Adsorption of a mixture of sodium dodecyl sulphate and dodecyl alcohol on a mercury electrode and its effect on the electrochemical processes of cadmium(II) in sodium chloride solution

Abstract

The adsorption behaviour of a mixture of the surfactants sodium dodecyl sulphate (SDS) and dodecyl alcohol (DOH) in 0.5 mol dm −3 NaCl has been studied on a hanging mercury drop electrode (HMDE) on the basis of capacity measurements using phase-sensitive ac voltammetry with prior accumulation of the surfactants at a potential E=−0.6 V (vs. SCE). Investigations were carried out in a wide concentration range of SDS (2.08×10 −7−5.20×10 −3 mol dm −3) and in the presence of trace quantities of DOH (0.5 and 1.4 wt%) in the mixture. The effect of surfactants on the electrode reaction of cadmium was studied by the same technique, based on faradaic current measurements. The standard rate constants of the electrode reaction of cadmium, k s, in the absence and presence of surfactants were estimated from the shape and height of ac voltammograms using the theoretical curves obtained by digital simulation. The results show that the adsorbed film on the mercury surface, which is formed by the mixture of sodium dodecyl sulphate and dodecyl alcohol, has different properties from those of films formed by individual components of the mixture. Two very interesting phenomena were observed: an increase of the electrode double-layer capacity of the mixture at the potential of maximum adsorption in comparison with the values of individual components, and an increased permeability of the mixed adsorbed layer for cadmium ions. Both phenomena were observed in a higher concentration range of surfactants (near the CMC value of SDS). It may be assumed that these phenomena are connected with the solubilization of dodecyl alcohol, which is sparingly soluble in water, and its penetration into the polylayer or micellar film of sodium dodecyl sulphate on the electrode surface.