Isostructural Self-Assembled Monolayers. 1. Octadecyl 1-Thiaoligo(ethylene oxides)

Abstract

The self-assembled monolayers (SAMs) of a series of octadecyl 1-thiaoligo(ethylene oxide)x disulfides {[S(CH2CH2O)xC18H37]2}, where x = 4−8, were assembled on gold and characterized by reflection−absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), electrochemical impedance spectroscopy (EIS), synchrotron X-ray reflectivity (XRR), and atomic force microscopy (AFM). For x = 6−8, the EIS data indicate stable, highly ordered, nearly defect-free SAMs that behave as ideal capacitors in an electrochemical environment, comparable to well-packed SAMs of octadecanethiol. For these SAMs, the RAIRS data show that the 1-thiaoligo(ethylene oxide) (TOEO) segments, oriented normal to the substrate, adopt the ordered 7/2 helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide) (PEO) and the hydrocarbon segments, tilted ∼30° to the normal, adopt a nearly all-trans conformation. AFM measurements on x = 6 yield reciprocal lattice points that are consistent with the √3 × √3R30°...