Association of counterions with micelles

The properties of solutions of ionic surfactants depend on the nature of counterions, as their association at the micellar surface compensates electrostatic repulsion between the charged heads, thus stabilizing the micellar phase. Above critical micellization concentration the conductivity of the surfactant solution is due to the presence of free surfactant ions and counterions, but also due to the charged micellar species. Analysis of the conductivity data, together with measurements of the hydrodynamic size and aggregation number of micelles, yields micellar charge and electric mobility, as well as the electrostatic potential at the interface. Such analysis was applied to aqueous solutions of a perfluoropolyether surfactant for which Gambi et al. have recently provided basic micellar features using SANS. The results were analyzed on the basis of the Surface Complexation Model, and the equilibrium constant for association of counterions with charged surfactant heads was obtained.