Structural Variations and Ordering Conditions for the Self-Assembled Monolayers of HS(CH₂CH₂O)₃_-₆CH₃

Abstract

The structure and ordering conditions of the self-assembled monolayers (SAMs) of HS(EO)xR, where R = CH3, EO = CH2CH2O, and x = 3−6, on polycrystalline gold (Au) were determined by reflection−absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy (EIS). For x = 5 and 6, RAIRS and SE data show that the 1-thiaoligo(ethylene oxide) [TOEO] segments adopt the highly ordered helical structure of the folded-chain crystal polymorph of crystalline poly(ethylene oxide) and are oriented normal to the substrate. RAIRS and SE data obtained as a function of immersion time show that x = 6 attains the highly ordered, all helical conformation faster than x = 5. For x = 3 and 4, RAIRS and SE data indicate largely disordered structures, with some all-trans conformation but little evidence of the helical conformation. EIS measurements, in an aqueous environment, indicate a significant presence of water in all SAMs with x < 6 while x = 6 remains essentially free of water and/or ions for extensive periods (2 h). These data suggest that a minimum of five ethylene oxide units are necessary for TOEO SAMs to adopt the helical conformation and that SAMs with x < 6 are penetrated by water or ionic species in an aqueous environment, regardless of the order of the TOEO segment in air.