Probing the Solid-Liquid Interface of Fluorinated Thin Films with Sum Frequency Generation Spectroscopy
Deleon, Daniela 1991-
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This dissertation investigates the interactions at the solid-liquid interface between contacting liquids and model fluorinated surfaces using self-assembled monolayers (SAMs) on gold. Sum frequency generation (SFG) spectroscopy was employed due to its inherent interface selectivity, which allows it to probe buried interfaces. In the first study, SFG spectroscopy was used to understand the influence of dipole-terminated surfaces on the orientation of contacting liquids by supplying the vibrational spectra of the molecules at the solid-liquid interface. Modulation of the dipole of the solid surface was achieved through selective fluorination of the molecules used for the self-assembly. The work presented here was the first in-situ analysis showing a polar liquid reorient in direct response to the orientation of the surface dipole. To understand the effect of contacting liquids on the structure of the SAMs, SFG spectroscopy was used to determine the orientation of the terminal functional groups of specifically fluorinated SAMs at the solid-liquid interface. In this study, a difluoromethylene moiety was systematically buried into the film and analyzed with several contacting liquids. The orientation and orientational distribution of the terminal functional groups showed that wetting liquids significantly perturb the interfacial structure of the film. A comparison between the conformation of the monolayer in contact with liquids and the macroscopic contact angle suggested that disordered monolayers yielded the most wettable surfaces. Finally, the effect of changing the substrate to which molecules adsorb on the structure of the SAM was determined by terminal group orientation of SAMs generated from three series of molecules bound to gold and underpotentially-deposited silver surfaces. The orientation of the terminal functional group of a SAM on Au changed depending on the length of the alkane chain, and the observed "odd-even" effect in the wettability of the films was reversed by changing the substrate. However, the solid-liquid interface spectra using two contacting liquids did not show any differences, with respect to the substrate. Therefore, although monolayer structure is an important factor in influencing the wettability of the resulting thin film, other factors must be considered to obtain a complete understanding of the macroscopic wetting phenomena of surfaces.