Double-layer capacitance measurements of self-assembled layers on gold electrodes

Abstract

The double-layer capacitance of self-assembled unsubstituted and ω-substituted n-alkanethiol layers on gold electrodes was studied using a potential step technique. A four-element equivalent circuit model was used for the evaluation of the current response. The double-layer capacitance and the conductivity were both found to decrease with increasing chain length for alkanethiol homologues. Substitution with COOH, OH, and NH2 in the ω-position caused the double-layer capacitance to rise dramatically, whereas the general pattern of decreasing capacitance with increasing chain length was confirmed for a series of homologue ω-mercaptoalcohols. The effect of polar substituents is explained by the introduction of permanent dipoles into the self-assembled layers. The capacitance was independent of the applied potential for dodecanethiol but increased with voltage for butanethiol when tested in the range of −200 to 500 mV versus Ag/AgCl. Buffer strength had little influence on the double-layer capacitance above 5 mM (phosphate buffer, pH 7.0). The self-assembly of mercaptoethanol was completed within 3 minutes from an aqueous solution and that of alkanethiols within 10 minutes using acetone solutions. The electrodes were stable for at least several hours.