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 (DELTAgamma(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 (approximately 0.7 muF 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-degrees to 110-degrees, corresponding to a decrease in gamma(SL) of 7mN m-1. The observed relationship between DELTAgamma(SL) and E can be conveniently described by interfacial thermodynamics.