We report the preparation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of partially fluorinated hexadecanethiols (CF 3 (CF 2 ) n-1 (CH 2 ) 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 CO 2 . 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 CH 2 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 ∼1.7 and ∼2.3, respectively, and were comparable to the values for the pure polymer counterparts.

We report the formation of self-assembled monolayers (SAMs) on gold substrates by exposure to n-alkanethiols [CH3(CH2) n - 1SH; n = 8, 10, 12, 16, and 18] in liquid and supercritical carbon dioxide. The results of this novel study show that an environmentally friendly solvent can be used to form highly crystalline SAMs with few gauche defects and that pressure as well as exposure time can be used to affect the structural and barrier properties of the monolayer film. Reflectance infrared spectroscopy, electrochemical impedance spectroscopy, and wetting measurements were used to characterize the SAMs. The effects of pressure (76−300 bar) and adsorption time (3−90 min) on the formation of the SAMs were explored. The overall chain density of these SAMs was greater than that for SAMs formed in common organic solvents such as ethanol. The properties of the SAMs were slightly affected by the pressure during formation. At 35 °C, as the carbon dioxide pressure increased (from 76 to about 140 bar), the packing density and resistance of the SAM increased. SAMs prepared at higher pressures ranging from about 140 to 300 bar exhibited similar resistances, capacitances, and canted structures. There was also no significant difference in using liquid (25 °C and 103 bar) or supercritical (35 °C and 103 bar) carbon dioxide for SAM formation. Supercritical carbon dioxide also enabled the formation of SAMs using polar adsorbates (−OH- and −CO2H-terminated thiols) to prepare high-energy surfaces that are wet by water.

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