Burying the Inverted Surface Dipole: Self-Assembled Monolayers Derived from Alkyl-Terminated Partially Fluorinated Alkanethiols

The direction and magnitude of surface dipoles directly affect the interfacial properties and can be tuned through molecular design. This article examines the effect of a hydrocarbon-fluorocarbon, "HC-FC", dipole on the structural and interfacial properties of self-assembled monolayers (SAMs) as the dipole is buried into the film. A series of selectively fluorinated alkanethiols with a progressively extended alkyl chain atop six fluorocarbons and an alkyl spacer of 11 hydrocarbons, H(CH2)(n)(CF2)(6)(CH2)(11)SH, where n = 1-7 (HnF6H11SH) were prepared and used to generate SAMs on evaporated gold, allowing for the systematic burying of the HC-FC dipole into the film. Structural analyses of the films revealed well-ordered films with slight disorder/loose packing in the top alkyl chains. In addition, odd-even effects were observed in the orientation and wettability of the SAMs corresponding to the number of carbon atoms in the top alkyl chain, leading to the conclusion that the fluorinated segment behaves as a surrogate surface. As for the effect of the dipole on the wetting behavior of the films, the effect appears to be minimized after three methylene units; however, the structural features of the monolayers were also found to influence the wettability of the films.