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Adsorption isotherms for sodium dodecyl sulfate (SDS) and octylphenolpoly(oxyethylene) (Triton X100 or TX100) on acidic γ-alumina from single aqueous surfactant solution and from mixed surfactant solutions are reported. Classical depletion adsorption experiments are employed. Analysis methods for both surfactants in the mixtures are independent of the presence of the second surfactant. Strong synergism is observed for the two adsorbing surfactants. In the Henry (low surface density) region of the adsorption isotherm, the two surfactants adsorb independently. TX100, which has very low adsorption densities in the absence of SDS, is strongly co-adsorbed in the hemimicellar region for SDS. Surfactant adsorption isotherms show remarkable inverse curvatures for SDS at low SDS:TX100 ratio, and for TX100 at high SDS:TX100 ratio. This behavior is best explained by a three-dimensional representation of the adsorption isotherms. Above the mixed micellar cmc, SDS adsorption does not reach a plateau, but continues to increase, while TX100 adsorption decreases. This follows the behavior of free surfactant concentrations calculated using regular solution theory. |
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