ω-Alkoxy-n-alkanethiols (HS(CH2)nOR) adsorb from solution onto the surfaces of evaporated silver and gold films and form oriented self-assembled monolayers (SAMs). For many of these SAMs (R ≥ propyl), the wetting properties of the SAMs by various polar and nonpolar liquids are indistinguishable from those of SAMs derived from n-alkanethiols and suggest that the presence of the ether oxygen atom is not sensed by contacting liquids. The structures of the SAMs that form from these adsorbates on silver and gold are different from, but reminiscent of, the canted structures that form upon adsorption of n-alkanethiols (CH3(CH2)xSH) onto these metal surfaces. The structural differences that exist between the SAMs on the two metals do not affect the wetting properties of the SAMs. The structure of the SAMs on the two metals has been determined using X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and sum-frequency generation spectroscopy (SFS). Application of these techniques indicates that the ether oxygen atom causes a local disordering and increases the population of gauche conformations. The magnitude of this disordering depends sensitively on the position of the oxygen atom along the chain. When the oxygen atom was located ∼2 or more methylene units away from the chain end, the terminal methyl group in the SAMs exhibited the same molecular orientations as is found in SAMs that do not contain the heteroatom. This observation suggests that this type of substitution constitutes a weak perturbation of chain ordering, and one which need not affect the structure of an extended chain.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry