Adsorption of sodium dodecylsulphate on mercury as an example of micellization within a multilayer interphase

Abstract

The adsorption of sodium dodecylsulphate from aqueous electrolytic solutions on a polarized mercury electrode was studied by means of differential capacitance measurements over a wide range of concentration and potential, and in the presence of different supporting electrolytes with different ionic strengths. An interpretation of the differential capacitance vs. applied potential curves is given, based on theoretical treatments developed previously. It is shown that at concentrations below the cmc, two-dimensional aggregates are formed on the electrode surface within a polarization region which is bounded by two capacitance peaks at extreme positive and negative polarizations. This film is not particularly stable and is transformed into a compact layer at polarizations close to the potential of maximum adsorption, resulting in a capacitance pit. With increasing bulk concentration the aggregation process extends across the interphase and at least two layers of aggregates—micelles are formed at concentrations around and above the cmc. This three-dimensional aggregation is characterized by the appearance of deformed and/or split capacitance peaks which determine the polarization region wherê this phenomenon occurs.