Structure, wettability, and electrochemical barrier properties of self-assembled monolayers prepared from partially fluorinated hexadecanethiols

We report the preparation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of partially fluorinated hexadecanethiols (CF3(CF2)(n-1) (CH2)(m)SH; n = 2, 4, 6, and 8; n + m = 16) onto the surface of gold. The quality of the SAMs, as measured by electrochemical impedance spectroscopy, was found to be sensitive to the solvent used as SAMs prepared in dichloromethane exhibited higher resistances and lower capacitances than those prepared in liquid CO2. The extent of fluorination was observed to influence the wettability, structure, and capacitance of the films without significantly affecting their charge-transfer resistance or their stability against exchange by a competing adsorbate. Reflectance-absorption infrared spectra showed that the fluorocarbon chains were oriented more normal to the surface for n = 8 but more parallel to the surface for n = 2. Advancing contact angles of hexadecane increased sharply with fluorination from n = 0 to 4 but slightly with additional fluorination, suggesting that hexadecane is insensitive to CH2 groups that are buried by >4 fluorinated carbon atoms. We also were able to model the dielectric properties of these partially fluorinated SAMs as two capacitors in series: one due to the outer fluorocarbon region and one due to the inner hydrocarbon region. The dielectric constants of the fluorocarbon and hydrocarbon regions of these SAMs were quantified as similar to1.7 and similar to2.3, respectively, and were comparable to the values for the pure polymer counterparts.