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

The pH dependent electrochemical properties of anthraquinone-2,7-disulphoni c acid, 2,7-AQDS, have been investigated both in solution and as adsorbed m onolayers. In low pH electrolyte, the voltammetric responses observed for t he conversion of the quinone, Q, to hydroquinone, H(2)Q, are close to ideal for both diffusive and adsorbed reactants and redox switching is controlle d by electron rather than proton transfer. In unbuffered solution, reductio n of the quinone causes significant polarisation of the interfacial proton concentration causing a discontinuity in plots of E-o versus pH for both so lution phase and monolayer species. The pK(a) values for the H(2)Q/HQ(-) an d HQ(-)/Q(2-) 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 immobilis ed within dense monolayers. Measurements of the differential capacitance re veal that this difference arises predominantly because the effective dielec tric constant within the monolayer, epsilon (film), (8.0 + 0.2) is signific antly lower than that of bulk water. At low pH, epsilon (film) is insensiti ve 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 oxida tion state being approximately 13 +/- 0.7 and 16 +/- 0.5 for oxidised and r educed 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. DeltaE(p). T he apparent standard heterogeneous electron transfer rate constant for the solution phase reactant, k(Soln)(o) depends on the electrolyte pH decreasin g from 4.0 +/- 0.3 x 10(-3) to 8.1 +/- 0.5 x 10(-4) cm s(-1) as the electro lyte pH is increased from 1.4 to 4.2. For monolayers, the corresponding rat e constants are 1.4 +/- 0.1 x 10(3) and 3.4 +/- 0.2 x 10(2) s(-1). Signific antly, the normalised rate constant for the monolayers appears to be more t han an order of magnitude smaller than those found for the solution phase r eactants. This result suggests that adsorption is accompanied by either a s ignificant increase in the activation barrier for electron transfer or that the adsorbate is weakly electronically coupled to the electrode surface. ( C) 2001 Elsevier Science B.V. All rights reserved.