Adsorption of Alkanethiols on Gold Surfaces: PM-IRRAS Study of the Influence of Terminal Functionality on Alkyl Chain Orientation

Abstract

The present paper focuses on a simplified method to study the orientation and the anisotropy of two different alkanethiols self-assembled monolayers on gold surfaces. The alkanethiols of interest vary only by their terminal functionalities (COOH and COOCH₃), thus allowing one to highlight the influence of these ending chemical groups on the final orientation of the adsorbed molecules. 11-Mercaptoundecanoic acid [HS-(CH₂)₁₀-COOH] and the methyl-11-mercaptoundecanoate [HS-(CH₂)₁₀-COOCH₃] were grafted under adequate conditions to obtain a high grafting density on gold substrates. These latter, before and after the alkanethiol adsorption, were analyzed mainly by the polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique to access orientation angles, and by atomic force microscopy (AFM) to check the homogeneity of the grafted layer.

By applying an original PM-IRRAS simplified method, the results showed an orientation closer to the normal of the surface plane in the case of the acid thiol compared with the ester one, which is probably because of the lateral hydrogen bonds established between the adjacent COOH functions. This method offers a direct and simple way to quantify the orientation angles in the alkanethiol nanofilms.

KEYWORDS:

• Infrared spectroscopy
• Molecular orientation
• PM-IRRAS
• SAM
• Thiols

ACKNOWLEDGMENTS

This work was funded by the French Research Agency ANR (ACI NR183 BIOSYAMI). We thank Dr. Khaled Zahraman, Dr. Mohamad Roumie and Dr. Bilal Nsouli (from the LAEC crew-Lebanon) for conducting RBS analyses.

Notes

One of a Collection of papers honoring Wulff Possart, the recipient in February 2012 of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M.