On anion-induced formation of hemicylindrical and hemispherical surface micelles of amphiphiles at the metal/electrolyte interface

The transition from an adsorption monolayer to surface hemimicelles was inv estigated theoretically. The investigations were focused on the transition from a monolayer of perpendicularly adsorbed molecules to densely packed he micylindrical surface aggregates (hemimicelles) lying parallel on the surfa ce (stripe structure of surfactant aggregates) as well the formation of hem ispherical surface micelles. The time dependence of the degree of coverage of the aggregates was derived for both cases, and it resulted in an exponen tial law (Theta = 1 - exp(-bt). The model predicts a relative increase of t he double-layer capacity because of the phase transition by 1.27 for the fo rmation of hemicylindrical surface micelles and by 1.65 for the formation o f hemispherical surface micelles. The exponential law of adsorption layer f ormation and the ratios 1.27 and 1.65 were found for the long-term capacity -time transients of octanoic acid and octanol adsorbed at mercury/electroly te interface. Conditions were investigated for which hemicylindrical or hem ispherical surface aggregates form. In the case of octanoic acid and octano l, the formation can be explained with a short penetration of electrolyte a nions in the layer of the amphiphiles' headgroups because of a certain spec ific adsorption and consequently to an increase of the headgroups' effectiv e surface areas. This effect leads to surface packing parameters of 1/2 or 1/3 necessary for the formation of hemicylindrical and hemispherical surfac e aggregates, respectively.