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 adsorption properties of soluble, surface-chemically pure n-alkanoic acids at the air/water interface were investigated by evaluating equilibrium surface tension and surface potential versus concentration isotherms. There is no transition-like change in the adsorption isotherms of the n-alkanoic acids between n-pentanoic (C5) and n-undecanoic acid (C11). The isotherms are evaluated by the two-state approach to the adsorption equation and by the Gibbs equation. The nondissociated n-alkanoic acids' surface area demand per molecule adsorbed is not constant within the homologous series but decreases with increasing chain length until it approaches the (almost constant) value of the insoluble homologues. The limiting surface area demand per molecule adsorbed of the soluble n-alkanoic acids is compared with the corresponding data of the insoluble homologues obtained from surface pressure versus surface area isotherms as well as from crystal structure analyses. Standard free enthalpies of adsorption, limiting cross-sectional areas, and surface interaction parameters reveal a distinct effect of alternation within the homologous series. Interestingly, also the Henry constants are subject to the even/odd phenomenon. This is explained by the even and odd homologues' different surface arrangement of their terminal methyl groups with respect to the interface. The linear relationships describing the chain length dependences of the standard free enthalpy of adsorption and of the surface interaction parameter hold for chain lengths in the range 6 ≤ n C ≤ 11. n-Pentanoic acid has a somewhat different characteristic. Unlike the shorter chain homologues' adsorption, the adsorption of n-dodecanoic acid cannot be described by a monotonically proceeding process but seems to include also processes of association.