Surface tension and surface potential of Na n-dodecyl sulfate at the air-solution interface: Model and experiment

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 minim um. The results of surface tension measurements indicate that the minimum o ccurs within the concentration range that corresponds to the transition fro m the Henry regime of adsorption for low surface coverages to the one typic al for adsorption of amphiphiles at high surface coverages. We proposed a s imple model of adsorption of ionic surfactants at air-fluid interfaces base d on the assumption that surfactant headgroups and counterions can adsorb i n the Stern layer at the same Helmholtz plane. The electric potential in th e 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 po tential drop across the interface was assumed to consist of two contributio ns: (1) the potential drop in the diffuse and compact double layers, negati ve for n-alkyl sulfate ions adsorbed at the air-solution interface, and (ii ) a positive contribution due to the effective dipole moment of adsorbed su rfactant molecules attributed mainly to the terminal CH3 groups. Our model correctly describes the dependence of the surface tension and surface poten tial of SDDS solution on its concentration and the amount of added salt.