Inverted Surface Dipoles in Fluorinated Self-Assembled Monolayers

The presence of surface dipoles in selfassembled monolayers (SAMs) gives rise to profound effects on the interfacial properties of the films. For example, CF3terminated alkanethiolate films are surprisingly more wettable toward polar contacting liquids than analogous hydrocarbon SAMs due to the fluorocarbon-to-hydrocarbon transition (CF3−CH2) at the interface (i.e., the presence of a strong “FC−HC” surface dipole). This report explores the converse situation by analyzing partially fluorinated monolayers (FSAMs) in which the polarity of the surface dipole has been inverted through the creation of an “HC−FC” surface dipole. Thus, a new series of methyl-capped partially fluorinated alkanethiols, CH3(CF2)6(CH2)nSH (where n = 10−13), were designed and synthesized. Structural analyses of the new films show that these adsorbates give rise to well-ordered monolayers. As for the wetting behavior of various liquids on these FSAMs, the new films proved to be less hydrophobic than both the corresponding CF3-terminated and hydrocarbon SAMs and more oleophobic than their hydrocarbon counterparts. Furthermore, odd−even trends were observed in the wettability of the nonpolar and polar aprotic liquids on the new films in which the even FSAMs were more wettable than the odd ones for both types of liquids. However, an inverse odd−even trend was observed for polar protic liquids: odd FSAMs were more wettable than even. We attribute this latter effect to the resistance of highly hydrogen-bonded liquid molecules at the liquid−FSAM interface to adopt a more favorable orientation (on the basis of polarity) when in the presence of the inverted HC−FC dipole. ■ INTRODUCTION The ability to decorate the tailgroups of partially fluorinated chains with different functional moieties opens up new avenues for the use of fluorinated amphiphiles as seeds for a new generation of fluorinated materials. Modifications that have created a sandwich of the fluorinated segment, like the recently reported PEG-terminated fluorinated thiols, have given rise to water-soluble partially fluorinated gold nanoparticles. Furthermore, the ability to distribute fluorinated moieties spatially within a larger network have led to a better understanding of the influence that such defined levels of fluorination can have upon the larger assembly, with recent examples being the incorporation of perfluorinated aromatic tetrazoles and carboxylic acids in metal organic frameworks (MOFs), creating highly fluorinated MOFs with superhydrophobic character. Separately, research involving fluorinated films has also benefitted from such advancements in organic synthesis; for example, Cai and co-workers have used click chemistry to create microarrays on fluorinated surfaces. Investigations involving self-assembled monolayers (SAMs) of thiols on gold surfaces continue to serve as model systems for determining how the structural features of fluorinated amphiphiles can be used to tailor the physical and interfacial properties of monolayer films. Fundamental studies of fluorinated self-assembled monolayers, FSAMs, have shown that fluorinated alkanethiols surpass their hydrocarbon analogues in key characteristics such as chemical and biological inertness, thermal stability, and oleoand hydrophobicity. Such properties are inherent in these structures owing in part to the intrinsic characteristics of the C−F bond (extremely polar but with a reported bond strength of 105.4 kcal per mole), as well as the stiff helical geometry of the perfluorinated chains. In addition, extended perfluorocarbon chains have a larger surface area as compared to their hydrocarbon counterparts. More than a decade ago, Lee and co-workers introduced a series of CF3-terminated alkanethiols for thin-film research. 13,14 SAMs derived from these molecules on gold surfaces exhibited similar structural features to their normal alkanethiol counterparts. Despite the differences in the sizes of the terminal methyl groups, the underlying alkyl chains appear to possess the same arrangement as the alkyl chains of alkanethiolate SAMs on a gold lattice. However, the interfacial properties reflect the change in the chemical makeup of the tailgroup; the CF3-terminated SAMs are less hydrophobic than normal hydrocarbon SAMs due to the dipole residing at the fluorocarbon−hydrocarbon (FC−HC) junction. Colorado and Lee investigated this phenomenon using a series of CF3Received: September 1, 2015 Revised: October 7, 2015 Published: October 9, 2015 Article