Mechanical Properties of Solidifying Assemblies of Nanoparticle Surfactants at the Oil-Water Interface

The effect of polymer surfactant structure and concentration on the self-assembly, mechanical properties, and solidification of nanoparticle surfactants (NPSs) at the oil-water interface was studied. The surface tension of the oil-water interface was found to depend strongly on the choice of the polymer surfactant used to assemble the NPSs, with polymer surfactants bearing multiple polar groups being the most effective at reducing interfacial tension and driving the NPS assembly. By contrast, only small variations in the shear modulus of the system were observed, suggesting that it is determined largely by particle density. In the presence of polymer surfactants bearing multiple functional groups, NPS assemblies on pendant drop surfaces were observed to spontaneously solidify above a critical polymer surfactant concentration. Interfacial solidification accelerated rapidly as polymer surfactant concentration was increased. On long timescales after solidification, pendant drop interfaces were observed to spontaneously wrinkle at sufficiently low surface tensions (approximately 5 mN m^-1). Interfacial shear rheology of the NPS assemblies was elastic-dominated, with the shear modulus ranging from 0.1 to 1 N m^-1, comparable to values obtained for nanoparticle monolayers elsewhere. Our work paves the way for the development of designer, multicomponent oil-water interfaces with well-defined mechanical, structural, and functional properties.