Impact of NaCl concentration on equilibrium and dynamic surface adsorption of cationic surfactants in aqueous solution

To obtain a better understanding of different NaCl concentration effect on surface activity and dynamic surface adsorption of cetyltrimethylammonium chloride (CTAC) and cetyltrimethyl-ammonium bromide (CTAB), conductivity measurement, equilibrium surface tension, and dynamic surface tension technical associated with quantum chemistry calculation methods were performed. It demonstrates that Br− counterions are more effective in reducing surface tension and facilitating micelle aggregation than Cl− ions. At addition of NaCl introduces competitive substitution for Br− ions by excess of Cl− ions in diffusion layer and Stern layer, which inhibits surfactant adsorption, especially the effective counterion concentration in surface layer. And the higher NaCl concentration the larger substitute tendency, thus, NaCl more effectively promotes surface excess for CTAC/NaCl systems than CTAB/NaCl systems. On the other hand, the effect of electrolyte on dynamic adsorption is time-dependent. For short time inhibits surfactant adsorption, for long time accelerates adsorption. And also the addition of NaCl promotes dynamic adsorption more pronounced for CTAC than CTAB.