Modulating Surface Dipoles in Fluorinated Thin Films
Marquez, Maria DeJesus 1988-
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This dissertation investigates the effect a surface dipole has on the interfacial properties of partially fluorinated self-assembled monolayers (FSAMs). In the first study, the synthesis of a new type of partially fluorinated adsorbate of the form CH3(CF2)6(CH2)nSH where n = 10–13 was performed. On the surface, these adsorbates generate a novel hydrocarbon-fluorocarbon (HC-FC) dipole at the interface of the SAMs that has a profound effect on the wettability of the surfaces with various contacting liquids; specifically, the inverse odd-even effect observed with regard to the wettability of polar protic and aprotic liquids. Additionally, the properties of the HC-FC dipole were compared to those of a FC-HC (fluorocarbon-hydrocarbon) dipole derived from CF3-termianted alkanethiols. In efforts to understand further the effect of the new HC-FC dipole, an additional series of alkyl-terminated alkanethiols of the form H(CH2)n(CF2)6(CH2)11SH, where n = 1–7, was synthesized and used to form SAMs. In this study, the dipole was systematically buried in the film and analyzed with several contacting liquids. The effect of the dipole in these types of FSAMs appears to have a diminished effect after 3 hydrocarbons, after which an odd-even effect in the wettability was observed. The odd-even effects observed in the FSAMs were the opposite of that observed with normal alkanethiols of the same carbon length, which suggests that the orientation of the terminal methyl group is different from the normal alkanethiol. Finally, to explore further the effect of the direction of the FC-HC and HC-FC dipoles, evaporated gold surfaces were electrochemically modified with a monolayer of silver via underpotential deposition (UPD). The monolayer of silver has an effect on the structural features of the films caused by the different binding geometries of the sulfur on gold and silver. The structural difference between the two substrates inverts the odd-even effects for SAMs with a FC-HC dipole. For SAMs possessing a HC-FC dipole, the presence of silver on the gold surface also changes the orientation of the molecules on the different metals, influencing the physical and interfacial properties of the resulting films.