Vibrational Spectroscopic Examination of Langmuir-Blodgett Mono- and Multilayer Fatty Acids by High-Resolution Electron Energy Loss Spectroscopy

HREELS analysis of Langmuir-Blodgett mono- and multilayer systems was conducted at a primary electron energy of 6.5 eV and a specular angle of 60°. Results from the analysis of multilayers of stearic acid on silver for up to 15 layers (approaching 400 Å in thickness) demonstrate that successful HREELS spectra can be obtained with extreme surface sensitivity to the outermost methyl groups. Selective enhancement of relative intensities is observed for certain numbers of stearic acid layers, intimating that resonance scattering may be involved in the sampling process. HREELS analysis of oleic acid on Au, Ag, Ge, and A1 demonstrates a complex relationship between the role of surface oxide and morphology of the substrate in the character of the observed spectra. Radical differences are observed in the relative intensities of loss features as a function of substrate. Clear detection of olefinic C-H stretching features in these samples indicates that the substrate has no effect on the detection limit for this loss feature. The ability to detect and discriminate this olefinic stretch is demonstrated for a 3:1 molar mixture of stearic/linolenic acids transferred to a Ag substrate. Various loss features are assigned to C-H vibrations, deformations, and twists in light of IR and Raman data for fatty acids and other recent results of HREELS analyses.