52
Langmuir 1985,1, 52-66
Taken together, the data show that closest packed, oriented monolayers of n-alkanoic acids can be formed by adsorption from dilute solution. The formation of these quasi-two-dimensional organic surface phases is characterized by complicated kinetics in which surface and/or monolayer defects, as well as impurities, seemingly play important roles. The structures so obtained are dynamic in nature in that they undergo rapid exchange with comparable ligands in solution. These results, and those presented in the following paper, further suggest an im-
portant limitation to closest-packed, self-assembly on this substrate which is related directly to the length of the alkyl chain. The data further show that oleophobicity is an insufficiently sensitive probe to determine the formation of equilibrium closest packed structures. In a companion paper we describe these structures and their relationship to many of the above observations in detail. Registry No. n-Cl9H&O2H, 506-30-9; aluminum 1344-28-1; stearic acid, 57-11-4; hydrogen, 1333-74-0.
oxide,
Spontaneously Organized Molecular Assemblies. 2. Quantitative Infrared Spectroscopic Determination of Equilibrium Structures of Solution-Adsorbed n -Alkanoic Acids on an Oxidized Aluminum Surface David L. Allara* Bell Communications Research, Murray Hill, New Jersey 07974
Ralph G. Nuzzo* AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Received August 16, 1984 Infrared reflection spectroscopy has been applied to the determination of the structures of adsorbed monolayer films of n-alkanoic acids on oxidized aluminum substrates. Acids of 16-22 carbons, terminated by methyl, vinyl, or propargyl groups, were absorbed from hexadecane solution at 25 O C , using immersion times up to several days. These results,together with those for perdeuterated acids, indicate that close-packed assemblies are formed with extended alkyl tails oriented with their chain axes tilted away from normal to the surface with a value of 10” for the longer chains. Frequency shifts to higher values are observed for CH stretching modes of the vinyl and propargyl terminal groups measured in monolayer films compared to the bulk acids. These shifts are of similar magnitude to gas-liquid phase shifts for these groups and imply the environment of the ambient-monolayer interface exhibits very diminished intermolecular interactions for these terminal groups compared to the bulk. The spectra clearly show that chemisorption occurs by proton dissociationto form carboxylatespecies. Examination of both line width and peak positions of the carboxylate stretching modes suggests a variety of binding geometries of the carboxylate groups to the surface exists. These results lend support to the general concept of forming stable, oriented and ordered two-dimensional organic films by solution adsorption and show the utility of applying surface vibrational spectroscopy to structural determination in these films.
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1. Introduction
The structures, formation, and properties of organic molecular monolayer films are the subjects of much current interest. One system of recent particular interest to us has been spontaneously adsorbed n-alkanoic acid monolayer films on ambient metal surfaces. In a previous paper’ we presented extensive results on the formation, properties, and dynamics of solution-adsorbed n-alkanoic acid films on oxidized aluminum substrates. Central to a comprehensive understanding of those results is the development of the details of molecular structure. Such detail includes the nature of the individual molecular species, e.g., ion formation, surface orientations, and surface-adsorbate bonding, and the nature of the total monolayer assembly, especially as regards ordered packing arrangements. While a number of surface analysis tools are now available due to extensive developments in the surface physics community, in actual fact, few are applicable to the analysis of the subtle features of the chemical structures of compli(1) A h a , D. L.; Nuzzo, R. G. Langmuir, preceding paper in this issue.
cated molecular organic films. A very elementary problem is that most popular surface techniques require impinging ions or electrons as probes and this usually causes damage to organic molecules. The techniques that have been generally found useful for organic structures are those using photon probes, in particular, X-ray photoelectron, infrared, and Wan spectroscopies. Additionally, inelastic electron tunneling and electron energy loss spectroscopies have had success for organic monolayers. Past studies of the adsorption of medium- to long-chain n-alkanoic acids on ambient metal surfaces have utilized electron diffract i ~ n ,inelastic ~ - ~ electron tunneling (IETS), (2) A review of earlier work can be found in: Bowden, F. P.; Tabor, D. “The Friction and Lubrication of Solids”; Oxford University Press: London, 1968; Part 11, Chapter 19. (3) Menter, J. W.; Tabor, D. Proc. R . SOC.London, Ser. A 1957,204, 514-524. (4) Chapman, J. A,; Tabor, D. Proc. R. SOC.London, Ser. A 1957,242, 96-107. (5) Brockway, L. 0.; Karle, J. J. Colloid Sci. 1947,2, 277-287. (6) Cass, D. A.; Straws, H. L.; Hansma, P. K. Science (Washington, D.C.)1976, 192, 1128-1130.
0743-7463/85/2401-0052$01.50/0 0 1985 American Chemical Society
Spontaneously Organized Molecular Assemblies. 2
and infrared spectroscopygJO(IR). The latter two vibrational spectroscopy techniques are quite sensitive, even for submonolayer coverages and, in general, are quite informative with regard to the chemical nature of the adsorbed species, in contrast to electron diffraction. Recently infrared spectroscopyhas been applied to the determination of chain orientation" and ordering12of n-alkanoic acid salts deposited by the Langmuir-Blodgett technique. Thus for our initial studies we have selected IR spectroscopy as a general probe of adsorbate structures. In our previous paper,' we determined conditions for the formation of stable monolayer assemblies of n-alkanoic, alkenoic, and alkynoic acids on oxidized aluminum and measured such selected properties as film thicknesses, contact angles, and tendencies toward molecular exchange in a solution environment. Since it was found that acids with greater than approximately 12 carbons lead to more consistent results, we have chosen to present in this paper only our results on the longer chain acids. It is the purpose of this paper to apply quantitative IR reflection spectroscopy to the determination of molecular structure in these films. In particular, we establish evidence to lend support to a model of assemblies of C16and longer alkanoic acids bound to the oxide substrates by multiple alkanoate ion species with strongly tilted and twisted head-group orientations and extended chains oriented about loo away from perpendicular alignment to the surface. In addition, we show evidence for the environmental effects of the air-film interface relative to the bulk molecular environment in the cases of terminally unsaturated chains. Since our approach is to be quantitative, a section of the paper is devoted to collecting together and presenting the known principles of macroscopic dielectric electromagnetic theory which we utilized for our structural interpretations drawn from the IR spectra. As far as we are aware, this particular detailed approach has not been used by others and, accordingly, is presented here. 2. Experimental Section 2.1. Materials and Sample Preparation. Full details of the materials used and sample preparations are given in the previous paper.' The samples generally consisted of -200-nm films of evaporated aluminum (99.999%) on single-crystal silicon wafers polished to high optical quality. Adsorption of the alkanoic acids was done from dilute hexadecane solutions (generally 0.005 M) thermostated at 25.0 0.3 "C. Most of the substrates are pretreated with acetic acid solutions, and the adsorbed acetate is displaced by the chosen alkanoic acid. The IR spectra did not appear different for these two methods although some difference might have been expected because of the observation that not all acetate is displaced in the formation of longer chain acid monolayers.' Samples were generally immersed for 2 or 3 days before analysis, a point at which the films appeared to exhibit no change in structure with time. Storage of the samples under ambient conditions for extended periods did not induce changes in the IR spectra. 2.2. Infrared Measurements. IR spectra of adsorbates were taken by reflection of the incident beam at an angle of incidence of 87' (3" off glancing) using p-polarized radiation. A nitrogen-purged Digilab 15-B Fourier transform spectrometer was used with modified optics to permit focusing outside the instrument
*
( 7 ) Brown, N. M. D.; Floyd, R. B.; Walmsby, D. G. J . Chem. SOC., Faraday Trans. 2 1979, 75, 261-270. (8) Hall, J. T.; Hansma, P. K. Surf. Sci. 1978, 76, 61-76. (9) Boerio, F.J.; Chen, S. L. J. Colloid Interface Sci. 1980,73,176-185. (10)Golden, W.G.; Snyder, C.; Smith, B. J. Phys. Chem. 1982, 86, 4675-4678. (11) Allara, D. L.;Swalen, J. J . Phys. Chem. 1982, 86, 2700-2704. (12) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946-952. Chollet, P. A. Thin Solid Films 1978, 52, 343-360.
Langmuir, Vol. 1, No. I , 1985 53
2
Figure 1. Schematic representation of a multiple-phase, parallel-layer sample having a total of m phases. Light approaches the sample from infinite phase 1at an angle of incidence The directions of the electric field for s (out of plane) and p (in plane) polarizations are shown for incoming and outgoing propagation directions, designated by and - superscripts, respectively.
+
and equipped with stops and apertures to give an -f/15 beam focusing to an -3-mm spot. The collected beam was detected by a mercury-cadmium-telluride liquid-nitrogen cooled detector with a 1-mm2active area and a cutoff at -700 cm-'. Spectra were taken at 2-cm-' resolution with a mirror speed of 1.4 cm/s. The interferograms were collected in double precision and Fourier transformed with triangular apodization. Usually about 200-800 scans were signal averaged for acceptable signal/noise ratios. Reference spectra were obtained on freshly evaporated gold-covered silicon wafers. These samples were checked carefully for hydrocarbon contamination and cleaned further using hot HZSo4/30%HzOz(5:1, v/v) if necessary to assure that no features would be present in the final ratioed spectra. Dirty reference samples can cause intensity errors and for sufficiently dirty samples can result in "upside down" C-H stretching peaks in the final sample/reference ratio spectra. Such artifacts can be checked by changing polarization manually or by high-frequency modul a t i ~ n . ' ~ , 'Our ~ references were checked by the former method and no residual C-H features were found when our cleaning procedure was carefully followed. Samples were mounted in a separately purged compartment isolated from the spectrometer system by KBr windows. The nitrogen purge gas was passed through molecular sieve and charcoal filters. Spectra of bulk compounds were obtained using normal incidence transmission spectra of pressed KBr disks prepared in a dry nitrogen atmosphere.
3. Theory of t h e Optical Measurements and Calculations 3.1. Parallel Layer Model. The infrared experiments presented in this paper, as well as the ellipisometry measurements in the previous paper,l involve the interaction of a light beam with a macroscopically smooth, reflective solid material. The theory which relates the macroscopic variables of the experiment to the propagating incident and reflected electric fields has been thoroughly developed in terms of boundary value solutions to Maxwell's equations for the cases of parallel layer samples where each phase can be described by a spatially uniform dielectric f~nction.'"'~ Specific adaptations for reflection spectroscopy have been describedl8Jgas well as the importance (13) Dowrey, A.E.;Marcott, C. A. Appl. Spectrosc. 1982,36,414-416. (14) Golden, W. G.; Saperstein, D. D.; Severson, M. W.; Overend, J. J. Phys. Chem. 1984,88, 574-581. (15) The general principles can be found in: Born, M.; Wolf, E. "Principles of Optics", 5th ed.; Pergamon Press: New York, 1965. (16) Heavens, 0.S. "Optical Properties of Thin Solid Films"; Dover Publications: New York, 1965; Chapter 4. Am. (17) For a general treatment, see: Berremen, D. W. J . Opt. SOC. 1972, 62, 502-510.
Allara and Nuzzo
54 Langmuir, Vol. 1, No. 1, 1985
of the application of these principies to quantitative analysis of band shapes and intensities.*O A brief description will be given here to show the development of our calculations, particularly for the determination of adsorbate orientations and corrected spectral shifts from IR data. Figure 1 depicts the interaction of a parallel monochromatic light beam of frequency i j (in wavenumbers) with a parallel layer sample consisting of m phases each having complex optical functions 6, where j referes to the j t h phase. The quantity tij is defined by eq 1in terms of the A, = n, + ik, (1) real refractive index nj and the absorption constant k .. For each polarization, the incoming, transmitted and redected complex electric fields, El+, E,+, and E
