Isostructural Self-Assembled Monolayers, Part 1. Octadecyl 1-thiaoligo(ethylene oxides)

Published

April 1, 2003

Author(s)

David J. Vanderah, Richard S. Gates, Vitalii I. Silin, D N. Zeiger, John T. Woodward IV, Curtis W. Meuse, Gintaras Valincius, B Nickel

Abstract

The self-assembled monolayers (SAMs) of a series of octadecyl 1-thiaoligo(ethylene oxide)x disulfides {[S(CH₂CH₂O)_xC₁₈H₃₇]₂}, where x = 4 to 8, were assembled on gold and characterized by reflection-adsorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy (EIS). Our results show that the SAMs differ over this range of x with an isostructural subset for x = 6 to 8. For x = 6 to 8, the RAIRS data shows that the 1-thiaoligo(ethylene oxide) [TOEO] segments adopt the ordered 7/2 helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide) [PEO], oriented normal to the substrate, and the hydrocarbon segments adopt a nearly all-trans conformation tilted {approximately equal to} 30 degrees to the normal. The EIS data indicate highly stable, highly ordered, nearly defect-free SAMs that behave as ideal capacitors in an electrochemical environment. These SAMs may be described as highly uniform, nearly impervious films each composed of a single structural phase with the TOEO segment as described above and few domain boundaries or defects. As a result, SAM thicknesses should increase in a predictable fashion. SE calculations indicate SAM thickness increases by 0.30 0.02 nm/ethylene oxide (EO) unit for x = 6 to 8 in good agreement with the predicted {approximately equal to} 0.28 nm/EO unit from the unit cell dimensions of crystalline PEO. The absolute and relative thicknesses of these isostructural SAMs may be useful for a variety of surface science techniques.

Citation

Langmuir

Volume

19

Issue

7

Pub Type

Journals

Keywords

electrochemical impedance spectroscopy, infrared spectroscopy, self-assembled monolayers, spectroscopic ellipsometry, standard films

Citation

Ross, D. , Gates, R. , Silin, V. , Zeiger, D. , Woodward, J. , Meuse, C. , Valincius, G. and Nickel, B. (2003), Isostructural Self-Assembled Monolayers, Part 1. Octadecyl 1-thiaoligo(ethylene oxides), Langmuir (Accessed April 18, 2024)