Self-assembly of surfactants or nanoparticles at liquid-liquid interfaces is important due to their wide applications. The application of surfactants is ubiquitous, ranging from natural and industrial processes to commercial and personal care products. However, many industrial processes are performed in the presence of both surfactants and colloidal particles. In spite of the importance, interfacial adsorption when a system contains both surfactants and colloidal particles has not been extensively studied.

We have performed molecular dynamics (MD) simulations to investigate self-assembly at water-trichloroethylene (TCE) interfaces with an emphasis on systems containing modified hydrocarbon nanoparticles (1.2 nm in diameter) and sodium dodecyl sulfate (SDS) surfactants. The nanoparticles and surfactants 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 nanoparticles either at or in the vicinity of the interfaces. One unique feature is the “attachment” of surfactant molecules to the nanoparticle clusters in the water phase followed by the “detachment” at the water-TCE interfaces. At low concentrations of surfactants, the surfactants and nanoparticles co-equilibrate at the interfaces. 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 surfactants, but not the nanoparticles. The order of the surfactants at the interfaces increases with increasing surfactant concentration, but is independent of nanoparticle concentration. The simulation has also shown that surfactants can aggregate along the water-TCE interfaces, with and without the presence of nanoparticles. Finally, we have studied the charge effect on nanoparticles and outcomes in interfacial assembly.