Adsorption kinetics of tetrabutylammonium cations on Bi(0 1 1̄) plane

Electrochemical impedance spectroscopy has been employed for the study of the adsorption kinetics of tetrabutylammonium cations (TBN+) at the Bi(0 1 1̄)|0.05 M and 0.5 M Na2SO4 aqueous solution interfaces. Non-linear regression analysis has been used to fit the experimental complex impedance plane (Z″,Z′)-plots and it was found that the Frumkin–Melik-Gaikazyan equivalent circuit can be used with a good accuracy to describe the experimental data for dilute TBNI solution. More complicated equivalent circuits have been used for interpretation of the experimental impedance data for more concentrated TBNI solutions, where a two-dimensional association is possible. However, the influence of additional elements (charge transfer resistance, resistance and capacitance of the formation of a compact two-dimensional adsorption layer at some parts of the electrode) is comparatively small. Analysis of the Cole–Cole and other dependences shows that diffusion of the TBN+ cations to the Bi(0 1 1̄) electrode surface is probably the rate-determining step of the adsorption process (for less concentrated TBN+ solutions) in the region of maximal adsorption potential and at moderate ac frequency. At lower ac frequency, a two-dimensional association process of TBN+ is possible. Within the region of adsorption–desorption peaks at E<−1.6 V, a mixed kinetic mechanism (slow diffusion and adsorption steps with the relaxation time constants τD and τK (τD>τK)) has been established.