A coarse-grain molecular dynamics study of oil–water interfaces in the presence of silica nanoparticles and nonionic surfactants

An understanding of the interfacial tension (IFT) between two immiscible fluids in the presence of external agents such as, nanoparticles (NPs) and surfactants, enable the fluids to find applications in detergents, emulsions,... [ view full abstract ]

An understanding of the interfacial tension (IFT) between two immiscible fluids in the presence of external agents such as, nanoparticles (NPs) and surfactants, enable the fluids to find applications in detergents, emulsions, cosmetics, pharmaceutical, foams, coating industry and petroleum industry. In this work, we have studied the effect of hydrophilic silica nanoparticles (NPs), in the presence of nonionic surfactants (Triethylene glycol monododecyl ether and Tween 20), on the oil–water interfacial tensions at 300 K, using coarse-grained molecular dynamics simulations based on MARTINI force field. Simulation results indicate that silica NPs solely do not affect the IFT. However, the silica NPs may or may not increase the IFT of oil–water containing nonionic surfactant, depending on tendency of surfactant to adsorb on the surface of NPs. The adsorption occurs due to the formation of hydrogen bonds, and adsorption increases with decrease in pH, as seen in experimental studies. In this work we found that the oil–water IFT increases with increasing amount of adsorption of surfactant on NPs. At a fixed amount of adsorption of surfactant on NPs, the IFT behavior is indifferent to the change in concentration of NPs. However, the IFT decreases with increase in surfactant concentration. We present a detailed analysis of the density profile and intrinsic width of interface. The IFT behavior is found to correlate extremely well with intrinsic width of interface. The current study provides an explanation for the increase in IFT observed in a recent experiment [J. Phys. Chem. B 2016, 120, 7265–7274] for various types of NPs and nonionic surfactant systems.