Surface dipoles influence the wettability of terminally fluorinated organic films

The correlation of differences in the wettabilities of partially fluorinated self-assembled monolayers (SAMs) to changes in the chemical structure and composition of the films was explored by contact angle goniometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). SAMs of simple alkanethiols (CH3(CH2)(n)SH with n = 9-15) and their CF3-terminated analogs (CF3(CH2)(n)SH with n = 9-15) were prepared by adsorption from solution onto evaporated gold. Advancing contact angles of hexadecane were measured on both the terminally fluorinated surfaces and the hydrocarbon surfaces. These data were compared to those obtained using a series of polar aprotic contacting liquids. As expected, the contact angles of hexadecane were higher on the CF3-terminated SAMs than on the CF3-terminated SAMs. The contact angles of the polar aprotic solvents, however, were measurably lower on the CF3-terminated SAMs than on the CH3-terminated SAMs. These observations were rationalized on the basis that the introduction of the CF3 terminal groups yields oriented surface dipoles that interact with the dipoles of the polar contacting liquids. Further support for this model was provided by the observation of an inverse parity ("odd-even") effect in the wettabilities of the polar aprotic solvents on the CF3-terminated surfaces. Analysis by PM-IRRAS revealed that both types of films consist of predominately trans-extended alkyl chains with relatively few gauche defects in a densely packed arrangement. The high degree of order is consistent with the detection of the parity effect, where small changes in the orientation of the tail groups can be sensed by contact angle measurements only in highly ordered organic thin films. The significance of the dipole-oriented dipole interaction in describing interfacial wettabilities is discussed.