Heterogeneous or competitive self-assembly of surfactants and nanoparticles at liquid-liquid interfaces

We have performed molecular dynamics (MD) simulations to investigate self-assembly at water-trichloroethylene (TCE) interfaces with the emphasis on systems containing sodium dodecyl sulphate (SDS) surfactants and modified hydrocarbon nanoparticles (1.2 nm in diameter, non-charged and negatively charged, respectively). The surfactants and nanoparticles were first distributed randomly in the water phase. The MD simulations have clearly shown the progress of migration and final equilibrium of the SDS molecules at the water-TCE interfaces, with the non-charged nanoparticles either at or in the vicinity of the interfaces depending on surfactant concentrations. The non-charged nanoparticles co-equilibrate with the surfactants at the interfaces at low concentrations of surfactants; however, the surfactants, at high concentrations, competitively dominate the interfaces and deplete nanoparticles away from the interfaces. The interfacial properties, such as interfacial thickness and interfacial tension, are significantly influenced by the presence of the surfactant molecules, but not the non-charged nanoparticles. Interestingly, nanoparticle charge has a significant impact on interfacial assembly, structure and properties. The negatively charged nanoparticles co-equilibrate with the SDS surfactant molecules at the TCE-water interfaces, regardless of the surfactant concentration. Although the inclusion of the charged nanoparticles has a minor influence on the interfacial thickness, it significantly affects the distribution, ordering and effectiveness of the SDS surfactant molecules.