Langmuir zyxwvu 1989,5, 1147-1152 1147 Packing and Molecular Orientation of Alkanethiol Monolayers on Gold Surfaces Abraham Ulman,* James E. Eilers,* and Nolan Tillman Corporate Research Laboratories and the Information and Computer Technology Division, Eastman Kodak Co., Rochester, New York 14650 Received February 15, 1989 Preliminary calculations based on a simple model give a good description of the molecular orientation and packing of akanethiol monolayers on gold surfaces. These calculations suggest that alkanethiol molecules on gold have a zyxwvuts total alkyl chain zyxwvut axis tilt of approximately 38' in a plane that bisects the methylene H-C-H angles, followed by a rotation about the alkyl chain axis of -46'. The alkyl chain tilt is a function of the sulfur-sulfur spacing in a hexagonal crystal lattice layer and maximizes the attractive interactions between neighboring molecules. These results are in agreementwith the molecular orientations obtained from zyx grazing angle FTIR experiments on dodecanethiol monolayers on a (111) gold surface. Introduction The understanding of the interrelationships between the molecular structure of a surfactant and its organization on different surfaces is a fundamental problem in today's surface science. The packing and orientation of such molecules affect the surface chemistry of the monolayer and are responsible for boundary lubrication, corrosion inhibition, adhesion, and catal~sis.l-~ Furthermore, such understanding is essential for the future development of multilayer systems with useful properties. For example, an organic film for nonlinear optics applications should have a noncentrosymmetric arrangement of the molecular dipoles, preferably perpendicular to the monolayer surface. Thus, a better understanding of the monolayer bulk structure relationships should allow for molecular design to specifically engineer the packing, orientation, and sta- bility of such monolayer films. The packing and orientation of individual molecules in a monolayer of long-chain alkyl compounds have been suggested to be a function of the spacing between the molecular head groups'5 and the van der Waals and dipole interactions between these mole~uls.~~' In monolayer assemblies where the head-head spacing is greater than the touching van der Waals distance of the alkyl chains, these tails tilt in such a way as to maximize attractive van der Waals interactions between molecules and thus min- imize the free energy of the Alkanethiols form well-ordered monolayers when ad- sorbed from solution onto metal s u r f a ~ e s . ~ J ~ ' ~ In 1987, (1) Zisman, W. A. In Friction and Wear; Davis, R., Ed.; Elsevier: New York, 1959; p 118. (2) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976 and references cited therein. (3) Somorjai, G. A. Chemistry zyxwvutsrqp of Two zyxwvutsr Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981 and references cited therein. (4) Safran, S. A.; Robbins, M. 0.; Garoff, S. Phys. zyxwvutsrqp Reu. A 1986, 33, 2188. (5) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. zyxwvutsrq J. Am. Chem. SOC. 1987,109, 3559. (6) Kitaigorodskii, A. 1. Organic Chemical Crystallography; Consult- anta Bureau: New York, 1959; pp 177-217. (7) Garoff, S. hoc. Natl. Acad. Sci. U.S.A. 1987,84,4729 and refer- ences cited therein. (8) Langmuir, 1. J. Chem. Phys. 1933, 1, 756. (9) Epstein, H. T. J. Colloid Chem. 1950,54,1053. (IO) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P., Silvera, I. F. (11) Finklea, H. 0.; Avery, S.; Lynch, M.; Furtach, T. Langmuir 1987, (12) Strong, L.; Whitesides, G. M. Langmuir 1988, 4,546. (13) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. SOC. 1987, Reo. Sci. Znstrom. 1986,57, 1381. 3, 409. 109, 2358. Porter et al.5 reported on a detailed investigation of monolayers of alkanethiols adsorbed on evaporated gold films. They used ellipsometry and grazing-angle FTIR spectroscopy to characterize the monolayers and establish molecular orientation. Both in their report and in a pre- vious report by Nuzzo et aLl3 on the orientation of alkyl chains in monolayers of dialkyl disulfides, the authors discuss their spectroscopic results in terms of molecular coordinates, with no consideration of the molecular packing in the two-dimensional assemblies. In this report, we present our efforts to predict the ex- pected chain tilt for an assembly of alkanethiolswith a very simple model and compare our results with tilt angles that have been estimated by IR spectroscopy. We attempt to account for the observed chain tilting in terms of the likely crystallographic packing of the monolayer film and the spacing of the sulfur atom head group, and we suggest some general conclusions. The Model Electron diffraction studies of monolayers of docosane thiol on gold single-crystal foils with an exposed (111) surface have shown that the alkanethiols have hexagonal packing.12 The S-S spacing in these samples was found to be 4.97 A, a distance that would leave substantial free volume between molecules if they "stood up" normal to the plane of the lattice. One might investigate this packing problem in many ways and at several levels of complexity and sophistication. For example, one could consider a small patch (e.g., 10 X 10) of alkanethiols, with imposed periodic boundary con- ditions in two dimensions to eliminate edge effects, un- dergoing molecular dynamics until they fall into an ordered pattern and stay that way.14J6 However, while very so- phisticated, complex, and expensive computations may eventually be required for a thorough understanding of the process of monolayer formation, we think that the question "what is the most favorable packed arrangement for these molecules?" should be amenable to relatively simple analysis. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ FTIR spectroscopy and electron diffraction experiments suggest that alkanethiol monolayers on gold surfaces have crystal-like periodicity in two dimensions (for alkyl chain (14) van der Ploeg, P.; Berendsen, H. J. C. J. Chem. Phys. 1982, 76, (15) Cardini, G.; Bareman, J. P.; Klein, M. L. Chem. Phys. Lett. 1988, 3271. 145,493. 0743-7463/89/2405-ll47$01.50/0 0 1989 American Chemical Society