On the formation of surface micelles at the metal ∣ electrolyte interface

The long-term capacity–time dependence of sodium decyl- and sodium dodecyl sulfate were measured at the mercury ∣ electrolyte interface for 0.1 M Na2SO4 electrolyte in the temperature range from 20 to 50 °C at the potential of maximal adsorption. All capacity–time dependences exhibited a slow increase of the capacity after a sudden decrease in the short-term range. The corresponding long-term time dependence of the degree of coverage can be well described theoretically with a first-order surface reaction in which surface micelles are formed from a condensed film of perpendicularly interdigitated adsorbed molecules. The temperature dependence of the ratio of final equilibrium capacity to initial minimal capacity can be explained by the formation of hemispherical surface micelles in the case of sodium decyl sulfate and sodium dodecyl sulfate for the whole temperature range. A generalized packing parameter was introduced which involves the increase or decrease of the mutual distance of the head groups as well as the tail groups (and consequently their effective surface area) due to electrolyte ions or other surfactants. A necessary extension of the present electrochemical adsorption theory is discussed.