Protonation reactions of anthraquinone-2,7-disulphonic acid in solution and within monolayers

The pH dependent electrochemical properties of anthraquinone-2,7-disulphonic acid, 2,7-AQDS, have been investigated both in solution and as adsorbed monolayers. In low pH electrolyte, the voltammetric responses observed for the conversion of the quinone, Q, to hydroquinone, H2Q, are close to ideal for both diffusive and adsorbed reactants and redox switching is controlled by electron rather than proton transfer. In unbuffered solution, reduction of the quinone causes significant polarisation of the interfacial proton concentration causing a discontinuity in plots of E° versus pH for both solution phase and monolayer species. The pKa values for the H2Q/HQ− and HQ−/Q2− couples increase significantly from 7.6±0.2 and 10.6±0.2 in solution to 10.6±0.2 and 12.0±0.2 when 2,7-AQDS is immobilised within dense monolayers. Measurements of the differential capacitance reveal that this difference arises predominantly because the effective dielectric constant within the monolayer, εfilm, (8.0+0.2) is significantly lower than that of bulk water. At low pH, εfilm is insensitive to the oxidation state of the monolayer being 8±0.5. However, at pH 11.2, the effective dielectric constants are larger and depend on the oxidation state being approximately 13±0.7 and 16±0.5 for oxidised and reduced forms, respectively. The pH dependence of the rate of heterogeneous electron transfer to both diffusive and adsorbed 2,7-AQDS has been measured from the scan rate dependence of the peak-to-peak separation, ΔEp. The apparent standard heterogeneous electron transfer rate constant for the solution phase reactant, k°Soln depends on the electrolyte pH decreasing from 4.0±0.3×10−3 to 8.1±0.5×10−4 cm s−1 as the electrolyte pH is increased from 1.4 to 4.2. For monolayers, the corresponding rate constants are 1.4±0.1×103 and 3.4±0.2×102 s−1. Significantly, the normalised rate constant for the monolayers appears to be more than an order of magnitude smaller than those found for the solution phase reactants. This result suggests that adsorption is accompanied by either a significant increase in the activation barrier for electron transfer or that the adsorbate is weakly electronically coupled to the electrode surface.