Abstract
Most of the equilibrium surface tension (σ e ) - concentration (c) isotherms of soluble surfactants given in the literature show a more or less straight line within the region of higher concentrations when σ e is plotted against the logarithm of c. Although this phenomenon is observed with all kinds of surfactants it is contradictory to basic thermodynamic assumptions of adsorption. This behaviour was also observed with the non-ionic surfactant n-decyl-N, N' (2-hydroxyethyl)amine oxide. However, after having repurified the aqueous surfactant solutions by a particular technique and having confirmed surface-chemical purity of these solutions the questionable behaviour had disappeared. The adsorption isotherm could now be evaluated quantitatively in terms of molecular thermodynamic data. It is therefore concluded that the appearance of a straight line region within the surfactant adsorption σ e :c-isotherm is due to trace impurities of comparatively strong surface activity. One cannot obtain true surfactant adsorption parameters by evaluating adsorption σ e : c-isotherms with a straight line region.
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