Potential-Dependent Wetting of Octadecanethiol-Modified Polycrystalline Gold Electrodes

A Wilhelmy plate technique is used to characterize the potential-dependent wetting of octadecanethiol (C₁₈H₃₇SH)-modified polycrystalline gold electrodes in 10^-2 M K₂SO₄. This technique is relatively simple and provides information on the electrostatic component of the solid electrode/solution interfacial tension (Δγ_SL(E)) and the electrocapillary maximum (ecm). Wetting measurements are carried out simultaneously with differential capacitance measurements and cyclovoltammetry. In the potential range where only double-layer charging occurs, the adsorbed, self-assembled C₁₈H₃₇SH layer is found to be very stable. The extreme hydrophobicity, the low differential capacitance (~0.7 μF cm^-2), and the low double-layer current (a factor of 100 less than for clean gold) are all indicative of the insulating dielectric character of these monolayers. By scanning from the ecm at about -0.45 V (SCE) to +0.8 V (SCE), the advancing contact angle decreases from 116° to 110°, corresponding to a decrease in γ_SL of 7 mN m^-1. The observed relationship between Δγ_SL and E can be conveniently described by interfacial thermodynamics.