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

The self-assembled monolayers (SAMs) of a series of octadecyl 1-thiaoligo(ethylene oxide) x disulfides {[S(CH2CH2O) x C18H37]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° overlayer structure commonly found in SAMs of long-chain alkanethiols. Within the limits of our measurements, the x = 6−8 SAMs appear to be isostructural. As a result, SAM thicknesses should increase in a predictable fashion. SE and XRR measurements indicate thickness increases of 0.30 ± 0.05 nm/ethylene oxide (EO) unit, in good agreement with the expected increase of ∼0.28 nm/EO unit as derived from the unit cell dimensions of crystalline PEO. Films in which relative thicknesses can be calculated directly from crystallographic dimensions and are independent of refractive index should be useful materials for surface science metrology.