Molecular Dynamics Simulation of Anionic Surfactant at the Water/n-Alkane Interface

The structural and dynamic properties of anionic surfactant at the water/n-alkane (nonane, decane, and undecane) interface were investigated by the molecular dynamics simulation. The model anionic surfactant contained a benzene sulfonate group attached to the 4th carbon in the hexadecane backbone and was denoted as 4-C16. We analyzed the interfacial properties (density profile, interfacial tension, and radial distribution function) of the n-alkane-surfactant-water systems in different oil phases and under special inorganic salinity conditions. The simulation results indicate that a well-defined interface exists between the n-alkane and water phases in the equilibrated model systems. The interfacial tensions of the n-alkane-surfactant-water systems show little change when sodium chloride is added to the solutions. We find that a change in the trans-form fraction of the dihedral in 4-C16 is related to a subtle change in the interfacial tension at the water/n-alkane interface. Clearly, the structure of the surfactant at the interface plays an important role in reducing the interfacial tension. In addition, we also find that the polar head group of the surfactant molecules with sodiumions and water molecules undergo stronger interactions.