Conformational Order in Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold Determined by Soft X-ray Absorption

Near-edge X-ray absorption fine structure (NEXAFS) measurements were performed on self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG)-terminated alkanethiols adsorbed on gold. Films with a varying number of EG units were investigated. By comparing the experimental results with predictions for an "ideal film structure", the degree of conformational orientation can be determined. Our results are consistent with earlier findings that the ethylene glycol units adopt a mixture of helical and amorphous conformations and allow it to quantify the maximum possible fraction of shorter oligomers displaying the helical conformation. The experiments were performed under high-vacuum conditions, thereby eliminating the effects of water on the conformation of the EG units. A significant amount of molecules in the films displays amorphous conformations.