SELF-ASSEMBLY OF ALKYLSILOXANE MONOLAYERS ON FUSED-SILICA STUDIED BY XPS AND SUM-FREQUENCY GENERATION SPECTROSCOPY

Infrared-visible sum frequency generation (SFG) vibrational spectrosco py in the C-H stretching region was used to study a set of self-assemb led monolayers, formed from octyltrichlorosilane [CH3(CH2)(7)SiCl3], i ts partially fluorinated analog, afluoro-1,1,2,2-tetrahydrooctyl)-1-tr ichlorosilane [CF3-(CF2)(5)(CH2)(2)SiCl3] and their mixtures. SFG meas urements were complemented with transmission-absorption FTIR spectrosc opy and X-ray photoemission spectroscopy. Simultaneous self-assembly o f both chlorosilanes results in preferential deposition of the fluorin ated compound. The major reason for this behavior is the alkyl chain f luorination. There is a strong spectroscopic evidence of close coadsor ption of fluorinated and non-fluorinated molecules in mixed monolayers . It follows that the species in self-assembled monolayers of binary c omposition do not segregate into different phases as it is observed fo r similar Langmuir-Blodgett films. Distinct differences found in SFG s pectra of fluorinated and non-fluorinated samples indicate that SFG sp ectroscopy is much more sensitive than FTIR in detection of symmetry d istortions in the alkyl chain of the amphiphiles. Both SFG and FTIR sp ectroscopic data show the presence of hydrocarbon residue in fluorinat ed and mixed films that is very likely due to hexadecane used as a sol vent during monolayer deposition. (C) 1998 Elsevier Science B.V.