The surface tension of aqueous solutions of cetyltrimethylammonium cationic surfactants in presence of bromide and chloride counterions

An improved model of ionic surfactant adsorption was applied for description of CTABr (cetyltrimethylammonium bromide), CTACl and CTAHSO4 surfactants adsorption from their aqueous solutions containing KCl and KBr at liquid/air interface. The model is based on the assumption that the surfactant ions and their counterions undergo non-equivalent adsorption within the Stern layer. The surface tension isotherms for various concentrations of surfactant ions and Br− and Cl− counterions were calculated and compared with the results of surface tension measurements performed by the pendant drop shape analysis methods. The dependences of surface excess values of all ions on CTA+/Br−/Cl− bulk concentrations were calculated. Obtained results showed much larger affinity to the interface of Br− anions than Cl− anions. This was explained in terms of a larger repulsion of chloride anions from the interface than bromide anions. It may be caused by the difference in their excess polarizability and the van der Waals interaction with the interface.