Potential dependent adsorption of anthraquinone-2,7-disulfonate on mercury†

Abstract

Monolayers of anthraquinone-2,7-disulfonic acid, 2,7-AQDS, have been formed by equilibrium adsorption onto mercury electrodes. In low pH electrolytes, cyclic voltammetry is nearly ideal with a peak-to-peak splitting of 8 ± 2 mV, and a full width at half maximum of 57 ± 1 mV, being observed for scan rates less than 3 V s^–1. The dependence of the surface coverage of 2,7-AQDS as measured using voltammetry on its bulk concentration is described by the Langmuir isotherm over the concentration range 0.06–9 µM. A saturation surface coverage, Γ_s, of (8.7 ± 0.5) × 10^–11 mol cm^–2 and an adsorption coefficient, β, of (5.6 ± 0.5) × 10⁵ M^–1 are observed where the supporting electrolyte is 1.0 M HClO₄. Consistent with the formation of an organic film, the double layer capacitance, C_dl, as measured at –0.700 V, decreases from approximately 40 µF cm^–2 in the absence of dissolved 2,7-AQDS to 10 µF cm^–2 when the bulk concentration exceeds 10 µM. By measuring the potential dependence of C_dl as the bulk concentration of 2,7-AQDS is varied systematically, an insight into the potential dependence of the free energy of adsorption, ΔG‡_ads, has been obtained. These data reveal that ΔG‡_ads is similar for both fully oxidised and fully reduced monolayers. However, these quinonoid monolayers are least strongly bound (ΔG‡_ads = –28.5 kJ mol^–1) when the film exists in the quinhydrone form.