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

M. L. CAROT; V. A. MACAGNO; P. PAREDES-OLIVERA; E. M. PATRITO

Journal of Physical Chemistry C

ACS Publications

Año: 2007 vol. 111 p. 4294 - 4304

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 thiol composition of the forming solution. The presence of a single reductive desorption peak in the voltammograms of the mixed SAMs and the lack of phase segregation observed by STM support the formation of homogeneous SAMs despite the large chain length difference between MPA and MUA. To explain the driving force leading to the formation of a homogeneous mixture, intermolecular interactions within the SAM were investigated using density functional theory. The carboxyl groups of adjacent MPA and MUA molecules in a compact monolayer can form a stable head to head cyclic dimer with a hydrogen bond strength of 16.2 kcal/mol. The flexibility of the alkyl chain of MUA allows the carboxyl groups of adjacent MPA and MUA molecules to be located on the same plane. However, the carboxyls of adjacent MPA-MPA and MUA-MUA pairs form much 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 explains the 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.