Factors governing the reversible change in ionic permeability of a low-density monolayer

Abstract

Abstract The dynamic response of low-density self-assembled monolayers (SAMs) on gold has been investigated with electrochemical impedance spectroscopy (EIS) under non-faradaic conditions and contact angle measurements. The impedance of low-density 16-mercaptohexadecanoic acid (LD-MHA) SAMs is compared with that of densely-packed MHA and tetraethylammonium–MHA (TEA–MHA) ion-pair monolayers in 0.1 M KCl (pH 8.4). The films composed of ion-pairs and loosely-packed MHA have higher capacitance and are more permeable to ions than fully-packed MHA SAMs. The impedance phase angle of the LD-MHA SAM at 0.1 Hz decreases reversibly by 10° upon scanning the applied potential from −0.1 V to +0.3 V (vs. SCE). Fitting the impedance data to an equivalent circuit correlates the decrease in phase angle with an order-of-magnitude drop in monolayer resistance (i.e. increase in the ionic permeability of the monolayer). The ion-pair and dense MHA SAMs do not exhibit an increase in permeability upon scanning the applied potential from −0.1 V to +0.3 V (vs. SCE). The capacitances of the LD-MHA and TEA–MHA films exhibit a small, but significant (15%), change with applied potential between −0.1 V and +0.3 V (vs. SCE). The impedance of the LD-MHA SAM is found to be independent of the electrolyte concentration (0.01–1 M KCl), and displays hysteresis with the direction of the potential scan. Receding contact angle measurements performed at −0.1 V and +0.3 V using the same electrolyte show that the higher ionic permeability of the LD-MHA SAM at +0.3 V is caused by a voltage-induced structural change (molecular re-organization) within the film. The agreement between EIS measurements performed at pH 3.3 and pH 8.4 shows that electrostatic attraction between a negatively-charged thiol tailgroup and oppositely-charged gold electrode is not required for this structural change to occur.