On Packing Constraints in the Formation of Surface Micelles of Amphiphiles at the Metal/Electrolyte Interface—The Necessity of Introducing a Generalized Packing Parameter

The long-term capacity–time dependence of 1-decanol was measured at the mercury/electrolyte interface for the electrolytes KCl, NaCl, LiBr, and K2SO4, each of concentration 0.1 M, in the temperature range 20 to 50°C and at 35°C for electrolyte concentrations of NaCl in the region 0.05 to 0.4 M. 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 theoretically well described with a first-order surface reaction. The ratios of final equilibrium capacity to minimal capacity can be explained with the formation of hemispherical or bilayer surface micelles from an initial monolayer of perpendicularly adsorbed molecules. A generalized packing parameter that involves the increase or decrease of the head groups' distance due to electrolyte ions or other surfactants was introduced. In the case of bilayers of parallel adsorbed molecules, obviously a new type of transition, which will be discussed in detail, occurs.