Cationic surfactant adsorption in the presence of divalent ions

The effect of divalent sulphate SO42− anions and monovalent hydrosulphate HSO4− anions, which are simultaneously present in solution due to dissociation equilibrium, on the surface tension of cethyltrimethyl ammonium salts C16TABr (CTABr) and C16TAHSO4 (CTAHSO4), was investigated. The surface tension was determined by the pendant drop-shape analysis method. The previously proposed “surface quasi two-dimensional electrolyte” (STDE) model of ionic surfactant adsorption, based on the assumption that the surfactant ions and counterions can undergo non-equivalent adsorption within the Stern at layer at air/solution interface, was extended for the description the adsorption of ionic surfactants in presence of multivalent electrolytes. The results indicate that a very good description of experimental data by the developed model can be achieved. They show that monovalent anions like Br−, Cl− or HSO4− ions decrease more effectively the surface tension of investigated surfactant than SO42− ions if the solutions of the same ionic strength are compared. That can be explained in terms of low ability of divalent SO42− ions to penetrate the surface layer due to their strong hydration. The ability to decrease the surface tension of cationic surfactants by SO42− follow the place taken by this anion in the Hofmeister series.