Tween-80 on Water/Oil Interface: Structure and Interfacial Tension by Molecular Dynamics Simulations

Surfactants are used in many fields of the chemical industry in a wide range of applications. Generally, surfactants on water/oil interfaces reduce interfacial tension, enabling the formation of emulsions or providing greater stability to the emulsion formed. Although molecular dynamics has been extensively used and has achieved remarkable success in describing thermodynamic and molecular properties of systems with surface-active compounds, the traditional molecular simulation force fields considerably constrain the system size and the time scale of simulations. Here, we propose a coarse-grained model of polysorbate 80, a nonionic surfactant commercially known as Tween-80. Based on the proposed coarse-grained model, we evaluate the influence of the more internal ethylene oxide chain on the properties of the water/Tween-80/decane system. We verify with the simulation results that the model can reproduce the expected decrease in interfacial tension as the surfactant quantity increases in the simulations. Furthermore, we observe changes in the surfactant orientation as their quantity in the interface increases, indicating a preferential orientation for these molecules in the adsorption layer. We also assessed the partition coefficient of the surfactant between the two bulk phases by performing free energy calculations, which showed a higher affinity of Tween-80 surfactants with the water phase.