Langmuir 1985, 1, 45-52
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Spontaneously Organized Molecular Assemblies. 1. Formation, Dynamics, and Physical Properties of n -Alkanoic Acids Adsorbed from Solution 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 This study shows that closest packed, oriented monolayers of n-alkanoic acids can be formed on oxidized aluminum substrates by adsorption from dilute solution. The formation of these 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 ligands in solution. The data further suggest an important limitation to self-assembly in this system which is directly related to the length of the n-alkanoic acid tail. The influences of experimental variables on the formation of oriented monolayers is discussed.
Introduction The adsorption and spontaneous organization of amphiphilic molecules, especially the n-alkanoic acids, on smooth, ambient' metal surfaces, are the subject of a vast l i t e r a t ~ r e . ~ - 'The ~ interest in these surface adsorbate structures traditionally has been motivated by their relevance to such important technological areas as lubrication, corrosion, and catalysis. In recent years, speculative technological applications, involving such areas as optics and microelectronics,16 as well as the development of surface-modified electrodes for electrochemical applicat i o n ~ have ' ~ generated a renewed interest as to the nature (1)The term ambient is used to indicate that moat attention has centered on metals having oxide overlayers which may or may not have contained various levels of impurities and/or contaminants. (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)(a) Chapman, J. A.; Tabor, D. R o c . R. SOC. 1967,London, Ser. A 242, 96-107. (b) Brockway, L. 0.; Karle, J. J. Colloid Sci. 1947, 2, 277-287. (c) Menter, J. W.; Tabor, D.R o c . R. SOC.London, Ser. A 1961, 204,514-524. (d) Sagiv, J. J. Am. Chem. SOC.1980,102,92-98. (4)Walker, D.C.; Ries, H. E. Adv. Chem. Ser. 1964,No. 43,295-301. (5)Doyle, W. P.; Ellison, A. H. Adu. Chem. Ser. 1964,No. 43,268-274. (6)Gaines, G. L. J. Colloid Sci. 1960,15, 321-339. (7)Timmons, C.0.; Zisman, W. A. J. Phys. Chem. 1965,69,984-990. Patterson, R. L.; Lockhart, L. B. J. Colloid In(8)Timmons, C. 0.; terface Sci. 1968,26, 120-127. (9)Bornong, B. J. Surf. Sci. 1969,16,321-330. (10)Allara, D.L.; Tompkins, H. G. J . Colloid Interface Sci. 1974,49, 410-421. (11)Boerio, F. J.; Chen, S. L. J. Colloid Interface Sci. 1980, 73, 176-185. (12)Brown, N. M. D.; Floyd, R. B.; Walmsley, D. G. J. Chem. SOC., Faraday Trans. 2 1979,75,261-270. (13)Lewis, B. F.; Mosesman, M.; Weinberg, W. H. Surf. Sci. 1974.41, 142-164. (14)Golden, W. G.; Snyder, C.; Smith, B. J. Phys. Chem. 1982,86, 4675-4678. (15)Cass, D.A,; Strauss, H. L.; Hansma, P. K. Science (Washington, D.C.)1976,192,1128-1130. (16)For example, see: (a) Vincett, P. S.; Roberts, G. C. Thin Solid film 1980,68,135-171.(b) Special issue on molecular electronics: Proc. IEEE 1983,230,197-263. (17)For example, see: Faulkner, L. R. Chem. Eng. News 1984,28-45 and references therein.
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of quasi-two-dimensionalorganic assemblies and the processes by which they can be prepared. As regards the latter point, spontaneous adsorption methodologies are to be contrasted with the Langmuir-Blodgett transfer process'* in which ordered, multilayer assemblies can be produced for the longer chain acids &e., fatty acids), particularly when the total number of carbon atoms is greater than 18. In the case of spontaneous adsorption, no preassembling of the adsorbate molecules is carried out; the substrate is exposed directly to the adsorbate in solution or the gas phase and any ordering that occurs is thus considered to be spontaneo~s.'~The questions of and related to the adsorption mechanism(s) (reflecting kinetics) and equilibrium structures (reflecting thermodynamics)are of great interest, but past studies have been intended to concentrate almost exclusively on the latter. It is not clear in all cases, however, whether true equilibrium structures, comprised solely of the intended adsorbate(s), in fact have been studied. Most studies of the adsorption of carboxylic acids on metals have involved surfaces exposed to air prior to adsorption and thus largely refer to adsorption on native oxide overlayers. These studies, which largely complement the voluminous data on bulk oxide powders, can be divided into three general functional classes: (1)acids chemisorbed to form metal carboxylate salts, processes that may involve substrate corrosion and multilayer formation; (2) acids chemisorbed via proton transfer to a lattice oxygen atom to yield monolayer structures; (3) acids chemisorbed (perhaps physisorbed is a more appropriate description) with no proton transfer. Representative examples of these three are carboxylic acid chemisorption on copper,6,10J1 aluminum,'2-16 and siliconFOrespectively. Although the cases of multilayer formation are quite important and of significant interest, the less complicated cases of monolayer chemisorption are more attractive as beginning points for (18)For example, see: Gaines, G. L. 'Insoluble Monolayers at Liquid-Gas Interfaces"; Interscience: New York, 1966. (19)As we shall detail, spontaneous, in this context, cannot be used interchangeably with the word instantaneous. (20)Iler, R. K. "The Chemistry of Silica"; Wiley: New York, 1979; Chapter 6.
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Allara and Nuzzo
careful structural studies of the first formed layer. A concern with regard to many of the studies cited above, in particular those that provide contrasts with Langmuir-Blodgett techniques, is whether the term selfassembling is in fact appropriate. These concerns focus on two issues. First, are monolayers with oriented (or perhaps ordered), closest-packed structures of low surface free energy indeed formed? Second, to what extent do the various experimental variables-solvent, temperature, carbon-chain length, substrate, and concentration as representative examples-influence the nature of the surface phases that are formed? The classic study of Timmons and Zisman7concluded that for chain lengths of C14 and longer, close-packed monolayers can be prepared on Pt; on NiO substrates, chain lengths as low as Clo were successfully employed. An earlier study of Zisman21 presented the interesting observation that the coefficient of friction of a glass surface, covered with a homologous series of fatty acids, drops to a minimum (and nominally constant) value for chain lengths of C14or greater. Borno~$ observed that coverages of fatty acids (C6--c30)adsorbed from the melt on chromium substrates were less than that expected for a Langmuir-Blodgett-type film; no discontinuities of the type described above were observed. Various studies of specific longer chain acids (CI6and above) seem to offer conflicting results as to whether submono-, mono-, and/or multilayer coverages are obtained for ambient metal surfaces such as copper416J1and aluminum.6J4 For example, Gaines reported that, for the latter, greater than monolayer coverages were obtained for stearic acid (C1J in sharp contrast to the results of Golden, Snyder, and Smith14 which show submonolayer coverages for arachidic acid (C20). The literature is repleat with such discrepancies despite an impressive array of experimental techniques (electron diffraction, reflection infrared spectroscopy, radioisotopic labeling, etc.) which have been brought to bear on this problem. Thus, while many additional comparisons can be made, no general conclusions can be drawn. These continuing significant and puzzling differences, in our opinion, reflect the need for a detailed, self-consistent reexamination of this classic and widely reported chemisorption system. It has been our contention22that spontaneous adsorption, under the appropriate conditions, out to be capable of producing monolayers with structural features akin to those of the Langmuir-Blodgett type. This contention is not new to us, having been suggested by previous authors%3 and recently broadly demonstrated by Hubbard et al.23*24 for the adsorption of nonamphiphilic molecules (differing structurally from the type reported herein) on metal surfaces. In the present and the following paperz5 we report the results of our detailed reexamination of the adsorption of carboxylic acids on native aluminum oxide surfaces. From reflection infrared spectroscopy,z6ellipsometry, isotopic labeling (both 2H and 3H), X-ray photoelectron spectroscopy (XPS), and contact angle ~~~
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(21)Zisman, W.A. In "Friction and Wear"; Davies, R., ed.; Elsevier: New York, 1959;pp 110-148. (22)Nuzzo, R. G.;Allara, D. L. J. Am. Chem. SOC.1983, 105, 4481-4483. (23)Soriaga, M. P.;Hubbard, A. T. J. Am. Chem. SOC.1982,104, 3937-3945 and references cited therein. (24)Hubbard, A. T.Acc. Chem. Res. 1980,13, 177-184. (25)Allara, D.L.;Nuzzo, R G. Langmuir, following paper in this isaue. (26)For discussions of the application of infrared spectroscopy to the determination o f alkyl chain orientation and ordering in monolayer structures, see: Allara, D. L.; Swalen, J. D. J . Phys. Chem. 1982,86, 2700-2704. Rabolt, J. F.; Burns, F.C.; Schlotler, N. E.; Swalen, J. D. J. Chem. Phys. 1983,78,946-952.
wetting data, an improved understanding of the structure and dynamic character of the self-assembly of n-carboxylic acid monolayers on aluminum has been obtained. Our results suggest an extremely important role for kinetics in the formation of equilibrium closest packed structures. Through an examination of both a homologous series of high-purity acids (C,-c24) and vinyl- and propargyl-terminated C22acids, the influence of adsorbate structure on monolayer structure has been detailed. Our results further demonstrate that these monolayer structures are capable of undergoing extremely facile exchange reactions with materials in solution. Taken together, these results develop a rich though complicated coordination chemistry for carboxylic acids chemisorbed on this (and presumably other) oxide surface.
Experimental Section Materials. The n-alkanoic acids from C6 and above were obtained from Sigma Chemical Co. and were used without further purification. GC analyses were obtained for the liquid acids and showed purities of >99.9%. The melting points of the higher acids were checked and all showed melting ranges of less than 2 "C. The terminally substituted acids CH2=CH(CH2)lgC02H and CH=C(CH2),,C02H were synthetic samples supplied by Prof. George Whitesides, Harvard University, and had melting points of 70.0-70.5 and 78.5-79.5 "C, respectively. Hexadecane, obtained from Aldrich Chemical Co., had a nominal purity of 99%. It was further purified by slow elution through a column of activity 1 alumina and stored under nitrogen. Deuterated alkanoic acids were obtained from Cambridge Isotope Laboratories (Cambridge, MA) and were used without further purification. Acetic acid was obtained from J. T. Baker Chemicals as the Ultrex grade (99.9%) and was used as received. Aluminum used for the substrate preparation was 99.999% purity. Tritiated acetic anhydride was obtained from New England Nuclear Co. (Boston, MA). Sample Preparation and Treatment. Aluminum substrates were prepared by the evaporation of aluminum from resistively heated tungsten boats onto 2-in.-diameter single-crystal silicon wafers which had been polished to C40-A surface roughness. Depositions were carried out in the lo4 to torr range in an oil diffusion pumped system equipped with a liquid nitrogen trap assembly. After melting the aluminum, a shield was removed, thus exposing both the samples and a quartz crystal thickness monitor which was placed nearby in order to determine the thickness of the deposited film. Approximately 200 nm of A1 was deposited and the chamber then backfilled with high-purity research-grade oxygen to 1 atm. The samples were removed and treated in one of two ways: (1) the samples were examined quickly by ellipsometry and, within several minutes, placed into the adsorbate solution; (2) the samples were placed directly into an ethanol solution of acetic acid (0.05 M) for -15 min, removed, rinsed with pure ethanol on a spinner (Headway photoresist type), examined by ellipsometry, and then placed in the adsorbate solution. The acid solutions were made by dissolving the weighed portion of acid in hexadecane contained in a Pyrex flask which had been precleaned in ethanolic KOH followed by repeated water and methanol rinses. For the acids C18 and above, solution only could be obtained by warming the contents to -35-40 "C. The solutions were transferred to wide-mouth, screw-top polypropylene containers, precleaned with chloroform and methanol washes, and maintained a t 25.0 f 0.3 "C during sample immersion. The solutions of acids CISand above tended to remain supersaturated at 25 "C although occasionally one would form crystals and require rewarming. Following immersion, the samples were removed and the surface rinsed quickly on the spinner with HPLC grade hexane. Infrared Measurements. IR spectra of adsorbates were taken by reflection using a Digilab 15B spectrometer with modified optics. Details are given elsewhere.25 Ellipsometry Measurements. Ellipsometric measurements were made using a Rudolph 423 null ellipsometer equipped with a Babinet-Solet compensator set a t 7r/4 retardation and with the optical axes at 45" to the instrument verical axis. Measurements were made with an 1-mm beam at 623.8 nm (He-Ne laser) and
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at 442.0 nm (He-Cd laser) using a 70" angle of incidence at the sample. Little difference was seen, however, in the results at the two wavelengths, and for convenience,most measurements were done at 632.8 nm. Spatial variations across the sample were usually within the experimental errors of m-fl.5 A in terms of the final film thicknesses calculated from the readings. Contact Angle Measurements. Static contact angles were measured with a Ram6-Hart goniometer. Drops were placed on the surface in the ambient environment and read several times on both sides of the drop. Angles were read to -&loand were reproducible from sample to sample to
