Self-Assembled Monolayers of Alkanethiolates on Thin Gold Films as

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Langmuir 1998, 14, 4782-4789

Self-Assembled Monolayers of Alkanethiolates on Thin Gold Films as Substrates for Surface Force Measurements. Long-Range Hydrophobic Interactions and Electrostatic Double-Layer Interactions Thomas Ederth,*,† Per Claesson,† and Bo Liedberg‡ Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden and Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden, and Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ ping University, S-581 83 Linko¨ ping, Sweden Received February 3, 1998. In Final Form: June 12, 1998 Surfaces prepared by self-assembly of alkanethiolates onto thin (10 nm) gold films supported on glass have been used as substrates for surface force measurements between macroscopic surfaces. Surface roughness, the order in the monomolecular film, wetting properties, and their stability in aqueous electrolyte solutions have been investigated using atomic force microscopy, infrared absorption spectroscopy, and contact angle measurements. Direct force measurements have been performed with a noninterferometric bimorph surface force apparatus, using surfaces with differently functionalized thiolates; measurements were made in air, water, and aqueous electrolyte solutions. Results from force measurements between hydrophobic and neutral hydrophilic surfaces in air and water are presented and discussed, as well as some results from measurements with surfaces exposing carboxylic groups in various aqueous electrolyte solutions. It is demonstrated that alkanethiol monolayers self-assembled onto thin gold films on macroscopic surfaces are well-suited as substrates for direct measurements of long-range surface forces. The utility of these surfaces as substrates for investigations of short-range phenomena is limited until the effect of the roughness on the adhesion between the surfaces has been properly quantified and the contribution of the van der Waals force to the total interaction has been calculated. The latter is difficult to assess due to the large number of layers in the system and the presence of conducting surfaces.

Introduction Self-assembled monolayers (SAMs) of ω-substituted alkanethiols covalently attached to gold have in recent years been used as a mean of creating surfaces with widely varying properties.1-3 The well-defined structural and chemical properties of these SAMs have attracted scientists from diverse areas to employ them as model organic interfaces for basic studies of, for example, micro-4 and macroscopic5 wetting, surface-induced nucleation,6 protein adsorption,7 and so forth. They have also been used in applied research areas including corrosion protection,8 * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +46 8 208998. Phone: +46 8 7909978. † Royal Institute of Technology and Institute for Surface Chemistry. ‡ Linko ¨ ping University. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-assembly; Academic Press: San Diego, CA, 1991. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) (a) Nuzzo, R. G.; Zegarski, B. R.; Korenic, E. M.; Dubois, L. H. J. Phys. Chem. 1992, 96, 1355. (b) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570. (c) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257. (d) Engquist, I.; Lestelius, M.; Liedberg, B. J. Phys. Chem. 1995, 99, 14198. (5) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (b) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (6) Gavish, M.; Wang, J.-L.; Eisenstein, M.; Lahav, M.; Leiserowitz, L. Science 1992, 256, 815. (b) Popovitz-Biro, R.; Wang, J.-L.; Majewski, J.; Shavit, E.; Leiserowitz, L.; Mahav, M. J. Am. Chem. Soc. 1994, 116, 1179. (7) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164.

patterned surfaces,9 molecular recognition,10 biosensors,11 and biomaterials.12 Among the materials that have proved to be suitable as substrates for surface force measurements, mica has by far been dominating, and few other materials have been used.13-15 Due to the limited number of materials available, modification of them is required if surface properties such as wetting and polarity is to be changed. To that end, various methods have been empolyed: surfactant adsorption from solution,16 Langmuir-Blodgett deposition,17-19 silanization,20 cold plasma treatment,21 spin coating,22,23 and so forth. Since SAMs formed from alkanethiols provide a combination of stability, flexibility, (8) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (9) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (10) Ha¨ussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (11) (a) Johnsson, B.; Lo¨fås, S.; Lindqvist, G. Anal. Biochem. 1991, 198, 268. (b) Rickert, J.; Go¨pel, W.; Beck, W.; Jung, G.; Heiduschka, P. Biosens. Bioelectron. 1996, 11, 757. (12) (a) Lestelius, M.; Liedberg, B.; Lundstro¨m, I.; Tengvall, P. J. Biomed. Mater. Res. 1994, 28, 871. (b) Lindblad, M.; Lestelius, M.; Johansson, A.; Tengvall, P.; Thomsen, P. Biomaterials 1997, 18, 1059. (13) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404. (14) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413. (15) Parker, J. L. Langmuir 1992, 8, 551. (16) Israelachvili, J.; Pashley, R. Nature 1982, 300, 341. (17) Marra, J.; Israelachvili, J. N. Biochemistry 1985, 24, 4608. (18) Claesson, P. M.; Carmona-Ribeiro, A. M.; Kurihara, K. J. Phys. Chem. 1989, 93, 917. (19) Wood, J.; Sharma, R. Langmuir 1994, 10, 2307. (20) Rabinovich, Y. I.; Derjaguin, B. V. Colloids Surf. 1988, 30, 243. (21) Cho, D. L.; Claesson, P. M.; Go¨lander, C.-G.; Johansson, K. J. Appl. Polym. Sci. 1990, 41, 1373.

S0743-7463(98)00131-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998

Alkanethiolate SAMs in Surface Force Measurements

and ease of preparation seldom attained with these methods, we believe that the use of such surfaces in surface force measurements will be particularly rewarding in the study of, for example, hydrophobic interactions (especially in high salt concentrations) and ion-specific adsorption phenomena or in any study where the wetting properties and surface charge needs to be modified. The use of atomic force microscopy (AFM) using a colloidal particle attached to the cantilever has led to the use of several new materials in force measurements (e.g., titanium oxide,24 zinc sulfide,25 polymers,26 and also thiolates adsorbed onto gold27-29). However, adding to the materials available for surface force methods using macroscopic surfaces is still desirable, since these methods provide some advantages, like optical inspection of the contact area and very precise quantitative measurements; the geometry of interaction is well-defined, and the stiffness of the measuring spring can be determined with high accuracy. The use of metal substrates in surface force measurement would allow, among other things, direct control of surface potential, capacitive distance measurements,30,31 and in situ electrochemical studies,32 or as is our present purpose, it could provide substrates suitable for selfassembly of alkanethiolate monolayers. However, only few works involving metals have been reported, starting with early attempts using crossed filaments,33 solid spheres34 or plates,35 and thin films in a variety of setups including the surface force apparatus,31,32,36,37 AFM,38 and others.30 Also, some instrumentation issues to facilitate the use of metal surfaces have been treated.31,39 The main difficulty in using metals is the inherent roughness of polycrystalline metal surfaces; thus, characterization of surface roughness and its relation to surface force measurements has received some attention, for long-range van der Waals forces,40-42 for adhesion forces in general,43-45 and for metal adhesion in particular.46,47 Since there is an interest in producing smooth metal films also for other purposes, for example, scanning probe microscopy, efforts have been taken to find ways of processing (22) Neuman, R. D.; Berg, J. M.; Claesson, P. M. Nord. Pulp Pap. Res. J. 1993, 8, 96. (23) Koehler, J. A.; Ulbricht, M.; Belfort, G., in press. (24) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885. (25) Toikka, G.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3783. (26) Karaman, M. E.; Meagher, L.; Pashley, R. M. Langmuir 1993, 9, 1220. (27) Thomas, R. C.; Tangyunyong, P.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. J. Phys. Chem. 1994, 98, 4493. (28) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (29) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. (30) van Blokland, P. H. G. M.; Overbeek, J. T. G. J. Chem. Soc. Faraday Trans. 1 1978, 74, 2637. (31) Frantz, P.; Agrait, N.; Salmeron, M. Langmuir 1996, 12, 3289. (32) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S.; McClure, D. J. J. Phys. Chem. 1988, 92, 199. (33) Derjaguin, B. V.; Voropayeva, T. N.; Kabanov, B. N.; Titiyevskaya, A. S. J. Colloid Sci. 1964, 19, 113. (34) Rabinovich, Y. I.; Derjaguin, B. V.; Churaev, N. V. Adv. Colloid Interface Sci. 1982, 16, 63. (35) Sparnaay, M. J. Physica 1958, 24, 751. (36) Parker, J. L.; Christenson, H. K. J. Chem. Phys. 1988, 88, 8013. (37) Coakley, C. J.; Tabor, D. J. Phys. D: Appl. Phys. 1978, 11, L77. (38) Biggs, S.; Mulvaney, P. J. Chem. Phys. 1994, 100, 8501. (39) Levins, J. M.; Vanderlick, T. K. Langmuir 1994, 10, 2389. (40) Mazur, P.; Maradudin, A. A. Phys. Rev. B 1981, 23, 695. (41) Mazur, P.; Maradudin, A. A. Phys. Rev. B 1981, 22, 1677. (42) van Bree, J. L. M. J.; Poulis, J. A.; Verhaar, B. J.; Schram, K. Physica 1974, 78, 187. (43) Greenwood, J. A.; Williamson, J. B. P. Proc. R. Soc. London A 1966, 295, 300. (44) Fuller, K. N. G.; Tabor, F. R. S. Proc. R. Soc. London A 1975, 345, 327. (45) Maugis, D. J. Adhes. Sci. Technol. 1996, 10, 161. (46) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1992, 96, 10405. (47) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067.

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metals so as to make surfaces with as little roughness as possible.48 One of the few general conclusions, though, is that the roughness scales with the thickness of the metal layer; consequently, a simple way to reduce roughness is to make the films thinner. In this study, we have prepared and characterized thin gold films, supported by glass substrates, with low enough roughness to make them suitable as substrates for direct measurements of surface forces at separations larger than a few nanometers. The gold films have been modified with self-assembled thiol monolayers. These modifications serve two purposes: First, the gold surface itself, when handled in air, attracts contaminants,49 and the thiol layers help in keeping the surfaces well-defined and reduce the need for cleaning. Second, and more importantly, thiol compounds are available with a wide variety of functional groups, thus providing a means to vary the chemical composition of the outermost molecular surface layer. The quality of these layers was studied using contact angle measurements and infrared reflectionabsorption spectroscopy, to ensure that the thin gold films were suitable supports for such thiolate monolayers. We present results from force measurements using thiohexadecane and thiohexadecanol modified gold in water, and also some results from a study using 16-thiohexadecanoic acid modified gold in various aqueous electrolyte solutions. The relevance of the method and the results for studies and understanding of the interaction among hydrophobic surfaces (for a recent summary of previous work in this area, see ref 50 and references therein) and to ionadsorption phenomena are also discussed. Experimental Section The surfaces used for the surface force measurements and for roughness studies by AFM were prepared as follows. Rods of borosilicate glass with 2-mm diameter were cut into 25-30-mm lengths, rinsed with ethanol, and using a butane-oxygen burner, melted in one end to give the end a drop shape with a radius of approximately 2 mm. For contact angle measurements, infrared absorption spectroscopy (IRAS), and some of the AFM measurements, silicon(100) wafers with native oxide were cut into 20 × 40 mm pieces for the IRAS measurements, or otherwise in 15 × 20 mm pieces, and cleaned in a mixture of 5/7 H2O, 1/7 30% H2O2, and 1/7 25% NH3 at 80 °C for 10 min. The flame-polished glass surfaces and the silicon flats were mounted in an electron-beam ultrahigh-vacuum evaporation system (Balzers UMS 500P) and treated identically from then on. Deposition of a 1-nm titanium adhesion layer preceded the 10-nm gold layer. Evaporation rates were set to 0.1 and 0.5 nm/s for titanium and gold, respectively, and the evaporation was automatically interrupted when the required film thickness was attained, as measured with a quartz film thickness monitor with 0.1-nm resolution. The base pressure was typically below 5 × 10-9 Torr before evaporation started, and the pressure during the gold evaporation step rarely exceeded 5 × 10-8 Torr. The gold substrates were removed from the evaporator and directly immersed in the thiol solutions without any intermediate cleaning, and with as little contact with laboratory atmosphere as possible. The substrates were left for adsorption for at least 15 h, using 1 mM solutions in 99.5% ethanol (Kemetyl, Stockholm). 16-Thiohexadecane (C16) (Fluka, >95%), 16-thiohexadecanol (C16OH) and 16-thiohexadecanoic acid (C16OOH) (gifts from Pharmacia Biosensor, g99.5%) were used without further purification. Adsorptionsas well as transporta(48) (a) Buchholz, S.; Fuchs, H.; Rabe, J. P. J. Vac. Sci. Technol., B 1991, 9, 857. (b) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312. (c) Clemmer, C. R.; Beebe, T. P. Scanning Microsc. 1992, 6, 319. (d) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. (e) Stamou, D.; Gourdon, D.; Liley, M.; Burnham, N. A.; Kulik, A.; Vogel, H.; Duschl, C. Langmuir 1997, 13, 2425. (49) Schrader, M. E. J. Colloid Interface Sci. 1984, 100, 372. (50) Hato, M. J. Phys. Chem. 1996, 100, 18530.

4784 Langmuir, Vol. 14, No. 17, 1998 tion and storage of the substratesswas made in beakers made from polypropylene and polytetrafluoroethylene. Before use, the surfaces were removed from the thiol solution, ultrasonicated in ethanol for 10 min to remove excess thiols physisorbed on the surface, and finally dried in a gentle flow of dry nitrogen. Water was purified with a Millipore Milli-Q Plus 185 water purification system, and for surface force measurements with C16-modified substrates, deaerated using a water jet pump for 2 h immediately before use. Hexadecane (HD) (Fluka, >99.5) was chromatographed over alumina before use in contact angle measurements. NaCl, MgCl2, and CaCl2 (Merck, >99.5) were used as received. Contact angles with water and hexadecane were measured using a goniometer (Rame´-Hart NRL 100) under ambient conditions. The flat solid surface was moved perpendicular to the line of sight with a translation stage, while a sessile droplet (3-4 mm diameter) was formed from a fixed syringe and thus dragged over the surface. Advancing and receding contact angles were measured at each side of the droplet, while moving relative to the surface. The AFM (Nanoscope III, Digital Instruments) was used in tapping mode to characterize the roughness of the glass and gold surfaces. Before roughness calculations were performed, all images (256 × 256 points) were plane-fitted using a secondorder polynomial to compensate for the curvature of the surfaces and the nonlinear lateral motion of the surfaces during scanning. All surfaces were examined in air. The quality of the thiolate monolayers was monitored using Fourier transform infrared reflection-absorption spectroscopy (IRAS), in a grazing angle reflection setup, with an angle of incidence of 83°. The resolution of the spectrometer (Bruker IFS113v) was 2 cm-1, and 1000 scans were collected for both the sample and the reference surface, the reference being a gold surface identical to the sample surface, but without the thiol layer. The device used for the surface force measurements is based on a single cantilever bimorph deflection sensor,15,51,52 and does not rely on interferometry to determine distances. The instrument can be used to measure forces between any two bodies, irrespective of composition and shape, provided they are smooth enough to make data interpretation possible in the separation range of interest. In this case spherical surfaces with radii of approximately 2 mm were used. One surface is mounted on a bimorph deflection sensor, and the other is attached to a piezoelectric displacement transducer. Using motor control the two surfaces are approached to a separation of some hundred nanometers, after which the upper surface is moved toward the lower in a continuous manner, using the piezoelectric tube. The approach rate is chosen to be low enough to eliminate hydrodynamic effects; usually half the rate at which an effect of hydrodynamics is detectable, as tested with hydrophilic surfaces in pure water. Further decreasing of the approach rate has not been observed to affect interactions of other kinds and is usually not done, since the bimorph interfacing electronics makes it desirable to keep measurement times as short as possible.51 When the surfaces have met in a (supposedly) hard wall contact, the upper surface is withdrawn again. During the process the bimorph output signal, which is proportional to the deflection, as well as the displacement of the upper surface is monitored, as measured with an linear variable differential transformer (LVDT) displacement transducer parallel to the piezotube. The information provided is of the same kind as that given with an AFM used for force-distance measurements, but with some advantages; the geometry of interaction is well-known due to the macroscopic surfaces, quantitative measurements can be made with high precision since the stiffness of the spring can be measured with high accuracy, and problems due to nonlinearities in the motion of the piezotube are eliminated since the displacement is measured directly. Since the displacement of the upper surface is known, the sensitivity of the bimorph is easily calculated (under the assumption that the surfaces are incompressible). Hence, the deflection of the lower surface is known at each instant, and the force exerted on it is calculated by multiplication with (51) Stewart, A. M. Meas. Sci. Technol. 1995, 6, 114. (52) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.

Ederth et al. the spring constant of the bimorph. The procedure is similar to the analysis of force versus distance curves in AFM. The distance resolution of the instrument is approximately 0.2 nm, the minimum detectable force in these experiments was of the order of 10-8 N, and the normalized force resolution about 20 µN/m. Force versus distance curves are presented without any averaging (to make the graphs less cluttered, data are presented as solid lines connecting the data points, rather than as sequences of individual data points). Unless stated, no corrections for deformation have been made in the graphs; the point of zero separation corresponds to the location of the “hard wall”, as selected in the analysis. All forces have been normalized with the average radius R of the spheres, where R ) R1R2/(R1+R2); R1 and R2 are the radii of the two spheres. The resulting normalized force F/R is related to the interaction free energy per unit area G between the plane surfaces through the Derjaguin approximation,53 F/R ) 2πG; thus, the results can be compared directly with results from measurements employing the common crossed-cylinder or sphere-on-flat geometries. To account for deformation, the theory of contact mechanics by Johnson, Kendall, and Roberts has been implemented.54 With R as above, this theory (henceforth referred to as “JKR”) relates the radius of the flattened contact area, a, to the interfacial energy, γ:

a3 )

3(1 - ν2)R (F + 3πγR + [6πγRF + (3πγR)2]1/2) 2E

where E is Young’s modulus, ν is the Poisson ratio, and F is the applied load. At zero applied load, the central displacement δ (i.e., the compression along the symmetry axis of the spheresphere contact) is

δ)

[

]

(1 - ν2) a2 - 4πγa R E

1/2

On applying a negative load, the spheres will separate when the “pull-off” force, Fa/R, equals -3πγ/2.

Results Surface Preparation. The flame-polishing procedure produces very smooth glass surfaces as a result of the high surface energy of the glass (Figure 1). The peakto-valley distance, taken as the height from the highest to the lowest point on a 1 × 1 µm2 AFM image, was typically 1.0 nm, with root-mean-square (rms) roughness around 0.10 nm. The addition of the titanium and gold layers increases surface rougness, though the increase is not dramatic (the total thickness of both metal layers is approximately 11 nm; the quartz film thickness monitor readings after completed evaporation showed Ti thicknesses between 1.0 and 1.6 nm and Au thicknesses between 10.1 and 10.4 nm). Peak-to-valley distances on gold surfaces were in most cases less than 2 nm over a 1 × 1 µm2 image, and typically about 1.5 nm, with rms values in the 0.15-0.20 nm range (Figure 1). By analysis of the power spectrum density of the images, the rms corrugation length (i.e., the grain size) was estimated to be 5-15 nm. These values compare well with what has been reported for surfaces prepared in a similar manner.55,56 Compared to thicker (200 nm) gold films prepared and analyzed similarly, these roughness values are about 10 times smaller.57 In no case did the AFM investigations reveal any indication of island (53) Derjaguin, B. Kolloid-Z. 1934, 69, 155. (54) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A 324, 301. (55) Maeda, M.; White, H. S.; McClure, D. J. J. Electroanal. Chem. 1986, 200, 383. (56) DiMilla, P. A.; Folkers, J. P.; Biebuyck, H. A.; Ha¨rter, R.; Lo´pez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225. (57) Ederth, T. unpublished data.

Alkanethiolate SAMs in Surface Force Measurements

Figure 1. Cross sections of AFM images of a flame-polished glass surface (top) and a surface with 1 nm of Ti and 10 nm of Au deposited onto flame-polished glass (bottom).

formation of the metals on the surface; consequently, we conclude that the gold films are continuous. AFM measurements were performed on gold films deposited both on the spherical surfaces intended for surface force measurements and on flat silicon surfaces. No differences were found between the two in any respect, which was taken to justify the use of flat surfaces instead of the spherical surfaces for the investigation of some of the surface properties; the spheres used for surface force measurements are not well-suited for either contact angle measurements or infrared spectroscopy. The C16 and C16OH SAMs prepared on the 10-nm thick gold films were analyzed with infrared spectroscopy to reveal their chain orientation and conformation. The intensity pattern of the CH2 and CH3 modes in the 30002800-cm-1 region is in good agreement with the very best C16 SAMs prepared on thicker gold films, suggesting that the polymethylene chains are tilted approximately 30° away from the surface normal.58 The exact frequencies of CH modes can also provide information about the presence of disordered components in the assembly. Snyder et al.59 established correlations between the vibrational frequencies and line shapes of the CH2 modes and the relative population of trans/gauche conformers along the chains. The very best SAMs on thick gold films with a strong (111) texture60 displayed asymmetric CH2 (d-) and symmetric CH2 (d+) modes at 2918 and 2850 cm-1, respectively. These positions are characteristic of crystalline-like, all trans, assemblies.59 The corresponding mode frequencies and line shapes observed for our C16 and C16OH SAMs prepared on thin gold films occur in the range 2919-2920 cm-1 (d-) and at 2850 cm-1 (d+), supporting a model of organized all trans assemblies. The (58) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (59) (a) Snyder, R. G. Spectrochim. Acta 1963, 19, 85. (b) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1982, 88, 334. (c) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (60) (a) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (b) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996. (c) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882.

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slight increase in the d- mode indicates a minor disorder in the outermost positon of the polymethylene chains. The contact angles (advancing and receding) of water on C16 surfaces are 107° ( 1° and 92° ( 1°, and the results for hexadecane (HD) on the same surfaces are 50° ( 1° and 46° ( 1°. With regard to the order in the thiol layer, these data are essentially in agreement with the IR data, showing that most chains are in an upright conformation where few CH2 groups are exposed to the surface. We observe that the values reported are a few degrees smaller than the contact angles reported by many others.61,62 If this difference is significant, and not a result of the chosen measuring procedure, we believe its cause could be an effect of the smaller roughness of the thin gold substrates used, but since, in general, contact angle data depend on the method used for the measurements, and also considering that the atmosphere was not controlled during the measurements, extensive speculation about the differences seems inappropriate. We observed unexpectedly high contact angles on C16OH surfaces with both water (29° ( 2° and 15° ( 1°, advancing and receding) and HD (13° - 21° and