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: (1) 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.