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Self-assembled monolayers of alkanethiols (CnH2,,+lSH; n = 3, 8, 12, 18, and 22) adsorbed on gold(ll1) are investigated with (atomically resolved) scanning tunneling microscopy (STM) and wetting measurements. The characteristic depressions observed in these monolayers with STM are proven to be holes in the underlying top gold surface layer rather than defects in the thiol monolayer itself. The holes originate from an etching process of the gold during adsorption of the thiol molecules: a correlation is obtained between the number of holes observed with STM and the amount of gold measured with atomic absorption spectroscopy in the thiol solution after adsorption. The erosion process is found to vanish as soon as complete self-assembly is observed in STM and wetting. For a dodecanethiol monolayer on gold adsorbed from a diluted methanoic thiol solution, selfassembly is observed within 10-min adsorption time. The average amount of gold in the thiol solution after 10 min corresponds to dissolution of 2% of a monolayer Au( 11 1). The erosion strongly increases when the dodecanethiol adsorbs from undiluted thiol. The amount of holes also increases with decreasing thiol chain length as a result of a lower degree of self-assembly. The surface gold atoms underneath the thiol layer are highly mobile, which is manifest in STM tip-induced reorganization of the thiol layer and in penetration of evaporated gold through the thiol layer. This mobility is believed to be cruciai in the etching process. Due to the mobility of thiol molecules during the adsorption process prior to acquiring a complete self-assembled structure, gold dissolves, probably in the form of a gold thiolate complex.

A Wilhelmy plate technique is used to characterize the potential-dependent wetting of octadecanethiol (Ci8H37SH)-modified polycrystalline gold electrodes in 10-2 M K2S04. This technique is relatively simple and provides information on the electrostatic component of the solid electrode/solution interfacial tension (AtslCE)) and the electrocapillary máximum (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 CigH37SH layer is found to be very stable. The extreme hydrophobicity, the low differential capacitance (~0.7 µ¥ 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 tsl of 7 mN nr1 11. The observed relationship between Aysl and E can be conveniently described by interfacial thermodynamics.

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