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Surface potential vs concentration isotherms of Na n-dodecyl sulfate (SDDS) adsorbed at the air−solution interface, measured using the vibrating plate method at various concentrations of added salt, exhibit a pronounced minimum. The results of surface tension measurements indicate that the minimum occurs within the concentration range that corresponds to the transition from the Henry regime of adsorption for low surface coverages to the one typical for adsorption of amphiphiles at high surface coverages. We proposed a simple model of adsorption of ionic surfactants at air−fluid interfaces based on the assumption that surfactant headgroups and counterions can adsorb in the Stern layer at the same Helmholtz plane. The electric potential in the electric double layer was calculated according to the Gouy−Chapman model for the diffuse part of the double layer and a modified Stern model for the inner layer with corrections for the discrete charge effects. The total potential drop across the interface was assumed to consist of two contributions:  (i) the potential drop in the diffuse and compact double layers, negative for n-alkyl sulfate ions adsorbed at the air−solution interface, and (ii) a positive contribution due to the effective dipole moment of adsorbed surfactant molecules attributed mainly to the terminal CH3 groups. Our model correctly describes the dependence of the surface tension and surface potential of SDDS solution on its concentration and the amount of added salt.

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The equilibrium surface tension of anionic surfactant n-decyl sulfate (DS-) for various monovalent (alkali) counterions was investigated. It was found that surface activity of surface chemically pure DS- significantly increases with decreasing hydrated size of the counterion. We describe our experimental results in terms of the previously developed adsorption model, which assumes that the counterions may penetrate the Stern layer where the surfactant headgroups are adsorbed. The headgroups and counterions have a finite size that leads to the surface exclusion effects in the adsorption isotherm. The model is improved by explicitly taking into account the electric interactions between adsorbed ions in the adsorbed layer. We obtain a good correlation between the relative counterion size in the Stern layer, the measure of the area excluded by the adsorbed counterions, and the effective radius of the hydrated counterion in the solution. The limiting areas per molecule at the critical micelle concentration for the adsorbed decyl sulfate for various counterions are in good agreement with those measured by neutron scattering.

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