Varying the Counterions at a Charged Interface

The influence of different counterions on the adsorption behavior of the ionic soluble surfactant dodecyl-dimethylammonium-pyridimium bromide is investigated. The addition of potassium halogenides to aqueous solutions of the surfactant modifies the surface activity of the amphiphile and has a profound influence on the surface tension isotherms. The measured critical micelle concentration follows the order of the periodic table of elements which is in strong contrast to the surface excess. The number density of the adsorbed surfactants at the cmc does not depend on the amount of counterions in the solution but on the nature of the counterion. Furthermore, evidence is provided that the surface region is depleted on fluoride ions. Surface second harmonic generation and ellipsometry have been used to gain direct structural information which complement the thermodynamic considerations. The combination of both optical techniques yields the number density of the condensed counterions within the compact layer. A strategy to retrieve selected parameters of the ion binding model of Radke et al. is presented. The analysis of the optical data reveal the existence of a phase transition towards a surface condensed state with increasing salt condensation.