Effect of sodium alkyl sulfate chain length on foam stability: A molecular dynamics study

The alkyl chain length of surfactant plays an important role on its own performance. In this paper, the effect of sodium alkyl sulfate (SAS) with different alkyl chain lengths on the stability of foam film was studied by molecular dynamics (MD) method. The results showed that the increase of alkyl chain length enhances the interaction between SAS molecules, and increases the thickness of hydrated shell around the polar head. To investigate the effect of alkyl chain length on the drainage and rupture process of foam film, the steered molecular dynamics (SMD) study of foam liquid film with two external forces was also carried out. One external force is parallel to Z-axis and perpendicular to the liquid film, which can reflect the difficulty of foam drainage. The other is parallel to the horizontal direction of X-axis, which represents the stability of foam film. It was found that the longer the alkyl chain of SAS, the greater the external force required to destroy the foam surface. In addition, the free energy change profile of SAS from aqueous phase to SAS layer further indicates that the growth of alkyl chain can enhance the stability of foam film. This work aims to further understand the mechanism of foam film rupture and explore industrial methods to control foam stability.