Effect of mixed surfactants on foam stabilization: A molecular dynamics simulation

Mixed surfactants have antagonistic or synergistic effects on the air/liquid interface of foams, representing two opposite interactions about destroying or stabilizing foams. In this paper, mixed surfactant systems containing non-ionic surfactant polyoxyethylene alkyl ether (AEO) and anionic surfactant sodium dodecyl sulfate (SDS) were discussed by molecular dynamics simulation to study the mechanism of decreasing stability of SDS foam after AEO was added. The results showed that with the addition of AEO, the interaction between SDS molecules is weakened, and the thickness of hydrated shell around the polar head of SDS decreases. In order to simulate the process of foam drainage and rupture, steered molecular dynamics (SMD) containing two external forces on the foam film was performed. One is about the z-axis vertical external force, representing the stability of SDS hydrated layer at the interface; the other is about the external force in horizontal direction, representing the rupture process of the liquid film. The simulated results showed that the higher the concentration of AEO in the mixed surfactant interface layer, the smaller the external force to destroy the foam film interface. In addition, the free energy change of AEO molecules from the water phase through the SDS layer to the air phase further showed that the addition of AEO can reduce the stability of SDS foam films and play a defoaming role in the studied system. The simulated conclusions were consistent with the experimental results about the decreasing stability of SDS foam after AEO was added.