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

The interfacial wettabilities of self-assembled monolayers (SAMs) on gold g enerated from trifluoromethyl- and methyl-terminated alkanethiols were eval uated using a combination of non-polar, polar protic and polar aprotic cont acting liquids. The wettabilities for the nan-polar hydrocarbon liquids ind icate that the fluorinated films are wet less than the hydrocarbon films, d emonstrating the non-ideal nature of dispersive interactions between hydroc arbons and fluorocarbons. In contrast, the wettabilities for the polar liqu ids revealed that the fluorinated films were wet more than the hydrocarbon films. The presence of interactions between the dipoles of the liquid molec ules and oriented CF3-CH2 dipoles at the monolayer surface was proposed to rationalize the observed trends. Furthermore, the wettabilities of the pola r aprotic liquids exhibited an inverse odd-even trend that supports the exi stence of oriented dipole effects upon wettability. The influence of the di poles on the interfacial wettabilities was further examined using a series of SAMs generated from terminally fluorinated hexadecanethiols having an in creasing degree of fluorination. As the dipoles were buried further into th e monolayer surface, their influence on the wettabilities decreased. The in terfacial energies of wetting for these films were evaluated in terms of th eir works of adhesion. These analyses provided evidence for the contributio n of oriented dipoles to the interfacial interactions of organic thin films , in addition to the commonly recognized contributions of dispersive and ac id-base interactions. Copyright (C) 2000 John Wiley & Sons, Ltd.