Effect of salt on the adsorption of ionic surfactants at the air-water interface

Abstract

• NaCl has significant effect on the equilibrium and dynamic surface tension of ionic surfactant solutions. • The SRT model predicts the adsorption dynamics more accurately than the DKC model. • Salt bridging leads to the compact packing of ionic surfactant at the air-water interface. The combined effect of ionic surfactants and inorganic salts on the interfacial properties is of great importance to various industrial processes. In this work, the effect of ionic surfactants (SDS and CTAB), in the presence of inorganic salt (NaCl), on the surface tension is systematically investigated using experiments, theoretical models, and all-atom molecular dynamics simulations. The equilibrium and dynamic surface tension of surfactant-salt-water systems in a salinity range of 0.01–0.1 M are measured using the pendant drop method. The theoretical models viz., statistical rate theory and diffusion kinetic controlled, suggest that adsorption drives the migration of surfactant molecules to the interface. Finally, the molecular dynamics simulations illustrate increased surfactant packing at the air-liquid interface in the presence of salt due to counterions bridging between the surfactant molecules, which results in a lower value of surface tension for both the surfactants.