ELECTRICAL DOUBLE-LAYERS ON THIOL-MODIFIED POLYCRYSTALLINE GOLD ELECTRODES

The solid\liquid interfacial tension, capacitive current and differential capacitance of alkyl-thiol-modified gold electrodes in K2SO4 are measured simultaneously as a function of the electrode potential. The chain length of the thiols is varied between C10H21SH and C22H45SH and the ion concentrations range from 10(-4) to 10(-2) M K2SO4. It is found that these alkyl thiols form extremely densely packed self-assembled layers. The potential-dependent wetting of the thiol-modified electrodes depends strongly on the chain length of the thiol: the shorter the chain, the stronger the influence. From the potential dependence of the contact angle, the helmholtz energy of the electric double layer is derived. It is found that measured double-layer capacitances are consistent with the model derived from the wetting method: a large and linear potential decay takes place within the thiol layer and a diffuse charge develops at the electrolyte side of the interface. The relative permittivity is independent of the chain length and is about 2. The K2SO4 concentration affects the measured double-layer capacitance in a consistent manner, but it does not influence the wettability significantly. It is concluded that the dependence of the wettability on the electrode potential originates in the formation of an electrical double layer and that potential-induced conformational changes within the thiol layer are insignificant.