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| Untersuchte Arbeit: Seite: 32, Zeilen: 7-17 |
Quelle: Ulman 1996 Seite(n): 1539-1540, Zeilen: 1539:right col. 14ff. - 1540:left col. 1ff. |
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| The work of Bain et al. showed at relatively dilute solution (10-3 M) two distinct adsorption kinetics steps [Bai89] for the adsorption of alkanethiols an gold:
1) a very fast step which takes a few minutes, by the end of which the contact angles are close to their limiting values and the layer thickness reaches about 80-90% of its maximum [Ulm91a]. 2) a slow step, which lasts several hours, at the end of which the layer thickness and the contact angles reach their final values. The initial step is depended on concentration of the sulfur compounds and is described by diffusion-controlled Langmuir adsorption [Bai89]. The second step can be described as a surface crystallization process, where alkyl chains get out of the disordered state and into unit cells, thus forming a two-dimensional crystal. Therefore, the kinetics of the first step is governed by the surface-head group reaction, and the activation energy may depend on the electron density of the adsorbing sulfur. The kinetics of the second step is related to chain disorder (e.g. gauche defects), the different components of chain-chain interaction (VDW, dipole-dipole, etc.) and the surface mobility of chains [Ulm96]. [Bai89] Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. [Ulm91] Ulman, A. An Introduction to Ultrathin Organic Films, Academic Press: Boston, 1991, Chap. 2. [Ulm96] Ulman, A. Chem. Rev. 1996, 96, 1533. |
[page 1539]
Kinetic studies of alkanethiol adsorption onto Au(111) surfaces have shown that at relatively dilute solutions (10−3 M), two distinct adsorption kinetics can be observed: a very fast step, which takes a few minutes, by the end of which the contact angles are close to their limiting values and the thickness about 80-90% of its maximum, and a slow step, which lasts several hours, at the end of which the thickness and contact angles reach their final values.143 The initial step - described well by diffusion-controlled Langmuir [page 1540] adsorption - was found to strongly depend on thiol concentration. At 1 mM solution the first step was over after ∼1 min, while it required over 100 min at 1 μM concentration.143 The second step can be described as a surface crystallization process, where alkyl chains get out of the disordered state and into unit cells, thus forming a two-dimensional crystal. Therefore, the kinetics of the first step is governed by the surface-head group reaction, and the activation energy may depend on the electron density of the adsorbing sulfur. On the other hand, the kinetics of the second step is related to chain disorder (e.g., gauche defects), the different components of chain-chain interaction (VDW, dipole-dipole, etc.), and the surface mobility of chains. It also was found that the kinetics is faster for longer alkyl chains, probably due to the increased VDW interactions.143 (1) 1. Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (143) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. |
Though nearly identical nothing has been marked as a citation. There is no entry for [Ulm91a] in Bsi's list of references. |
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