Self-assembly of alkylsiloxane monolayers on fused silica studied by XPS and sum frequency generation spectroscopy

Abstract

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy in the C-H stretching region was used to study a set of self-assembled monolayers, formed from octyltrichlorosilane [CH3(CH2)(7)SiCl3], its partially fluorinated analog, (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane [CF3-(CF2)(5)(CH2)(2)SiCl3] and their mixtures. SFG measurements were complemented with transmission-absorption FTIR spectroscopy and X-ray photoemission spectroscopy. Simultaneous self-assembly of both chlorosilanes results in preferential deposition of the fluorinated compound. The major reason for this behavior is the alkyl chain fluorination. There is a strong spectroscopic evidence of close coadsorption of fluorinated and non-fluorinated molecules in mixed monolayers. It follows that the species in self-assembled monolayers of binary composition do not segregate into different phases as it is observed for similar Langmuir-Blodgett films. Distinct differences found in SFG spectra of fluorinated and non-fluorinated samples indicate that SFG spectroscopy is much more sensitive than FTIR in detection of symmetry distortions in the alkyl chain of the amphiphiles. Both SFG and FTIR spectroscopic data show the presence of hydrocarbon residue in fluorinated and mixed films that is very likely due to hexadecane used as a solvent during monolayer deposition. (C) 1998 Elsevier Science B.V.