Structure of Mixed Carboxylic Acid Terminated Self-Assembled Monolayers: Experimental and Theoretical Investigation

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• 作者：M. L. Carot ; V. A. Macagno ; P. Paredes-Olivera ; and E. M. Patrito
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：March 22, 2007
• 年：2007
• 卷：111
• 期：11
• 页码：4294 - 4304
• 全文大小：1012K
• 年卷期：v.111,no.11(March 22, 2007)
• ISSN：1932-7455

文摘

The structure and stability of mixed self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA)and 11-mercaptoundecanoic acid (MUA) prepared by immersion in ethanolic solutions were studied by cyclicvoltammetry, electrochemical impedance spectroscopy, ellipsometry, and STM as a function of the thiolcomposition of the forming solution. The presence of a single reductive desorption peak in the voltammogramsof the mixed SAMs and the lack of phase segregation observed by STM support the formation of homogeneousSAMs despite the large chain length difference between MPA and MUA. To explain the driving force leadingto the formation of a homogeneous mixture, intermolecular interactions within the SAM were investigatedusing density functional theory. The carboxyl groups of adjacent MPA and MUA molecules in a compactmonolayer can form a stable head to head cyclic dimer with a hydrogen bond strength of 16.2 kcal/mol. Theflexibility of the alkyl chain of MUA allows the carboxyl groups of adjacent MPA and MUA molecules tobe located on the same plane. However, the carboxyls of adjacent MPA-MPA and MUA-MUA pairs formmuch weaker hydrogen bonds because steric constraints avoid the formation of the stable cyclic dimer.Therefore, the prevalence of MPA-MUA interactions over MPA-MPA and MUA-MUA interactions explainsthe homogeneous mixing of MPA and MUA. The potential chemical switchability properties of mixedmonolayers of mercaptoalcanoic acids of different chain lengths as a function of the solution pH are discussed.