Microscopic mechanisms of MgCl2 affecting anionic surfactant adsorption kinetics on the air-water interface

Inorganic salts can significantly affect the interfacial tension (IFT) of surfactant solutions, which is a key factor influencing the interphase mass transfer and reaction in multiphase systems. The change in IFT is directly related to the adsorption behavior of surfactants. However, the mechanisms underlying the effects of salt on surfactant IFT and adsorption kinetics are not yet clear. Here, taking MgCl2 and sodium dodecyl sulfate (SDS) as examples, the IFT characteristics of surfactant solutions under different salt concentrations were investigated, and the adsorption behavior and interface characteristics of SDS with or without MgCl2 were further analyzed by molecular dynamics simulations. We find that surfactant adsorption involves mixed diffusion-kinetic controlled adsorption. The addition of MgCl2 can obviously accelerate the adsorption kinetics and increase the adsorption capacity of SDS. Under the influence of magnesium ions, more "vacant sites " exist on the interface. Meanwhile, the orientation of the headgroups can be rapidly changed during the adsorption process to achieve a reasonable adsorption configuration faster, and the double electric layer (EDL) becomes thinner, reducing the electrostatic barrier. In addition, our findings offer new insights into anionic surfactant adsorption kinetics in a saline environment.