Sum-frequency vibrational spectroscopic study of mixed alkylsiloxane monolayers self-assembled on fused silica surface

Abstract

Self-assembled monolayers of fluorinated and non-fluorinated alkylsiloxanes were prepared on fused silica substrates and studied by both conventional IR and nonlinear sum-frequency vibrational spectroscopy complemented with XPS. Simultaneous self-assembly of the two alkylsiloxanes results in preferential adsorption of the fluorinated compound and formation of the mixed monolayer. Different species are found to be closely coadsorbed in the mixed monolayers which distinguishes self-assembly from Langmuir-Blodgett deposition where the similar species tend to separate into different phases. Nonlinear sum-frequency spectroscopy of C-H stretching region detects symmetry perturbations of the alkyl chain induced by trans-cis defects and partial fluorination and clearly distinguishes between them. All three characterization methods consistently indicate the presence of a hydrocarbon residue in all deposited monolayers, which is ascribed to the hexadecane used as a solvent during the self-assembly process. (C) 1997 Elsevier Science S.A.