One-Dimensional Nucleation−Growth−Collision in the Formation of Surface Hemimicelles of Amphiphiles

Abstract

The transition from an adsorption monolayer to surface hemimicelles will be 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). The aggregates were assumed to grow from their fronts linearly at constant growth rate, i.e., a one-dimensional growth proceeds. The overlapping of the aggregates during growth was considered with the Avrami theorem. The time dependence of the degree of coverage of the aggregates was derived for instantaneous and progressive nucleation as well as nucleation according to the exponential law. The increase of double layer capacity with time in long-term transients of octanoic acid adsorbed at mercury−electrolyte interface can be described with one-dimensional instantaneous nucleation-growth-collision of hemicylindrical admicelles. The ratio 1.27 of the equilibrium capacity to the minimal initial capacity, predicted by the model, was corroborated by the experiment. Conditions were investigated for which hemimicylindrical surface aggregates form. The role of geometric constraints in self-assembly is discussed here, especially the role of the surface packing parameter G. This parameter should fulfill the condition 1/3 < G < 0.5 for formation of hemicylindrical surface aggregates. The influence of the electrolyte ions on the shape of the surface aggregates was considered.