Adsorption kinetics of dodecyl sulfate anions on the bismuth [formula omitted] plane

Abstract

Electrochemical impedance spectroscopy has been employed for a quantitative study of dodecyl sulfate anion adsorption kinetics at the Bi (01 1 ̄ ) ∣ 0.05 M Na 2SO 4 aqueous solution interface. Analysis of the impedance data demonstrates that the adsorption process of the dodecyl sulfate anion at high negative surface charge density (i.e. in the region of adsorption–desorption peaks) is limited by the rate of the diffusion and heterogeneous adsorption steps (mixed kinetics) of the organic anion to the electrode surface. In the region of maximal adsorption the slow diffusion stage seems to be the rate-determining step of adsorption. At small ac frequency and higher dodecyl sulfate concentrations, two-dimensional association of the dodecyl sulfate anions is possible in the adsorption layer. Nonlinear regression analysis has been used for fitting the experimental complex plane ( Z″, Z′) plots. It was found that, to a first approximation, the Frumkin–Melik-Gaikazyan equivalent circuit (where C true is the double layer capacitance and Δ C is the adsorption capacitance, Z W is the Warburg-like diffusion impedance and R el is an electrolyte resistance at ac frequency→∞) can be used for fitting the experimental complex plane plots for dilute dodecyl sulfate sodium salt solutions. For more concentrated solutions and within the region of maximal adsorption, other more complicated equivalent circuits (and elements) have been used for fitting the experimental impedance data. The parameters C true, Δ C and diffusion resistance R D depend on the electrode potential as well as on the adsorbate concentration.