Adsorption of cationic surfactants on a neutral polymer adsorbent: Investigation of the interactions by using mathematical modeling

Adsorption of benzyldimethyldodecyl ammonium chloride and benzyldimethyltetradecyl ammonium chloride on polymeric XAD-16 adsorbent was studied by varying the surfactant concentration, the ionic strength of the solution, and the temperature. The data were correlated with eight isotherm models based on different assumptions (number of adsorbed layers, heterogeneity of the surface, lateral interactions between the adsorbates, formation of surface aggregates) in order to assess the role of various interactions on the adsorption equilibrium.Below the CMC, the experimental adsorption isotherms exhibit a transition from convex to nearly linear shape. A “Langmuir-linear isotherm” was derived to describe this phenomenon. The physical interpretation of the model is the formation of hemimicelles on the adsorbent surface. Above the CMC, the adsorption of surfactants competes with micellization. The experimental data obtained under such conditions were successfully correlated by combining the Langmuir-linear isotherm with a simple micellization equilibrium model.Increasing the ionic strength of the solution was found to have a larger effect (increased affinity) on the adsorption of surfactants directly onto the surface of the polymer than it has on the formation of hemimicelles. Similarly, temperature was found to decrease the strength of the adsorbate–adsorbent interactions (i.e., adsorption directly on the surface) more than it decreases the adsorbate–adsorbate interactions (i.e., formation of hemimicelles).