Influence of Solvation and the Structure of Adsorbates on the Kinetics and Mechanism of Dimerization-Induced Compositional Changes of Mixed Monolayers on TiO2

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• 作者：Jonathan R. Mann ; Jeremy S. Nevins ; Gregory R. Soja ; David D. Wells ; Seth C. Levy ; David A. Marsh ; David F. Watson
• 刊名：Langmuir
• 出版年：2009
• 出版时间：October 20, 2009
• 年：2009
• 卷：25
• 期：20
• 页码：12217-12228
• 全文大小：1071K
• 年卷期：v.25,no.20(October 20, 2009)
• ISSN：1520-5827

文摘

Mixed monolayers of thiol-terminated (T) and methyl-terminated (Me) carboxylic acids on nanocrystalline TiO₂ films underwent dimerization-induced compositional changes. At short reaction times, the compositions of mixed monolayers were kinetically controlled and mirrored the compositions of coadsorption solutions. On time scales up to several hours, well after the establishment of saturation surface coverages, the monolayers relaxed to thermodynamically controlled compositions through the displacement of Me by T. Equilibration was driven by the formation of intermolecular disulfide bonds between thiol groups of adsorbed T, which yielded polydentate dimeric adsorbates that were bound more strongly than monomeric adsorbates to TiO₂. The rate of compositional changes increased with decreasing solvent viscosity and decreasing alkyl chain length of T, suggesting that the rate of adsorption of T to TiO₂ strongly influenced the overall kinetics under certain conditions. Steric bulk within adsorbates and the strength of surface-attachment interactions also influenced the rate of compositional changes. A kinetic model, derived on the basis of Langmuir adsorption and desorption kinetics, accounts for key aspects of the mixed-monolayer compositional changes. The rate-determining step in the overall mechanism involved either the adsorption of T or the formation of disulfide bonds, depending on the conditions under which monolayers were prepared. Our findings illustrate that dimerization and other intermolecular interactions between adsorbates may dramatically influence the composition and terminal functionalization of mixed monolayers.