Influence of Thiol Coverage and Functionality On the Potential-Dependent Permeability of Monolayers

The dynamic response of loosely-packed self-assembled monolayers (SAMs) on gold has been investigated with electrochemical impedance spectroscopy (EIS) under non-faradaic conditions over a large range of applied potentials. The electrochemical response of loosely-packed 16-mercaptohexadecanoic acid (MHA) SAMs is compared with that of densely-packed MHA, hexadecanethiol (HDT), and tetraethylammonium-MHA ion-pair monolayers. We find that the films composed of ion-pairs and loosely-packed MHA have a higher capacitance and are less electrochemically stable than fully-packed SAMs. In sharp contrast to the other SAMs investigated, the loosely-packed MHA SAM exhibits an order-of-magnitude decrease in monolayer resistance (from 107 Ω•cm2 to 106 Ω•cm2) and 15% increase in capacitance as the voltage is scanned in the positive direction from -0.1V to +0.3V (vs SCE). These changes in monolayer resistance and capacitance are reversible, upon scanning the applied potential in the opposite direction, and independent of the electrolyte concentration (0.01 – 1M KCl). Therefore, the change in insulating properties of the loosely-packed MHA film is caused by a voltage-induced structural change (molecular re-organization) within the film itself. Non-faradaic impedance measurements conducted with the loosely-packed MHA film in the de-protonated state (pH 8.5) and protonated state (pH 3) display the same dynamic resistance and capacitance behavior, which suggests that electrostatic attraction between a negatively-charged thiol tailgroup and oppositely-charged gold electrode is not required for conformational change.