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, Γs, 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, Cdl, 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 Cdl 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.