Molecular Dynamics Simulation on Behavior of Common Surfactants at the Oil/Water Interface in Complex Systems-

In order to explore the behavior of common surfactants on oil/water interface in complex systems, five models were designed to study using the molecular simulation method. Except a blank model, four different surfactants were placed in the same complex systems as follows: sodium dodecyl benzene sulfonate (SDBS), dodecyltrimethylammonium bromide (DTAB), nonylphenol ethoxylates (NPE), and betaine. These four surfactants represented the anionic surfactant, the cationic surfactant, the nonionic surfactant, and the zwitterionic surfactant, respectively. For these models, radial distribution function(RDF) of surfactants and the oil molecules, mean square displacement (MSD) along z direction of oil molecules, oil/water interfacial tension (IFT), interaction energy between the oil layer and the mineral stone layer, the distribution of relative concentration of oil molecules along z direction, etc. were analyzed. For the SDBS, NPE, Betaine system, at the beginning, the surfactants arranged regularly at the interface of oil/water, the lipophilic parts of the surfactants were partially inserted into the oil phase, the hydrophilic parts were stretched into the water phase. Then the hydrophilic parts of the surfactants got together and formed to a micelle gradually. The micelle, which of its interior is hydrophilic and its exterior is lipophilic, moved to the oil layer gradually. For DTAB, at first, the surfactants were distributed regularly at the interface of oil/water, they gradually became disorderly as the simulation time increased. But the lipophilic head stretched into oil phase and the hydrophilic head distributed at the oil/water from the beginning to end. The order of the interaction between surfactants and oil molecular were: Betaine approximate to DTAB < SDBS < NPE; The order of oil displacement efficiency of different surfactants were: Betaine > SDBS > NPE > DTAB > None. Above all, the model systems constructed were rationalized. The results obtained from the simulation were agreement with the experimental data. The research gave partly the illustration of actual oil displacement efficiency of different surfactants at the molecular level.