Electrochemical Characterization of Functionalized Alkanethiol Monolayers on Gold

The stability of functionalized alkanethiol monolayers (HS(CH 2 ) n-1 X, with X = CH 3 , OH, CN, and Cl (n = 12)), adsorbed on polycrystalline gold electrodes is investigated with capacitive current, differential capacitance and electrowetting measurements in indifferent electrolyte. It is found that these monolayers are highly stable upon cycling of the modified electrode in a potential region where only double layer charging occurs. The difference in the capacitance Cx of the thiol monolayers with terminal group X is determined by the dielectric properties of this group and increases in the following sequence : C CH3 < C Cl < C OH < C CN . No indications are found that Cx is affected by a difference in ordering or in packing density of the molecules. The potential E e in the electrocapillary maximum of the functionalized thiol monolayer can be obtained from electrowetting measurements. The sequence found is E e CH3 < E e Cl < E e CN < E e OH . The differences in E e are ascribed to the different dipole moments of the terminal group X. From the electrowetting measurements, in combination with interfacial thermodynamics, it is concluded that the chemical component of the interfacial tension of the thiol/electrolyte is a function of the applied potential for X = CN, OH, and Cl. This is interpreted as a change in the orientation of the molecules, which attempt to align the dipoles parallel to the direction of the electric field across the thiol layer.