Equilibrium surface tension (σ e ) versus bulk concentration ( c ) isotherms of aqueous, surface-chemically pure solutions of various alkali perfluoro- n -octanoates were measured at 295 K. These 1:1 ionic surfactant systems belong to the pseudo nonionic ones. Evaluating the different σ e vs log c isotherms by basic adsorption equations reveals that they follow ideal surface behavior. The novelty of this investigation exists in the fact that the surface area demand per molecule adsorbed calculated from the experimental σ e vs log c isotherms is identical to that of the hydrated alkali cation. Thus, as long as the counterion’s cross-sectional area is greater than that of its amphiphilic anion, the amphiphile’s total surface area demand will exclusively be governed by that of its alkali counterion. This, in turn, means that the counterion is nonrandomly bound to the amphiphilic anion in the adsorption layer. Furthermore, the size of the hydrated alkali counterion in the adsorption layer does not differ from that in the bulk phase.
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Lunkenheimer et al. (2017) studied this question.
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