Physical organic probes of interfacial wettability reveal the importance of surface dipole effects

Abstract

The interfacial wettabilities of self-assembled monolayers (SAMs) on gold generated from trifluoromethyl- and methyl-terminated alkanethiols were evaluated using a combination of non-polar, polar protic and polar aprotic contacting liquids. The wettabilities for the non-polar hydrocarbon liquids indicate that the fluorinated films are wet less than the hydrocarbon films, demonstrating the non-ideal nature of dispersive interactions between hydrocarbons and fluorocarbons. In contrast, the wettabilities for the polar liquids revealed that the fluorinated films were wet more than the hydrocarbon films. The presence of interactions between the dipoles of the liquid molecules and oriented CF3–CH2 dipoles at the monolayer surface was proposed to rationalize the observed trends. Furthermore, the wettabilities of the polar aprotic liquids exhibited an inverse odd–even trend that supports the existence of oriented dipole effects upon wettability. The influence of the dipoles on the interfacial wettabilities was further examined using a series of SAMs generated from terminally fluorinated hexadecanethiols having an increasing degree of fluorination. As the dipoles were buried further into the monolayer surface, their influence on the wettabilities decreased. The interfacial energies of wetting for these films were evaluated in terms of their works of adhesion. These analyses provided evidence for the contribution of oriented dipoles to the interfacial interactions of organic thin films, in addition to the commonly recognized contributions of dispersive and acid–base interactions. Copyright © 2000 John Wiley & Sons, Ltd.