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

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 (n-octane-water, n-dodecane-water and n-hexadecane-water) interfacial tensions (IFTs) at 300 K, using coarse-grained molecular dynamics simulations based on the 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 the tendency of the surfactant to adsorb on the surface of NPs. The adsorption occurs due to the formation of hydrogen bonds, and adsorption increases with a decrease in pH, as seen in experimental studies. In this work, we found that the oil-water IFT increases with an increasing amount of adsorption of the surfactant on NPs. At a fixed amount of adsorption of the surfactant on NPs, the IFT behavior is indifferent to the change in concentration of NPs. However, the IFT decreases with an increase in surfactant concentration. We present a detailed analysis of the density profile and intrinsic width of the interface. The IFT behavior is found to correlate extremely well with the intrinsic width of the interface. The current study provides an explanation for the increase in IFT observed in a recent experiment [N. R. Biswal et al., J. Phys. Chem. B 120, 7265-7274 (2016)] for various types of NPs and nonionic surfactant systems.