Surface Modification of Self-assembled Monolayers of Thiol- and Disulfide-terminated Organosilanes by UV/ozone Treatment toward Fabrication of Damaged-Hair Surface Models

Self-assembled (SA) monolayers of pendant-tail alkoxysilanes, 3-mercaptopropyltrimethoxy-silane and bis[3-(triethoxysilyl)propyl]disulfide, were prepared on silicon wafers and exposed to a UV/O3 environment for chemical modification of sulfurous groups. The change in chemical state and the decomposition of film molecules due to the UV/O3 treatment were investigated by X-ray photoelectron spectroscopy (XPS). The UV/O3 exposure induced oxidation of the terminal thiol and disulfide groups into predominantly sulfo or sulfonate groups. The monolayer degradation, indicated by the decrease in surface concentration of carbon species, gradually progressed as prolonging the exposure time, but a surface with a desired fraction of the oxidized sulfurous groups could be prepared with a minimal decomposition degree under an appropriate exposure time. The XPS spectra were also measured for human hair surfaces. It was implied that the oxidized SA monolayers have the same oxidized sulfur species as those on the damaged hair surfaces in terms of the binding energy. This work demonstrates that the UV/O3 exposure to the sulfurous group-terminated monolayers is an easy and useful approach to obtain the surfaces covered with the highly oxidized sulfurous groups. Such surfaces are utilizable as models of damaged hair surface for a study on adsorption characteristics of hair-conditioning agents.