Nanoparticles as novel materials have unique properties due to their incredibly small sizes. Ensembles of nanoparticles not only collect their intrinsic properties but also generate new ones when nanoparticles are sufficiently close. One important way of forming nanostructures entails the assembly of nanoparticle monolayers at liquid interfaces. It is important to understand how the iron oxide nanoparticles transport in a liquid phase and on a liquid/liquid interface and self-assemble into nanostructures over time. As a preliminary research topic before the comprehensive small angle X-ray scattering (SAXS) study, real-time optical reflection of incident p-polarized light near Brewster’s angle shows that after drop-casting iron oxide nanoparticle heptane dispersion on top of a diethylene glycol (DEG) liquid substrate, an iron oxide nanoparticle layer forms at the DEG/heptane interface, and it self-limits to one monolayer even when there are excess nanoparticles dispersed in the upper heptane phase.

As is needed for the high time resolution and X-ray exposure minimization requirements of kinetics studies, a new cell with walls at angles is designed to significantly reduce the size of the meniscus, which enables the collection of much larger signals in the SAXS images of ordered arrays of nanoparticles at liquid/air interfaces, along with the observation of extremely high degrees of order. Spatial and temporal SAXS scans show that 8.6 and 11.8 nm iron oxide nanoparticles in heptane drop-cast on top of a heptane layer atop a DEG layer are trapped at the DEG/heptane interface to generally form a single ordered, hexagonally close-packed monolayer, and this occurs long before the heptane evaporates. The morphology of the monolayer is independent of the number of nanoparticles used in the formation process. Many nanoparticles remain dispersed in the heptane after this nanoparticle assembly. Assembly occurs faster than expected from considering only the diffusion of nanoparticles from the drop-cast site to this liquid/liquid interface.

And, on the same time scale there is a concomitant decrease in the SAXS form factor from disordered nanoparticles. X-ray beam transmission at different vertical heights characterizes the heptane and DEG bulk and interfacial regions, while monitoring the time dependence of SAXS at and near the DEG/heptane interface gives a clear picture of the evolution of nanoparticle assembly at this liquid/liquid interface. These SAXS observations of self-limited nanoparticle monolayer formation at the DEG/heptane interface are consistent with those using the less direct method of real-time optical reflection monitoring of that interface.