Interactions at the solid-liquid interface have a large impact on the macroscale wetting properties of thin films. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and can be used to determine the orientation of molecules at the interface. This presentation utilizes SFG spectroscopy to study the interaction between liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF3(CH2)17SH and CD3(CF2)6(CH2)10SH on gold. The C–N and C–H stretching regions, 2000–2300 cm-1 and 2800–3050 cm-1, respectively, were used to probe the solid-liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive- or negative-pointing peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the presence of dipoles at the top of the films. Thus, the orientation (up or down) of the acetonitrile peaks were ascribed to changes in the FSAM dipole direction. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided evidence for the dipole-influenced wettability behavior of these unique surfaces.

Probing the influence of surface dipoles on the structure of contacting liquids with sum frequency generation spectroscopy

2019 Spring National Meeting

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