Chain Length Dependence of the Structure and Wetting Properties in

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Langmuir 1995,11, 3882-3893

3882

Chain Length Dependence of the Structure and Wetting Properties in Binary Composition Monolayers of OH- and CHs-Terminated Alkanethiolates on Gold Sundar V. Atre,' Bo Liedberg,**$and David L. Allara**ts$ Department of Materials Science and Engineering and Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, and Laboratory of Applied Physics, Linkoping University, S-581 83 Linkoping, Sweden Received February 21, 1995. I n Final Form: June 26, 1995@ Equimolar,binary-component,self-assembledmonolayers(SAMs)of -S(CHz)15CHzOH and -S(CHz)(lb+m)CH3, with m varied systematicallyfrom -6 to +6, were prepared on evaporated gold films by self-assembly from ethanol solutions of the corresponding thiols. Single-wavelength ellipsometry, X-ray photoelectron spectroscopy, and infrared reflection spectroscopy (IRS) were used to monitor the coverage, composition, and chain conformational order, respectively, in the air1SAM structures, while liquid drop contact angle measurements with hydrogen-bonding and hydrocarbon liquids were used t o probe the structural and chemical characteristics at the 1iquidSAM interface. Both the IRS C-H stretching mode characteristics and the hydrocarbon liquid contact angles show sharp changes as m increases through the region of -4 t o 0, while the hydrogen-bonding liquid contact angles remain essentially constant across the range of m values. The combined data lead to a general structural model in which monolayer structures shift from randomly placed, protruding HO-terminated chain segments which screen the underlying CH3 surface at low m to contiguously placed, protruding CH3-terminated chains, organized in clusters and extending away from the underlying CHzOH surface at high m. These structures require that the monolayers approach random mixing at low m but be phase-segregated at high m.

1. Introduction Recently, there has been a growing interest in controlling the chemical properties of solid surfaces by variation of the molecular composition in multicomponent (mixed) organic monolayers.' A variety of preparations are possible,2-6 but the most widely used one is multicomponent solution self-assembly of w-functionalized alkanethiols, HS(CHZ),X, onto gold surface^^,^ with a large variety ofX groups such as CH3, COzH, OH, CN, C02CH3, and ferr~cenyl.~ Such films allow many possibilities for systematic studies of a wide range of interfacial phenomena including wetting,1° electron t r a n ~ f e rprotein ,~ adsorption,l' molecular recognition,12and multilayer film f0rmati0n.l~Alkanethiolate/Au self-assembled monolay-

* Corresponding authors. Department of Materials Science and Engineering, The Pennsylvania State University. Department of Chemistry, The Pennsylvania State University. 8 Linkoping University. Abstract published in Advance ACS Abstracts, September 1, 1995. (1)(a) Bain, C. D.; Whitesides, G. M. J . Am. Chem. SOC.1988,110, 6560-6561. (b)Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1988, 110,3665-3666. (c) Bain, C. D.; Whitesides, G. M. Science 1988,240, 62-63. (d) Bain, C. D.; Biebuyck, H. A,; Whitesides, G. M. Langmuir 1989, 5, 723-727. (e) Laibinis, P. E.; Fox, M. A,; Folkers, J. P.; Whitesides, G. M. Langmuir 1991,7,3167-3173. (0 Evans, S. D.; Sharma, R.; Ulman, A.Langmuir 1991,7,156-161. (g) Siepmann, J. I.; McDonald, I. R. Molec. Phys. 1992,75,255-259. (2)(a)Overney, R. M.; Frommer, J.;Brodbeck, D.; Liithi, R.; Howald, L.; Guntherodt, H. J.;Fujhira, M.; Takano, H.; Gutoh, Y. Nature 1992, 359,133-135. (b) Mohwald, H. J . Mol. Electron. 1988,4,47-55. (3)Biebuyck, H.A.;Whitesides, G. M. Langmuir 1993,9,17661770. (4)Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, D. L. Langmuir 1988,4,365-385. (5)Laibinis, P. E.; Whitesides, G. M. J . A m . Chem. SOC.1992,114, 1990-1995. (6)Silberzan,P.; LBger, L.; AusserrB, D.; Benattar, J.Lungmuir 1991, 7,1647-1651. (7)Dubois, L. H.; Nuzzo, R. G. Annu. Reu. Phys. Chem. 1992,43, 437-463. ( 8 ) b j Bain, C. D.; Evall, J.; Whitesides, G. M. J . Am. Chem. SOC. 1989,111,7155-7164.(b)Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1989,111, 7164-7175. +

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ers (SAMs) are particularly convenient because of their ease of preparation and extensive characterizati~n.~,~~-~' One particularly useful structural variation involves mixed chain lengths to provide controlled vertical and horizontal distributions of functionality with respect to the surface plane, as suggested by the cartoons in Figure 1. This strategy has been used in several wetting studies of mixed chain length S A M S ~ in ~ which J ~ ~ it~was , ~ observed ~ that protrusion of long-chain segments into the wetting medium decreases the wetting effects of the shorter chain termini. While such structural variations are quite important in determining the response of a surface to physical, chemical, and biological stresses,23the fundamental structure/property correlations remain largely unknown and the preparation conditions required for the synthesis of specific architectures remain unclear. In this paper, we focus on the preparation and characterization of partially hydroxylicsurfaces prepared from mixed chain length CH3- and OH-terminated alkanethiolate SAMs. The combination of the strongly hydrophobic and hydrophilic characters of the CH3 and OH groups, respectively, as well as the ability to derivatize the OH groups with biologically active groups, makes these SAMs of specific interest for applications to ongoing studies in our laboratories of water and molecular adsorption and biological surface interactions. From a structural point of view, the relative vertical (Figure 1,B-E) placements of the OH and CH3 groups within the monolayer matrix are of particular importance since these could regulate the availability of a specific functional group to approach(9)(a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990,112,4301-4306. (b) Chidsey, C. E. D. Science 1991,251,919-922. ( c ) Collard, D. M.; Fox, M. A. Langmuir 1991,7,1192-1197. (d) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7,2307-2312.(e) Hickman, J.J.;Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991,252,688-691. (10) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989,5, 13701378. (b) Ulman, A,; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J . A m . Chem. SOC.1991,113,1499-1506. (11)(a) Prime, K. L.; Whitesides, G. M. Science 1991,252, 11641167. (b)Pale-Grosdemange, C.;Simon, E. S.;Prime, K. L.;Whitesides, G. M. J . Am. Chem. SOC.1991,113,12-20.

0 1995 American Chemical Society

OH- and CH3-Terminated Alkanethiolates on Gold

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Langmuir, Vol. 11,No. 10, 1995 3883

correlations. In a more recent study,26varied composition mixed SAMs of S(CH2)&H20H (Rl6OH)and S ( C H ~ ) ( I ~ + ~ ) CH3 (RnCH3,where n = 15 m ) were prepared on gold with values of m of -4, -2, and 0. In this case, infrared spectroscopy (IRS) was used to follow the derivatization of the OH groups, thus yielding limited structural data, but the wetting properties were not reported. The present study was designed to partially bridge the above gaps in structure and surface properties by providing a systematic study of the wetting properties of the above class of mixed SAMs in which the structures were carefully characterized by IRS. Toward this end, monolayers were synthesized with m varied between -6 to +6 in steps of 2. This particular m-value range was chosen in order to keep the differential chain length to a value for which a protruding polymethylene chain segment (CH2)lml would be unable to form more than one complete reverse fold at best, since '4 CH2 units are required to make such a tight fold.27In order to contain the size of the sample base, the OWCH3 surface compositions were maintained constantly at a 1:l mole ratio in all SAMs. The dry monolayers were characterized by single-wavelength ellipsometry (SWE) and X-ray photoelectron spectroscopy ( X P S ) in order to monitor the coverage and composition and by IRS to characterize the essential features of the chain structure. Liquid drop contact angle measurements were used to probe the general structural and chemical character of the wetting interface. The combination of the wetting data, which show sharp variations with remarkably small changes in differential chain lengths, with the IRS data, which show corresponding variations in the conformational ordering of the chains, reveals that shifts in both the lateral and the vertical distribution of functional groups arise with changes in differential chain lengths.

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A

B

C

D

E

Figure 1. A drawing of structural arrangements in 1:l composition mixed monolayers of long and short alkanethiolate chains on a gold surface. The four pictures represent limiting cases of phase-segregated and well-mixed surface distributions. The top views (A) reveal t h e locations of the S atoms of each chain on a hexagonal superlattice at t h e gold surface. The corresponding side views (B-E) reveal a cross section of the film across a line of chains. B and D represent adjacent chains extended above t h e short chains as ordered rods and splayed across the short chain surface, respectively. C and E similarly represent nonadjacent (randomly placed) chains extended above the short chain surface with clustered termini and splayed across the short chain surface, respectively.

ing reactants or interactive species, particularly water, whbse local structure at surfaces is critical to biological phenomena, e.g., in the formation of tertiary structures of proteins24and cell adhesion.25 Since at the outset it was not clear how the exact preparation procedure would affect the S A M structures and properties, the simple method of room-temperature self-assembly from multicomponent ethanol solutions was chosen in order to connect the present data with earlier studies. While the latter1aJbJ0$22 reported significant changes in wetting on OWCH3 mixed S A M s prepared with large differences between the chain lengths of the components, the lack of systematic, incremental chain length variation and direct S A M characterization precluded meaningfbl structural (12)Haussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Znt. Ed. Engl. 1991,30,569-572. (13)(a)Sanassy, P.; Evans, S. D. Lungmuir 1993,9,1024-1027.(b) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995,99,9996-10008. (14)Electron diffraction (TEM): (a) Strong, L.; Whitesides, G. M. Langmuir 1988,4, 546-558. (b) Chidsey, C. E. D.; Loicano, D. N. Lungmuir 1990,6,682-691, (15)X-ray diffraction (XRD): Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 437-447. (16)IRS: NUZZO, R. G.; Dubois, L. H.; Allara, D. L. J . Am. Chem.Soc. 1990,112,558-569. (17)Raman spectroscopy: Bryant, M. E.; Pemberton, J. E. J . Am. Chem. SOC.1991,113,8284-8293. (18)Molecular dynamics simulations: Hautman, J.; Klein, M. L. J . Chem. Phys. 1989,91,4994-5001. Low-energy He atom diffraction studies: (a) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J . Chem. Phys. 1989,91,4421-4423.(b) Camillone, N., 111; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J . Chem. Phys. 1991,94,8493-8502. (19)Scanning tunneling microscopy (STM): (a)Widrig, C. A.; Alves, C. A.; Porter, M. D. J.Am. Chem. SOC.1991,113,2805-2810. (b)Kim, Y.-T.; Bard, A. J. Langmuir 1992,8,1096-1102. (c) Sun, L.; Crooks, R. M. Langmuir 1993,9,1951-1954. (d) Edinger, K.;Giilzhauser, A> Demota, (K.;Wo11, C.; Grunze, M. Langmuir 1993,9,4-8. (e) Poirier, G. E.; Tarlov, M. J. Langmuir 1994,10,2853-2856.

2. Experimental Section 2.1. Materials. The sample of 16-mercaptohexadecanol (courtesy of Pharmacia, AB, Uppsala, Sweden) was purified by double recrystallization from hexane (mp 53-54 "C). The solid alkanethiols, n-eicosanethiol, n-docosanethiol (available from previous studies2*),and n-octadecanethiol (Aldrich, 98%), were purified by recrystallization from ethanol (mp's, 39-40,47-48, and 29-30 "C, respectively), while n-decanethiol (Lancaster), n-dodecanethiol (Aldrich, 98%), n-tetradecanethiol (Lancaster), (20)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. (21)Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, R. G. J . Am. Chem. SOC.1989,111,321-335. G. M.; NUZZO, (22)(a)Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992,8,1330-1341. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J . Adhesion Sci. Technol. 1992,6,1397-1410. (23)For example, it has been noted in electron-transfer studies involving ferrocenyl- and CHs-terminated alkanethiolate SAMdAu that the electroactive ferrocenyl groups need to be well-separated from each other by the n-alkyl component in order to ensure observation of identical, isolated electrochemical event^.^ On the other hand, in order to mimic domains of receptor sites such as coated pits on cell surfaces, lateral distributions of the order of about 100 nm would be typically required [Goldstein, J. L.; Anderson, R. G. W.; Brown, M. S. Nature 1979,279,679-6851.Finally, the extension of functional groups away from a surface is relevant to biological interfaces where functional group mobilities are thought to be important [(a)Brier-Russell, D.; Salzman, E. W.; Lindon, J.; Handin, R.; Merrill, E. W.; Dincer, A. K.; Wu, J.-S. J . Colloid Interface Sci. 1981,81,311-318.(b)Reichert, W. M.; Filisko, F. E.; Barenburg, S. A. J.Biomed. Mater. Res. 1982,16,301-312.(c) Merrill, E. W.Ann.N. Y.Acad.Sci. 1987,516,196-203.(d)Haussling, L.; Knoll, W.; Ringsdorf, H.; Schmitt, F.-J.; Yang, J. Makromol. Chem. Macromol. Symp. 1991,46,145-1551. (24)Creighton, T. E. Prog. Biophys. Mol. Biol. 1978,33,231-297. (25)(a)Hanein, D.; Sabanay, H.; Addadi, L.; Geiger, B. J . Cell Sci. 1993,104,275-288. (b) Hanein, D.; Geiger, B.; Addadi, L. Langmuir 1993,9,1058-1065. (26)Bertilsson, L.; Liedberg, B. Langmuir 1993,9,141-149. (27)(a) Groth, P. Acta Chem. Scand. 1979,A33,199-201. (b) Kay, H. F.; Newman, B. A. Acta Crystallogr. 1968,B24,615-624. (28)Sheen, C. W.; Martensson, J.; Shi, J.; Parikh, A. N.; Allara, D. L. J . Am. Chem. SOC.1992,114,1524-1525.

3884 Langmuir, Vol. 11, No. 10, 1995 and n-hexadecanethiol (Aldrich, 92%) were purified by vacuum distillation (lo+ kPa). Absolute ethanol (Pharmco, Weston, MO) was used without further purification. Purified, low organic content, high-resistivity (> 18.0 MQ/cm) water was obtained using a Millipore (Bedford, MA) purification system. Hexadecane (Aldrich, 99+% gold label) and dicyclohexyl (Aldrich, 99+%) were chromatographed over alumina (Aldrich, Brockman I) immediately before use. Formamide (Aldrich, 99.9+%) was used without further purification. 2.2. Sample Preparation. For the preparation of gold substrate films, 2-cm x 4-cm rectangular polished silicon wafers were first cleaned in a mixture of HzOz (30%) and concentrated HzS04 at 80 "C (caution: this is a strongly oxidizing mixture which can undergo explosion upon contact with organic materials), rinsed with deionized water and absolute ethanol, and finally dried by spinning. These substrates were then subjected to sequential resistive evaporations of -10 nm of chromium and -200 nm of gold (both 99.99% purity) at 0. Figure 9 shows the variation in the full width at halfmaximum (FWHM) of the d+- and d--mode peaks as a function of m. The measurement of the d- line width is somewhat complicated by the presence of an overlapping (45)Snyder, R.G.; Strauss, H. L.; Elliger, C. A. J . Chem. Phys. 1982,

--.(46)Snyder, R.G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M.

86. 5145-5150. ~ - ~ ~ - --

J. Chem. Phys. 1986,90,5623-5630. (47)Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J . Phys. Chem. 1992,96,5097-5105.

Langmuir, Vol. 11, No. 10, 1995 3889 component due to an r+ Fermi resonance mode lying on the high-frequency side of the band (refer to Figure 7). In the cases form 2 -2, the intensity ofthe Fermi component is relatively low and the FWHM of the d- mode could be obtained with reasonable accuracy without corrections to the observed peak. In the cases for m = -4 and -6, an estimate was made using a simple two-component curve fit48 for the contour of the Fermi resonance peak, and corrected values of the d- FWHM were generated. For both ofthe d modes, the largest FWHM values are observed for the smallest values of m and a minimum value is observed for m = 0. Further, the larger line widths are associated consistently with an asymmetric broadening of the high-frequency side of the d- band. It has been observed that FWHM values of the d+ and d- modes of conformationally ordered polymethylene chains are 7 ( f 2 ) and l l ( f 2 ) cm-l, respectively, while the corresponding values for conformationally disordered polymethylene units are 16(f2) and 23(+2) cm-l, r e s p e ~ t i v e l y . By ~~ analogy, the low values of 7-8 and 11-13 cm-l observed for the FWHM of the d+and d- modes, respectively, of the present m 0 monolayers thus imply the presence of predominantly conformationally ordered CH2 units in this m-value range. In contrast, the significantly higher values of 10-16 and 17-23 cm-l observed, respectively, for the d+ and d- modes for m < 0 indicate a considerable population of conformationally disordered CH2 units in these monolayers. These conclusions are consistent with those derived above from the d-mode frequencies. Figure 10shows the variation ofthe integrated intensity of the d+ and the d- modes as a function of m. The dintensities have been corrected by subtracting the estimated contribution of the high-frequency r+ Fermi resonance component in the total -2890-2950-cm-l peak envelope.48 Both d-mode intensities increase nearly monotonically with m, as would be expected for an increase in the total number of CH2 units in the monolayers. The d+band, which is relatively free of overlapping absorptions, is known to show a linear intensity dependence on n for ordered CH~(CH~),S/AU SAMs with n > In the present case, the apparent leveling off of intensity between m = -2 and +4 in Figure 10 suggests a change in the chain structures as m crosses this region, consistent with the conclusions drawn from the d-mode frequencies and line widths in Figures 8 and 9. Quantitative interpretations of the intensity data would require corrections for the exact fractional contributions of the protruding segments to the total spectra, but we do not believe that such corrections can be made with sufficient accuracy to be useful because of the difficulties in accurately separating effects of ~ r i e n t a t i o n ,c~~~n f o r m a t i o n ,and ~ ~ local electric fields52 on the protruding overlayer and the underlying matrix and the inherent difficulty in assigning accurate cross sections to segments of chains. ll.7,20132

(48)The observed peaks were decomposed using a standard curvefitting algorithm into two components with mixed Gaussian + Lorentzian line shapes: intense d-, 2916-2922 cm-l; weak, broad r+(Fermi), 2933-2938 cm-'. (49)Atre, S.V.;Allara, D. L.; Snyder, R. G.; Elliger, C. A. Manuscript in preparation. ( 5 0 ) Parikh, A. N.; Allara, D. L. J . Chem. Phys. 1992,96,927-945. (51)Snyder, R. G. Macromolecules 1990,23,2081-2087. (52) Cdculations were carried out using effective medium theory [generalized Maxwell-Garnett model: (a)Maxwell-Garnett, J. C. Phil. Trans. 1904,203,385. (b) Maxwell-Gamett, J. C. Phil. Trans. 1906, 205,2371in which the infrared optical functions for islanded and isotropic distributions of the protruding chains were determined starting from independently known values of the optical function spectra of the pure compounds (Shi, J.-X. M.S. Thesis, The Pennsylvania State University, University Park, PA). The calculations show that the d- intensities for equal coverage mixed 1:l films of long and short chains increase with increased islanding of the long chains. Details will be published elsewhere (Parikh, A. N., Allara, D. L. Manuscript in preparation).

3890 Langmuir, Vol. 11, No. 10, 1995

Atre et al.

CHs Modes. Figure 8 shows that the r+ and r- peak positions increase with increasing values of m. Starting from m = -6, where the peaks appear at -2876.5 and -2961.5 cm-l, respectively, the frequencies increase to reach maximum values of -2880 and 2967 cm-l, also, respectively, at m = +6. In contrast, the r+ and r- peak frequencies for spectra of single-component n-alkanethiolate SAMs remain constant with chain length variations within 1 cm-l at -2878 and 2965 cm-l, r e s p e c t i ~ e l y . ~ ~ ~ ~ ~ ~ ~ ~ Since the r+ mode is complicated by the presence of overlapping contributions of the d+ mode as well as the presence of a peak tentatively assigned to the CH2 group adjacent to the OH group (see above), further analysis of the r+feature was not made, and only the r- peak positions were analyzed. Because of a number of complexities associated with the r- mode53and because of the lack of any suitable form 3000 3200 3400 3600 3800 Wavenumber, cm-1 of a bulk alkyl compound with a low-density structure (-50% bulk) comparable to that of the partial coverage, Figure 11. High-frequencyinfrared spectra (0-H stretching protruding chains in the mixed SAMs, the most efficient and region) of 1:l mixed monolayers of -S(CH~)(E+~)CH~ way to derive structural features is by direct comparison -S(CHz)&HzOH. The spectrum of each of the pure RMOH and R15CH3 monolayers is also shown. ofthe reflection spectra ofthe present mixed S A M systems with analogous reflection spectra reported for simple, single-componentmonolayers of known structures. In this layer packing, other reported data do. In the adsorption regard, it is simplest to restrict the comparisons to samples of n-alkanoic acids on the aluminum (oxide), the rinvolvinghighly metallic substrates, such that differences frequency increases from 2960 to 2965 cm-l with increasin the anisotropy of the surface electromagnetic fields and ing surface coverage,j7and a parallel increase in the chain accompanying orientational effects on the spectra are conformational order is observed. Similarly, we have eliminated,50and to monolayers of closely similar strucobserved nearly identical shifts in R&H$gold SAMs as ture. coverages increase from 10% to complete.58 Such effects can be ascribed to the increased intermolecular interacFor the cases of the m < 0 mixed SAMs, for which the tions between CH3 groups as the chains become more CH3 groups are buried in a CH2 matrix, the best organized and densely packed. In this regard, it has been comparison is with the data47for the mixed monolayer of observed that thermal disordering of Langmuir-Blodgett S(CD2)17CD&lCH3 in which the shorter RllCH3 chains monolayers of cadmium eicosanoate on aluminum (native are observed to be highly extended and located in a denseoxide) substrates59shifts the r- peak to lower frequencies packed environment for all compositions. These data show as the conformational order of the chains decreases. The an r- frequency of 2961 cm-l, a value identical to that for above comparisons lead to the conclusion that the high rthe m = -6 monolayer. As m increases from -6, the CH3 frequency value of -2967 cm-l in the m > 0 monolayers groups approach the surface, and Figure 8 shows that the is quite consistent with the CH3 groups placed in a dense r- frequencies approach -2964 cm-l, in close corresponlocal environment approximately like that of a fully formed dence with the range of2964-2966 cm-l reported at room monolayer, as opposed to that of a disordered layer. The temperature for monocomposition, dense-packed, oriented most straightforward way to rationalize this conclusion monolayer films of n-alkanethiolates on gold7J6332and n-alkanoates on aluminum (native oxide c ~ v e r e d ) .For ~ ~ - ~ ~is on the basis that the protruding (CH2),CH3 chain segments are localized into densely-packed clusters. We the cases of m > 0 SAMs, the observed r- frequencies note that while the above correlations of r- frequencies remain constant at -2967 cm-I. In these cases, for which are useful for comparison purposes, the underlying cause the CH3 groups protrude above the surface at -50% of these shifts remains ~ n c l e a r . ~ ~ , ~ ~ coverage,comparisons are needed to account for the effects of both coverage and CH3 distribution (e.g., clumping). 0 - H StretchingRegion. Figure 11 shows the portion The data of Nuzzo et al.47for the mixed SAMs of S(CD2111CDa21CH3 show a broad peak centered at -2965(f1) (57) Sondag, A. H. M.; Raas, M. C. J . Chem. Phys. 1989,91,4926cm-l near 1:l S A M composition. Increases in the long493 1. (58) Dunbar, T. E.; Sheen, C. W.; Allara, D. L. Unpublished results. chain fraction shift the intensity to the higher frequency (59) Saperstein, D. J . Phys. Chem. 1986,90, 1408-1412. side of the peak, but the average frequency remains (60) There are (at least) two types ofdensity effects to consider. First, roughly constant. While these data are not able to reveal downward r- shifts of -2-3 cm-I arise from contact of CHs-terminated SAMs/Au with liquids such as water and methanol (IRS: Stole, S. M.; very clear correlations between r- frequencies and mono(53) The r- spectra of alkyl chains depend upon both spatial and temporal effects and ared strongly temperature and orientationally dependent (MacPhail, R. A.; Snyder, R. G.; Strauss, H. L. J . Chem. Phys. 1982, 86, 1118-1137). The two orthogonal components with vibrations in (rl-) and out (la-) of the C-C-CH3 plane are broadened near room temperature to form an asymmetric singlet peak, typically at -2956 em-', for isotropic samples such as polycrystalline n-alkanes (MacPhail, R. A.; Snyder, R. G.; Strauss, H. L. J . Chem. Phys. 1982,86, 1118-1137) and a variety of alkyl compounds. Shifts of the ra-/rbintensity ratio can arise for a variety of reasons, including structural and orientationa150 effects, leading to apparent shifis in the average rpeak position. (54)Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. ( 5 5 ) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93-98. ( 5 6 ) Tao, Y.-T. J . Am. Chem. SOC.1993, 115, 4350-4358.

Porter, M. D. Langmuir 1990,6,1199-1202. Sumfrequencygeneration spectra: Ong, T. H.; Ward, R. N.; Davies, P. B.; Bain, C. D. J . Am. Chem. SOC.1992,114,6243-6245). The downward r- shifts observed with decreasing m values form SO in Figure 8 would be consistent with this trend, in part, on the basis of overlayer structures in which the protruding (CH&+1 OH segments increasingly cover the underlying CH3 group surface, analogous to a liquid layer. No such overlayer interactions can arise for the CHs groups with m > 0, consistent with the observed constant frequencies. Second, the r- peak frequency of n-octane shifts from 2959 to 2969 cm-1 in going from the liquid to the vapor phase at room temperature (Cameron, D. G.; Hsi, S. C.; Umemura, J.;Mantsch, H. H. Can.J . Chem. 1981,59,1357-13601, an observation attributed to increased rotational motion and reduced intermolecular interactions upon vaporization. While this shift is similar to that observed with the emergence of CH3 groups from the mixed S A M surface (increasingm), one must use caution since the average CH3-CH3 spacing (volume density) in the mixed SAMs is -2 orders of magnitude higher than for an ideal gas at room temperature.

OH- and CH3-TerminatedAlkanethiolates on Gold of the spectra of the mixed monolayers and pure R160H and R&H3 monolayers in the 3000-3800-cm-’ region. The very broad peak of the pure R160H spectrum is assigned to the 0 - H stretching vibration of intermolecularly hydrogen-bonded OH groups.61 Compared to the pure RI6OH monolayer spectrum, the mixed monolayer peak intensities are highly attenuated for the -6 Im 5 -2 cases and are nearly absent when m L 0. Similar observations have been reported for OWCH3 mixed monolayers with m = -4 and 0.26 The constant lack of an observation ofthe expected sharp free 0-H stretching mode between 3618 and 3636 cm-l 61 indicates that all OH groups are hydrogen-bonded over the range of m values. One should note that in a 1:l monolayer, even at the limit of random chain distribution (see Figure l ) , numerous multiplets of clustered alcohol groups will exist, and a significant degree of intermolecular hydrogen bondingis thus always expected. On this basis, the above loss in the H-bonded 0 - H stretching mode intensity for m L 0 cannot be ascribed to loss ofH bonds. Rather, other factors must be responsible such as changes in line widths and intrinsic oscillator strengths caused by shifts in the dispersion of H-bonding states. Such perturbations could be caused readily by shifts in the degree of clustering and relative vertical placement of the OH groups with changing m. While the 0-H stretching spectra appear to offer the possibility of detailed surface structure information, further study is needed to untangle these complex effects, and such studies are in progress. 4. Discussion

4.1. Variations of SAM Structures with Differential Chain Lengths (mValues). In this section, we show that the IRS and wetting data, in combination with other reported work, lead to the description of the high- and low-m-value SAMs in terms of the generic structures given in Figure 1B and E, respectively. The IRS data show that the air/SAM interface (dry) structures can be characterized in terms of two components: (1)an underlying assembly of S(CH2)15+,CH3 (-6 < m < 0) or S(CH2)1&H20H (m > 0) chains with a structure nearly identical to that of a comparable monocomposition alkanethiolate/Au SAM and (2)a 50% surface coverage overlayer of protruding chains whose structure changes sharply in the region of m = -2 to 0, the point at which the CH3-terminated chains are depressed slightly below the level of the OH-terminal groups. With increasing values of m for m > 0, the combination of the observed decreases in d- frequencies, narrowing of d- line widths, and increasing r- frequencies suggests an approach to a limiting air/SAM structure in which the (CH2),CH3 segments exist as clusters of contiguous, largely trans chains extended above the underlying OH surface, as depicted in Figure 1B. Structures with larger average chain spacings, such as given in Figure lC, are less likely possibilities a t high m values because of the higher inherent gauche contents. With decreasing values of m for m < 0, the increase of the d- frequencies to 2921 cm-l at m = -6 (Figure 8) and the parallel increase in line widths signal approach a limiting air/SAM structure in which the chains exhibit multiple gauche defects. Structures similar to those depicted by Figure lC, D, and E, particularly D and E, would be consistent with these data. The wetting data, which characterize the liquid/SAM interface (wet), show sharp changes at m = -2 to 0 for hydrocarbon liquids but are essentially invariant for hydrogen-bondingliquids. We first make interpretations (61)Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1975.

Langmuir, Vol. 11, No. 10, 1995 3891 of these data on the basis of static interfacial structures. Later, we consider liquid-dependent surface reconstruction. At high m values, the wetting data indicate approximately full exposure of the OH groups a t the interface with mixed exposure of the CH2 and CH3 groups but a domination by CH3 groups (Figure 5). Structures consistent with these data would have the protruding (CH2),CH3 segments extended into the liquid, similar to the structures depicted in Figure 1Bor C. At low m values, the complete loss ofoleophobic character (no CH3 exposure) with maintained hydrophilic character (OH exposure) is most simply interpreted in terms of an approach to limiting structures in which the protruding (CH2),+10H segments exit from the underlying (CH2)CH3 surface and lie relatively flat across the (CH21CH3 surface depicted as Figure 1Dor E. In the limit ofcompletely flattened chains, an initialgauche kink is required in the S ( C H Z ) ~ ~ C H ~ O H chain (at carbons 14, 12, and 10 in the m = -2, -4, and -6 SAMs, respectively). The combined wetting and IRS data in the m > 0 region restrict the possible wet and dry SAM structures to ones with extended (CHz),CH3 segments, similar to Figure 1B or C. The latter two structures differ by the degree to which the long alkyl chains are pinned at adjacent sites at the substrate surface. Further constraints on the extent of clustering of pinning sites can be set by analysis of the reported62behavior of a nearly exact experimental analogue in which O(CH2),$H3 chains, with systematically varied lengths, were covalently bonded at -50% coverage to fixed random sites on a hydrophilic (+3iOH-covered), amorphous Si02 substrate surface. With these monolayers, it was observed that for n I12, complete wetting of hexadecane occurs while the d- frequencies peak at -2924-2925 cm-’, whereas for n L 16, &ID attains a constant, strongly oleophobic value of 36(&2)Oand the dfrequencies move to -2919-2920 cm-l. Applying these observations to the present 0 < m 5 6 SAMs shows definitivelythat the -50% coverage protruding(CH2),CH3 chains cannot be both randomly pinned and give the observed strongly oleophobic wetting character and low d- frequencies, since m I6 is well below the required 12- 16 carbons for the onset of such behavior in randomly pinned structures. The predicted wetting behavior for randomly pinned alkyl chains in the m > 0 region is shown for convenience in Figure 5, indicated in terms of the CH3 group exposure, where it is seen that the prediction is in complete contrast to the experimentally derived points. The only reasonable conclusion from this analysis is that the S(CH2)(15+,1CH3 chains in m > 0 films are not randomly pinned on the gold surface; uiz.,them > 0 SAMs are phase segregated, similar to the type of contiguous chain structure depicted in Figure 1B. Precedence for the latter is given by recent STM63results, which show the existence of -2-5-nm monocomposition domains64 in mixed alkanethiolate/Au S A M s with similar components, and by TOF-SIMS data.65 In support of this conclusion, we note (62)Allara, D.L.;Parikh, A. N.; Judge, E. J.Chem. Phys. 1994,100, 1714-1717. ( 6 3 )Stranick, S.;Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994,98,7636-7646. (64)With regard to domain size, we note that the C-0 and 0-H stretching mode spectra (Figures 6 and 11,respectively) show that the peak-shapes and intensities are quite different from those observed in the pure RIGOHmonolayer and thus appear to preclude the possibility of large-scale, macroscopic phase segregation of the RxOH component in the mixed SAMs. (65)Recent time-of-flight secondary ion mass spectrometry investigations of RAuR’ cluster peaks from equal chain length OWCH3mixed S A M s indicate the existence of phase segregated domains on a nanometer scale (Wood, M.; Atre, S. V.; Liedberg, B.; Zhou, Y.; Allara, D. L.; Winograd, N. Manuscript in preparation).

3892 Langmuir, Vol. 11, No. 10, 1995 that recent molecular dynamics simulations66 of dry, mixed-chain-length n-alkanethiolate SAMs, similar to the present ones, at ambient temperatures show that the protruding segments of the longer chains, when pinned at adjacent sites, adopt extended, highly trans conformations, except for the perimeter chains which tend to be more disordered. In contrast, randomly pinned chains give rise only to highly disordered overlayer structures. The combined wetting and IRS data for them -= 0 SAMs are consistent with an approach to collapsed (CH2)(lml+l)OH segment structures similar to those in Figure 1D or E. The contiguouslypinned chain type ofstructure (Figure 1D) does not seem likely at the air/SAM interface since one would expect preferential clustering of the adjacent protruding (CH2)(l,l+l,0Hsegments, based on both chainchain van der Waals interactions and the possibility of H-bondinginteractions between the terminal OH groups. Further, as the analysis in the next section shows, highly collapsed, randomly pinned chain structures at the hydrocarbon/SAM interface are quantitatively consistent with the loss of oleophobicity in low-m structures. The reason for the shift in the intrinsic chain pinning pattern from clustering to random with decreasing m is unclear at the present time, but we believe that it is related to solvation effects during a s ~ e m b l y . Further ~ , ~ ~ work is in progress to uncover these effects. 4.2. Wetting and Monolayer Surface Structure. The limiting structure models proposed above imply specific wetting responses for surfaces with specific molecular topographies and composition. In this section, the underlying basis for such behavior is examined. Because the detailed molecular basis of wetting is not well understood,68the analysis begins with the simplest assumption of a wetting model in which the monolayer structure is similar at the air/SAM and liquid/SAM interfaces; uiz., reconstruction is not considered, the liquid is a continuum fluid, and the actual geometrical area occupied by a molecular group governs its contribution at the wetting interface. This model is clearly oversimplistic, and realistic deviations will be discussed at the end of the section. First, we examine the oleophobic behavior of the m < 0 SAMs, in particular, the sharp change in the CH3 group exposure (Figure 5). We start with a model of randomly pinned chains in which the protruding (CH2)(1,1+1,0H segments can statistically cover all underlying CH3 groups on average, similar to Figure 1E. A simple prediction follows that complete screening of the CH3 groups occurs when the total projected steric area of the methylene units of the overlying (CHz)(l,l+l,OHsegments is at least that ofthe total surfce area occupied by the CH3 groups. Using approximate dimensions of -5 and 1.25m A (m is used here as its absolute value) for the and length,70 respectively, of an extended polymethylene chain, the area covered per segment of length m is 6.25m Az. Given the equal number of protruding chains and underlying CH3 (66) Bhatia, R.; Garrison, B. J. Personal communication. (67) The mechanismb) by which phase segregation in SAMs arises remains unclear. Published STM experiments (ref 63) show that the surface motion ofalkanethiolate chains on a Au{ 111)substrate appears insufficient to significantly alter the distribution of a once-formed mixed SAM at room temperature. On this basis, it appears that phase segregation must arise duringthe deposition process and that solvation effects may be important. Experiments centering on solvent effects in the formation of mixed S A M structures using scanning tip microscopies are in progress and will be reported elsewhere [Dunbar, T . E.; Cygan, M.;Allara, D. L.; Weiss, P. S.Unpublishedresults. Coulman, D.; Parikh, A. N.; Allara, D. L. Unpublished results]. (68) de Gennes, P. G. Reu. Mod. Phys. 1986,57,827-863. (69) Billmeyer, F. W. Textbook of Polymer Science; John Wiley and Sons: New York, 1994. (70) Frommer, J.Angew. Chem., Int. Ed. Engl. 1992,31,1298-1328.

Atre et al. groups and using the known value of 21.4 A2 for the area the fraction of the surface which could per CH3 be covered by the protruding chains when completely flattened against the underlying CH3 surface was calculated, and the results are shown graphically in Figure 5. The fit to the observed CH3 exposure calculated from the wetting data, by either eqs 2 or 3, is striking and supports a randomly pinned, collapsed protruding chain structure at the hydrocarbodSAM interface, similar to Figure 1E. In this picture, CH3 screening is complete at m -4, so for surfaces with m s -4, the protruding (CH2)(lml+l)OH chain segments must overlap if they are to remain flattened. For example, for m = -6, the fractional total surface area covered is 0.88 [=(5 x 6 x 1.25)/(2x 21.411, which is equivalent to -75% (-100 x 0.38/0.50) chain overlap. Such overlap further implies that additional gauche kinks arise with decreasing m, in accord with the observed d- frequency trends (see discussion in section 4.1) observed for the dry surfaces. The success of the simple steric model above in fitting the hydrocarbon wetting data implies that the screening of the surface is complete over a liquid/molecule separation of the width of an alkyl chain, -5 A, and that actual surface areas and steric dimensions appropriately describe screening interactions of nonpolar alkyl chains and hydrocarbon solvents. The first conclusion is in excellent agreement with previous experimental evidence.71 Next, the effective surface coverage values in Figure 5 are examined for qualitative consistency with the limiting structures of the low- and high-m surfaces. The low-m limiting model (represented by Figure 1E) shows exposure of only OH and CH2 groups. Figure 5 shows -35-45% and 65-55% OH and CH2 effective coverages, respectively, at m = -6, and the OH value is close to the stoichiometric composition of50%. The higher CHz value suggests some steric screening of the OH groups by overlapping CH2 segments, consistent with the -75% chain overlap calculated above. In contrast to the low-m limiting model, the high-m one (cartoon in Figure 1B) shows a vertically featured topography with an apparent total effective surface area for wetting greater than the geometric area. .If the protruding (CHz),CH3 clusters were considered as simple vertical cylinders of hexagonally packed shells of -5-Adiameter chains, then the CH2 surface area would be the cylinder wall area, approximated from the diameter (number of chains across) and the height (-1.25m A, ignoring tilt). Using 21.4 Azas the area occupied per OH and CH3 groups (see above), the experimentally derived average CHZ, OH, and CH3 exposures of -10-20,35-45, and 45% (Figure51, respectively, form > 0 can be achieved with clusters in the range of -500 chains (-125-A diameter, giving -14% CH2 exposure) for m = +6 and -50 chains (-25 A, -12% CH2) for m = f 2 . However, these results appear unrealistic since it seems highly unlikely that there is any particular driving force during film formation to systematically change the cluster size with the m value in this way. This conclusion likely indicates a failure of some or all the initial assumptions, uiz., eqs 2 and 3 and a simple continuum wettingmedium, to adequately describe the wetting behavior ofthe high-m structures. The above analyses of the wetting data in terms of the simple predicted behaviors ofthe low- and high-m limiting models using eqs 2 and 3 show good success for the former but apparent unrealistic behavior for the latter. Among several possibilities, the two main deviations from the

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(71) Wilson, M. D.; Ferguson, G. S.; Whitesides, G. M. J.Am. Chem. SOC.1990, 112, 1244-1245.

OH- and CH3-Terminated Alkanethiolates on Gold

simple wetting model to consider are limited steric access of the wetting liquid to the S A M surface groups and wetting-induced reconstruction of the S A M surface. With respect to the former deviation, in the high-m SAMs, hexadecane and dicyclohexyl,which discriminate strongly between pure CH3 and pure CHZsurfaces, appear to have decreasing sensitivity, or, alternatively, decreasing access, to the growing CH2 surfaces of the clusters with increasing m as judged by the lack of an expected drop in the contact angles. In sharp contrast, the near independence of water and formamide contact angles on m in the m 2 0 region implies constant access of these liquids to the underlying OH surface. Since the sizes of water,35 f ~ r m a m i d edicyclohexyl,72 ,~~ and h e ~ a d e c a n are e ~ ~2.9,3.6, 10.4 (long axis), and 22.8 (extended dimension) or 13.7 A (rms collapsed chain diameter), respectively, any discrimination in access based purely on steric size would require fairly small cluster spacings and diameters.74In contrast, for the low-m surfaces, the wettingdata discussed earlier appear adequately understood on the basis of complete access of liquid to the surface groups. Some degree of liquid-induced surface r e o r g a n i ~ a t i o n ~ ~ should be expected, a priori, for the S A M surfaces, especially in hydrocarbon solvent where the surface constituent intermolecular forces are of a similar magnitude to the liquid-liquid interactions. This is an important consideration since the more extensive the reconstruction, the less the correlation between the dry (by IRS) and wet S A M structures. On the basis ofpacking density, the protruding overlayer chains will be more prone to reorganization than the underlying matrix of denselypacked chains. The latter should show relatively minimal reorganization, similar to a maximum coverage alkanethiolate S N A u where solvent perturbs the conformations of the chain terminPo but liquid penetration and consequent conformational reorganization fall off sharply with depth in the densely packed chain For the low m SAMs, the close fit of the hydrocarbon wetting data (Figure 5) to the predictions based on a simple steric screening model with flattened (CHz)(lm1+1,OH segments (Figure 1E) indicates that solvent reorganization is minimal. In the case of the high-m surface, described (72) Estimate based on space-filling models. (73) Smith, A. E. Acta Crystallogr. 1952, 5, 224-235. (74) Relevant to this point, it has been reported [(a)Bain, C. D.; Whitesides,G. M. J.Am. Chem.Soc. 1988,110,5897-5898. (b)Shafrin, E. G.; Zisman, W. A. J.Phys. Chem. 1962,66,740-7481 that water has a 3-A greater penetration range than hexadecane with respect to the wetting effect of the ether oxygen in the chains of CH3(CH2),0(CH2)16S/ Au SAMs (0 5 m 5 6). For fully dense monolayers, the maximum 0-0 spacings (to admit HzO molecules) would be given by those at tilt-phase boundaries, -19 A for the above mercapto ether given a 60"(=2 x 30") chain-chain angle along the phase boundary. (75) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G . M. J. Phys. Chem. 1995,99, 7663-7676.

Langmuir, Vol. 11, No. 10, 1995 3893

in terms of contiguously clustered chains, water would not be expected to do more than perturb the chain termini of the dense clusters, as for a full coverage S A M ,while hydrocarbon could be expected to penetrate and "loosen" the chains at the outer edges of the clusters, minimally affectingthe dense cores ofthe clusters in a fashion similar to a full coverage S A M . On this basis, the dry and wet high-m S A M structures will be closely correlated. Theoretical and experimental studies are in progress to better define the actual structure^.^^^^^ 6. Conclusions Infrared spectroscopy and wetting measurements indicate that when equimolar, binary-component monolayers of S(CHz)&HzOH and S(CHZ)(~~+~&!H~ moieties, with -6 Im 5 +6, are self-assembled on gold surfaces from ethanol solutions of the corresponding thiols, a spectrum of structures results which span two limiting extremes. At low m, the films approach a limiting structure of protruding HO-terminated chain segments, randomly exiting from and splayed across the underlying CH3 surface,while at high m, a limiting structure is approached in which adjacent CH3-terminated chain segments extend away from the underlying CHzOH surface as organized clusters. The underlying reason for these structural trends appears to be a shift in the substrate pinning pattern of the chains with increasing m from random to adjacent. These results demonstrate that designed molecular-scale architecture of functional groups in mixed self-assembled monolayers offers an approach for precise control of surface properties such as wetting, chemical reactivity, and biological response but also indicate that further work is needed in the development of controlled methods of surface preparation.

Acknowledgment. We thank Atul Parikh for many valuable discussions. This work has been supported by NSF (DMR-900-1270; D.L.A. and S.V.A.), the Procordia Science Foundation (B.L.), and the Swedish Research Council for Engineering Sciences (B.L.). LA950126N (76) Some indication ofthe reconstruction can be obtained from recent molecular dynamics simulations of hexadecane wetting of randomly distributed, 50% coverage arrays of CH3(CH*)!, chains (Kacker, N.; Kumar, S.;Allara, D. Unpublished results]. The initial results indicate that the dry surface has highly disorganized chains and that upon wetting, chain conformational reorganization is surprisingly slight. Further, the liquid penetration into the S A M is limited to less than a 20% additional incorporation of liquid chains at depths of less than several angstroms. Given that the simulation chains are terminated by CH3 groups rather than OH groups, further comparisons with the mixed S A M data are difficult a t this point. (77) Experiments involving in situ analyses of the liquiUSAM interface are underway using spectroscopic ellipsometry (SE) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett., in press.