Probing 2D Phase Separation in Mixed Silica and Gold Nanoparticle Assemblies

Interfacial assemblies of mixtures of different nanoparticles (NPs) can be used to obtain well-defined 2D phase separated morphologies that, by functionalization, generate surfaces with heterogeneous interaction sites. Yet, the kinetic pathways by which the 2D phase separation occurs are still poorly understood. We capitalize on nanoparticle surfactant (NPS) assemblies at a liquid-liquid interfaces to increase the binding energy of NPs to interfaces and to provide a platform for investigating densely packed, mixed silica(polystyrene) and gold NPSs by UV-Vis reflection spectroscopy and grazing incidence small angle x-ray scattering. Because the plasmon excitation energy depends on both the properties of individual gold NPs and the location and number of NPs in the ensemble, we can characterize the interparticle distances, degree of ordering, domain size, and structure of the NP assemblies. By tuning the size, number, and adsorption rate of each NP species in the assembly, we can control the morphology and kinetics that govern the phase separation and reveal structural correlations of larger gold NP clusters. We find that as inert NP concentration increases, the reduction rate of interparticle spacing decreases, are more ordered, and have smaller NP domains as the inert NPs act as barriers to intimate gold-gold contact. These complementary methods can be used to understand and control the nanoscopic factors that govern phase separation in two dimensions.