. For the homogeneously mixed SAM, 1, where only one peak appears at a potential between the two Ep’s for the corresponding single-component SAMs, the χMUDA was estimated from the Ep.20 A SAM-adsorbed surf gold substrate was mounted at the bottom of a cone-shaped cell by using an elastic O-ring whose diameter was 8 mm.23,25 All CVs for the reductive desorption of a part or the entire SAM were measured at the sweep rate, v ) 20 mV s-1, in a deaerated 0.5 M KOH at 20 ( 2 °C. All potentials were referred to a Ag|AgCl|saturated KCl electrode. Contact Angle Measurements. A drop of 10 mM phosphate buffer solution at pH 7 was put on the surface of a SAM-adsorbed gold substrate with a micropipet. The volume of a drop was usually 4 µL. A drop of 2 µL was used when the contact angle was less than 30°. The advancing contact angle of a water drop on a gold substrate was determined from the circumference of the drop, which was acquired with a video camera. The image (22) Bain, C. D.; Trooughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (23) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (24) Hobara, D.; Sasaki, T.; Imabayashi, S. K.; Kakiuchi, T. Manuscript in preparation. (25) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.
Figure 1. Advancing contact angle of water, cos θ, plotted against the mole fraction of COOH-terminated alkanethiol, χCOOH surf , for phase-separated binary SAMs of HDT and MPA (9) and homogeneously mixed binary SAMs of UDT and MUDA (O). The solid line was described using the Cassie equation. was digitized and the circumference was extracted after thresholding the gray level of each pixel. The values of the contact angle were averaged for at least triplicate measurements taken at different positions of a substrate. All measurements were made at 20 ( 2 °C and ambient humidity.
Results and Discussion Figure 1 shows the variation of cos θ as a function of χCOOH for two binary SAMs prepared by the coadsorption surf of two thiols. The error bars indicate 90% confidence interval. The cos θ on the homogeneously mixed SAM, 1 (plot A). This varia(O), linearly increased with χMUDA surf tion of cos θ is similar to that predicted by the Cassie equation26 (solid line in Figure 1). On the other hand, the cos θ on the phase-segregated binary SAM, 2 (9), little MPA depended on χMPA surf for χsurf < 0.5 and abruptly increased MPA MPA with χsurf for χsurf > 0.5 (plot B). STM images of the SAM surfaces of 2 indicated that the average size of the MPA MPA domains increases with χMPA surf for χsurf < 0.5. The inversion of the phases of the two thiols (i.e., the change in the continuous phase from the HDT-rich domain to the MPA19 The fact rich domain, takes place around χMPA surf ) 0.5. that the cos θ starts to increase in the range of χMPA surf where the inversion of the two thiol phases occurs suggests that water does not wet MPA domains when they are isolated with each other surrounded by HDT domains. One conceivable factor which causes the different cos plots for the two binary SAMs is the effect θ versus χCOOH surf of the domain size. In fact, Drelich et al. reported that the advancing contact angle of water on phase-separated heterogeneous surfaces in micrometer size was 7-10° lower than that calculated from the Cassie equation due to a corrugation of the three-phase contact line.27 On the other hand, the homogeneously mixed SAM, 1, is wellrepresented by the Cassie equation (Figure 1). The size of the phase-separated domains thus influences the wetting characteristics, and the phase separation in nanometer scale in the SAM, 2, might cause the observed large deviation (plot B) from the Cassie equation. Another plots possible factor for the different cos θ versus χCOOH surf is the height difference of the two thiol domains in the SAM, 2. Folkers et al. observed a similar concave plot of cos θ versus χOH surf for the binary SAM composed of 11-mercaptoundecanol and docosanethiol.12 They hypothesized that the OH groups are partially buried in the SAM and the wetting of the mixed SAM is insensitive to (26) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11-16. (27) Drelich, J.; Miller, J. D.; Kumar, A.; Whitesides, G. M. Colloids Surf., A 1994, 93, 1-13.
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Figure 2. Advancing contact angle of water, cos θ, plotted against the mole fraction of MUDA, χMUDA surf , for homogeneously mixed (O) and phase-separated (b) binary SAMs of UDT and MUDA. The solid line was described using the Cassie equation.
the OH groups, although the phase properties of the SAM in nanometer scale was not known. In the mixed SAMs prepared by the coadsorption of two alkanethiols, we cannot change the height difference of the two thiols and the state of mixing independently, as the phase behavior depends on the relative chain length of the alkyl chains as well as the type of the terminal group. Two thiols having similar alkyl chain lengths tend to form a homogeneously mixed monolayer, and it is impossible to prepare by the coadsorption a phaseseparated monolayer where two domains have similar heights. To examine the effect of the domain size on the contact angle separately from the effect of the height difference between the two thiol domains, we measured the contact angle on artificially phase-separated SAMs, 3, prepared by the selective replacement method.21 The for 3 (b in Figure 2) dependence of cos θ on χMUDA surf (plot C). exhibited a linear increase in cos θ with χMUDA surf The little difference between the plot A (replotted in Figure 2 as O) and the plot C means that the contact angle is not appreciably affected by the phase separation in which the domain size of the minor component is 5-30 nm.24 No significant deviation of the plot C from the Cassie equation clearly demonstrates that a corrugation of the three-phase contact line on heterogeneous patches in nanometer scale is not macroscopically detected as the change in θ. In Figure 2, the cos θ values for 3 tend to be greater for χMUDA < 0.4 than those for 1. These larger cos θ values surf (i.e., greater wettability) are likely to be ascribed to the through the exchange of UDT in the increase in χMUDA surf partially desorbed SAM with MUDA in an ethanolic solution during the preparation of 3. Further studies, however, will be required to determine the degree of the exchange of UDT with MUDA and the resulting change in the wetting of the SAM. The remaining factor which can bring about the plot (plot B) is the difference nonlinear cos θ versus χCOOH surf in the height of the thiol domains. We observed a similar nonlinear behavior for the binary SAMs, 4, as shown in Figure 3 (0). Folkers et al.’s data for the SAM of docosanethiol and 11-mercaptoundecanol are also reproduced in Figure 3 (∆),12 together with the data for the SAM, 2 (9). These three binary SAMs, where the lower domains consist of hydrophobic COOH or OH-terminated thiols, thus show a similar curvature, although the height difference between the higher and the lower thiol domains in 4 and the SAM of docosanethiol and 11-mercaptoundecanol is smaller than that in 2. The differences of 8 and 10 methylene units for 4 and the SAM of docosanethiol and 11-mercaptoundecanol correspond to the height
Imabayashi et al.
Figure 3. Advancing contact angle of water, cos θ, plotted against the mole fraction of the shorter alkanethiol, χshort surf , for binary SAMs of HDT and MPA (9), of UDT and MPA (0), and of docosanethiol and 11-mercaptoundecanol (∆).12
differences of 1 and 1.25 nm, respectively, for the thiol molecules oriented 30° from the surface normal.22,28-32 There are, in general, two possible explanations for the insensitivity of the contact angle for the SAM. First is the trap of air in the lower domains and second is the disorder of the outer part of the higher domains, projecting the longer thiols over the shorter thiols in the lower domains.6,11,12 This disorder can result in the exposure of the methylene part of the longer alkyl chains, making the lower domains more hydrophobic.6,11,12 The latter explanation does not seem to apply well to the case of 2. According to STM imaging of 2, most MPA domains are 5-20 nm across when χMPA surf < 0.5, where MPA is the minor component.19 The bending of the longer alkyl chains cannot fully cover the MPA domains whose diameter is mostly greater than 5 nm, and hence the significant portion of the lower domain should remain to be hydrophilic. The remaining explanation, the trapping of air bubbles in the lower domains, is not, however, self-evident and needs to be examined quantitatively. The trapping of air is believed to occur in the case of rough surfaces where the liquid advances over a roughness asperity and intercepts the next asperity, leaving air trapped between the two.1 The depth between the higher and the lower domains of 2 is no greater than 1.6 nm,22,28-32 which is much less than the typical diameter of the lower MPA domains, which we hereafter call pits for simplicity. In order that water advances without touching the bottom hydrophilic part, the capillary force supporting water from the edge of the pits should be large enough to prevent the touching of water with the bottom for the pits having the low aspect ratios. In our system 2, the inner wall of the pits is made of lipophilic hydrocarbon chains and is not therefore wetted by water. Then, a certain pressure must be applied, as water does not spontaneously penetrate in. This wetting problem would be likened to the compression of a liquid into a porous plug.33 The pit may be modeled as a capillary having the radius r. Then, the depth of the meniscus can be correlated with the surface tension through the YoungLaplace equation.1,2 When r is small and the shape of the (28) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (29) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-558. (30) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-564. (31) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (32) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (33) Bartell, F. E.; Walton, C. W., Jr. J. Phys. Chem. 1934, 38, 503511.
Effect of Nanometer-Scale Phase Separation
meniscus is approximated to be spherical, the YoungLaplace equation reduces to
∆p ) 2γ cos θ/R where ∆p is the difference in the pressure across the water-vapor interface, γ is the surface tension of the liquid, and R is the radius of curvature. In order that water can penetrate into the capillary, ∆p should be greater than 2γ cos θ/R. The depth of the water drop, h, measured from the edge of the capillary is geometrically related with r and R through
R)
h 2 + r2 2h
When h ) 1.5 nm and r ) 10 nm, R becomes 34 nm. If we use 72 mN m-1 for γ, ∆p is then ∼42 cos θ atm. Unless θ is not too close to π/2, the hydrostatic pressure exerted by a water drop on the substrate is much less than this value. This semiquantitatively accounts for the nonwetting of the bottom part of the pit, even though its aspect ratio is very small. The observed anomaly in the dependence of cos θ on the composition for 2 underlines the importance of the relative height of the domains, aside from the wettability of a single-component SAM, in designing the wettability using multicomponent SAMs. Troughton et al. found that the
Langmuir, Vol. 14, No. 9, 1998 2351
wettability of homogeneously mixed SAMs prepared from asymmetrical dialkyl sulfides with a long methyl- and short calboxylic acid-terminated alkyl chains is insensitive to that of buried groups.34 In the present study, this insensitivity holds even when the two thiol species are segregated into two types of domains of several tens of nanometers in diameter. This is intriguing in view of Drelich’s finding that the contact angle on phase-separated heterogeneous surfaces in micrometer size was 7-10° lower that that calculated from the Cassie equation.27 The present study also showed the insensitiveness of the domain size up to several tens of nanometers across for the wetting of binary SAMs composed of thiols having the similar lengths. These findings address the problem of finding a critical domain size which gives rise to the macroscopically distinguishable wetting behavior. Acknowledgment. Partial support of this research was provided by the Saneyoshi Scholarship Foundation (S.I.), Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (D.H.), the Ministry of Education, Science and Culture, Japan (T.K., Grant-in-Aid for exploratory research, No. 09875208), and Casio Science Promotion Foundation (T.K.). LA971377U (34) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385.
