Molecular dynamics simulations of surfactant and nanoparticle self-assembly at liquid–liquid interfaces

Abstract

We have performed molecular dynamics (MD) simulations to investigate self- assembly at water-trichloroethylene (TCE) interfaces with the 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. Finally, the simulation has shown that surfactants can aggregate along the water-TCE interfaces, with and without the presence of nanoparticles. (Some figures in this article are in colour only in the electronic version) Self-assembly of nanosized objects at liquid-liquid interfaces is of tremendous interest for various natural and industrial applications. For example, self-assembly of surfactant molecules or polymers at liquid-liquid interfaces is essential in the preparation and stabilization of conventional emulsions. The importance of conventional emulsions is reflected through their wide applications in the food, cosmetic, pharmaceutical, petroleum, fine chemical, and coating industries. Surfactant interfacial self-assembly is also critical in numerous processes such as lubrication, detergency, biological transferring, and polymer processing. Recently, there has been a growing interest in the self-assembly of nanoparticles due to their important applications. For example, self-assembled nanoparticles at a liquid-liquid interface serve as building blocks