Effects of interfacial molar ratios of anionic/cationic surfactant mixtures on properties at the gas-liquid interface: a molecular dynamics study

The gas-liquid interface properties and molecular adsorption of the binary mixtures of anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant dodecyl trimethyl ammonium chloride (DTAC) with different molar ratios were described through molecular dynamics (MD) method with the aim of providing supplements to the experiments at the molecular level. The simulation results showed that the anionic/cationic surfactant mixtures can form a denser interfacial monomolecular layer than single surfactants, and the interface will tend to form equimolar adsorption eventually. The entry barrier of the interface under different molar ratios was investigated, the system with a high SDS ratio has higher the entry barrier, which means that the initial non-equal molar ratio of the interface composition may be more conducive to prolonging the existence time of the interface. The rupture process of the microscopic liquid film was simulated, and it was found that the site not covered by surfactant molecules was the point where the liquid film was prone to rupture. The critical rupture thickness of each models was about 0.8 ∼ 1.2 nm.