On Anion-Induced Formation of Hemicylindrical and Hemispherical Surface Micelles of Amphiphiles at the Metal/Electrolyte Interface

• Published in: Langmuir 2001-08, Vol.17 (16), p.5039-5044
• Main Authors: Retter, U, Avranas, A
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0101649

The transition from an adsorption monolayer to surface hemimicelles was investigated theoretically. The investigations were focused on the transition from a monolayer of perpendicularly adsorbed molecules to densely packed hemicylindrical surface aggregates (hemimicelles) lying parallel on the surface (stripe structure of surfactant aggregates) as well the formation of hemispherical surface micelles. The time dependence of the degree of coverage of the aggregates was derived for both cases, and it resulted in an exponential law ϑ = 1 − exp(−bt). The model predicts a relative increase of the double-layer capacity because of the phase transition by 1.27 for the formation of hemicylindrical surface micelles and by 1.65 for the formation of hemispherical surface micelles. The exponential law of adsorption layer formation 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/electrolyte interface. Conditions were investigated for which hemicylindrical or hemispherical surface aggregates form. In the case of octanoic acid and octanol, the formation can be explained with a short penetration of electrolyte anions in the layer of the amphiphiles' headgroups because of a certain specific adsorption and consequently to an increase of the headgroups' effective surface areas. This effect leads to surface packing parameters of 1/2 or 1/3 necessary for the formation of hemicylindrical and hemispherical surface aggregates, respectively.