Mixing of Surfactin, an Anionic Biosurfactant, with Alkylbenzene Sulfonate, a Chemically Synthesized Anionic Surfactant, at the n-Decane/Water Interface

Biosurfactants show synergic effects with synthesized surfactant in reducing hydrophobic/hydrophilic interfacial tension, while the understanding of the synergistic mechanism is limited. In the present work, mixed monolayers of surfactin and branched alkylbenzene sulfonate at the n-decane/water interface were studied using atomistic molecular dynamics simulations, and the presence of surfactin affecting the microstructure and dynamic properties of the mixed monolayer was evaluated at molecular level. The density distributions of the surfactants along the direction normal to the interface, radial distribution functions of the surfactant head groups, hydrophobic contacts between surfactants, translational activities of both surfactants and counterions, and the dynamics of the hydrogen bonds formed between surfactant and water were calculated. The results suggested that the structure of the mixed monolayers was more compact than that of the individual system of alkylbenzene sulfonate and the interfacial tension was more efficiently reduced, and the translational activities of both surfactants within the mixed monolayers were much lower. The results implied that biosurfactant surfactin and alkylbenzene sulfonate mixed well at the n-decane/water interface, though they were both anionic surfactants.