Structural Modification of Partially Fluorinated Alkanethiols: Their Impact on the Properties of Fluorinated Self-Assembled Monolayers



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The ability to fine-tune structural parameters for partially fluorinated alkanethiols opens avenues for the fabrication of fluorinated films with certain targeted physical properties. This dissertation focuses on the modification of fluorinated alkanethiols in order to provide new types of fluorinated self-assembled monolayers (SAMs) as model films to study their physical properties. In one study, we developed a new fluorinated surfactant that bears a methyl terminal group; CH3(CF2)6(CH2)nSH (H1F6HnSH where n = 10, 11). This amphiphile created an unprecedented HC–FC dipole at the SAMs interface. When comparing the structural properties of SAMs of H1F6H11SH to those of octadecanethiol and 18,18,18-trifluorooctadecanethiol, films generated from H1F6H11SH were found to have a well-packed underlying hydrocarbon assembly. However, this property disrupts the orderliness of the perfluorinated segments, which gives rise to anomalies in the interfacial properties when comparing even and odd numbered chains. To draw a deeper understanding of the role of the underlying fluorinated segment, we synthesized and generated monolayers from alkyl-capped partially fluorinated alkanethiols; H(CH2)n(CF2)6(CH2)11SH (HnF6H11SH; where n = 3, 4, 5). These films revealed three important findings: 1) the HC–FC dipole ceases to affect the wetting behavior of the films as it is buried underneath the alkyl unit, 2) the orientation of the terminal methyl follows an odd-even trend as a function of the size of the upper alkyl segment and 3) a disordered interface is generated when extending the alkyl cap to five hydrocarbons. A separate study involves the manipulation of the interfacial properties of SAMs by introducing three new surfactants that are highly fluorinated with a small propyl unit as an alkyl spacer: F(CF2)n(CH2)3SH, FnH3, where n = 8, 10, 12. An evaluation of the performance of the FnH3 series in relation to the known SAMs using ellipsometry, XPS, and wettability analysis, indicate that the newly fluorinated surfactants not only behave similar to those formed with the shorter ethyl spacer, but they also provide an improved packing structure. Such work expands the scope of molecules that generate films that are close to a completely perfluorinated surface, while improving the packing ability of the adsorbates.



Fluorinated self-assembled monolayers (FSAMs)