Inverted Surface Dipoles in Fluorinated Self-Assembled Monolayers

Abstract

The presence of surface dipoles in self-assembled monolayers (SAMs) gives rise to profound effects on the interfacial properties of the films. For example, CF3-terminated 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.