Quantitative prediction of oil-water interfacial tension in surfactant systems using dissipative particle dynamics

We present a dissipative particle dynamics (DPD) model for surfactants at oil-water interfaces, parametrised directly against experimental data. The model is applied to pure and mixed systems of ionic and nonionic surfactants, including sodium dodecyl sulfate (SDS), dodecyldimethylamine oxide (DDAO), and polyoxyethylene alkyl ethers (C_iE_j). Our simulations reveal that exceeding the maximum interfacial packing density can lead to the spontaneous production of micelles from the interfacial surfactant layer into the bulk. This behaviour, together with our parametrisation scheme, enables quantitative prediction of interfacial tension as a function of surface concentration in excellent agreement with experiment. In addition, the model provides access to other interfacial properties such as maximum surface coverage and interfacial monolayer thickness. Moreover, we show that the model can be used to investigate synergistic effects in mixed surfactant systems, where combinations of surfactants yield lower interfacial tensions than either component alone. In particular, simulations of DDAO and SDS systems demonstrate a lower interfacial tension than the corresponding pure surfactants. Utilising these results, we propose that these synergistic effects result from a balance between the intrinsic ability of a surfactant molecule to lower the IFT and its achievable maximum packing at the interface.