Langmuir zyxwvu 1994,10, 3315-3322 3315 Direct Measurement of the Adsorption Kinetics of Alkanethiolate Self-Assembled Monolayers on a Microcrystalline Gold Surface D. S. Karpovich and G. zyxwv J. Blanchard* Department zyxwvutsrq of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 Received June 15, 1994@ We report measurements on the adsorption kinetics of alkanethiolate monolayers onto a microcrystalline gold surface. Our data indicate that the formation of this monolayer is rapid and is described by the Langmuir adsorption isotherm. The concentration dependence of the monolayer formation rate indicates that there exists an equilibrium for the adsorption/desorption process. We determine from these measurements AGads zyxwvutsrqp = -5.5 kcal/mol for 1-C18H37SHin n-hexane and cyclohexane adsorbing onto gold and AGads = -4.4 kcaymol for l-CsH17SH in n-hexane adsorbing onto gold. Introduction The study of organic interfacial monolayer systems has attracted a great deal of research attention over the past decade because of the potential utility of these interfaces in materials science and chemical separation applications. Initial work on the structural and chemical properties of interfacial organic monolayers supported on amorphous substrates required statistical treatment to understand the role of their intrinsic heterogeneity. Recent chemical advances in the formation of single molecular layers on highly ordered meta11-4 and dielectric5-12 surfaces have allowed the structure of these assemblies to be examined more directly by removing structural variability induced by the surface. Much ofthe recent work on self-assembling organic monolayers at metal surfaces has focused on the alkanethiolate/gold system, where alkanethiols adsorb spontaneously onto the metal surface to form a highly ordered a r r a ~ . l ~ - ~ ~ A variety of tools have been applied in studies of these self-assembled monolayers to elucidate ~~ * Author to whom correspondence should be addressed. @ Abstract publishedinAdvanceACSAbstracts, August 1,1994. (1) Allara, D. L.; Nuzzo, R. G. zyxwvutsrqpon Langmuir 1986,1, 45. (2) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1, 52. (3) Sondag, A. H. M.; Raas, M. C. J. Chem. Phys. 1989,91, 4926. (4) Chau, L. K.; Porter, M. D. Chem. Phys. Lett. 1990,167, 198. (5) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. (6) Lee, H.; Kepley, L. J.; Hong, H. G.;Akhter, S.; Mallouk, T. E. J. (7) Cao, G., Mallouk, T. E. J. Solid State Chem. 1991, 94, 59. (8) Akhter, S.; Lee, H.; Mallouk, T. E.; White, J. M.; Hong, H. G. J. (9) Hong, H. G., Mallouk, T. E. Langmuir 1991,7, 2362. (10) Cao, G.; Rabenberg, L. K.; Nunn, C. M.; Mallouk, T. E. Chem. (11) Kepley, L. J.; Sackett, D. D.; Bell, C. M.; Mallouk, T. E. Thin (12) Hong, H. G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, (13) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. 1990, (14) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. zyxwvut SOC. (15) zyxwvutsrqp Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, (16) Chidsey, C. E. D., Loiacono, D. N. Langmuir 1990, 6, 682. (17) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujscu, A. (18) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, (19) Bain, C. D., Whitesides, G. M. J. Am. Chem. SOC. 1989, 111, SOC. 1988,110, 618. Phys. Chem. 1988,92, 2597. Vac. Sci. Tech. A 1989,7, 1608. Mater. 1991 3, 149. Solid Films, 1992,208, 132. 3, 521. 112, 570. i990,112,55a. 93, 767. M. J. Am. Chem. SOC. 1990,112,4301. G. M.; Nuzzo, R. G. J. Am. Chem. SOC., 1989, 111, 321. 7155 . (20) Troughton, E. G.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; (21) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Allara, D. L.; Porter, M. D. Langmuir, 1988, 4, 365. Am. Chem. SOC. 1987, 109, 3559. (22)Whitesides, G. M., Laibinis, P. E. Langmuir, 1990, 6, 87. their molecular-scale structural properties. FTIR spec- troscopy has been used most commonly for the examina- tion of molecular ordering in these films, and these results have shown that the interface is composed of densely packed, virtually all-trans aliphatic chains that are tilted at -30" with respect to the surface normal.15 Ellipsometric and contact angle measurements have indicated that these films are uniform over macroscopic distances (several micrometers), and recent atomic scale microscopy has demonstrated the existence of highly regular structure on molecular length scale^.^^-^^ Based on this large body of information, the alkanethiolate/gold system has come to be viewed as largely immobile, once formed. McCarley's recent scanning probe microscopywork has called into question the extent to which these monolayers are "fixed"in place subsequent to a dsorpti~n.~~ The island structures formed by the alkanethiolate monolayers on an evaporated gold substrate exhibited macroscopic shape changes over periods as short as minutes, and the McCarley group has concluded that these shape-changes are associated with mobile defects in the gold surface and that the Au-S bond is labile.29 This finding indicates that either the formation of the alkanethiolate monolayer has a substantial effect on the mobility of the surface gold atoms or that the overlayer itself exhibits structural mobility. Virtually all examinations of the alkanethiolate/gold monolayer system have been performed on monolayers that were formed prior to examination, and little effort has been spent on understanding the elementary steps in the formation of the monolayers. We have chosen to examine the formation of these monolayers gravimetri- cally. We have measured the rate at which these monolayers form in real time by monitoring the change in mass of a quartz crystal microbalance made with evaporated gold electrodes. These data show that the alkanethiolate/gold system exhibits a measurable equi- librium between adsorbed thiolate and the free species. From our data we have determined that the free energy ~ ~~~ ~~ ~~ ~ (23) Camillone, N., 111; Chidsey, C. E. D.; Liu, G.-Y.;Putvinski, T. (24) Chidsey, C. E. D.; Liu, G.-Y.; Wang, J. Langmuir, 1990,6,1804. (25) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. (26) Widrig, C. A,; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC. 1991, (27) Liu, G.-Y.; Salmeron, M. B. Langmuir 1994, 10, 367. (28) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, (29) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, M.; Scoles, G. C.; Wang, J. J. Chem. Phys. 1991, 94, 8493. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. SOC. 1989,111, 5852. 113,2805. 97, 211. 9, 2775. 0743-746319412410-3315$04.5010 0 1994 American Chemical Society