Adsorption and Packing of Non-Spherical Nanoparticles at Liquid-Air Interfaces*

The near 2D assembly of interfacial ellipsoidal (NE) and disk-like (ND) nanoparticles (NPs) was examined by variable-pressure SEM, and single-particle resolution was achieved at tunable areal density for NPs adsorbed on low-volatility liquids such as ionic liquids, glycerol, and tetra-ethylene glycol. Out-of-plane orientation of the silica-coated hematite NPs was noted for both NEs and NDs of aspect ratios across the range 3 to 6, with this orientation affected by molecular weight (MW) of grafted PEG; in-plane orientation increased at higher MW.  In systems of mixed orientation, in-plane NEs segregated into locally ordered domains while out-of-plane NEs were ejected into smaller disordered domains.  At jamming (areal density ~0.6), the in-plane NEs assembled into slightly curved but closed-packed stacks of ~40-60 NPs. Differently, in-plane NDs remain disordered but out-of-plane NDs preferentially assembled into columnar closed-packed stacks, with NDs oriented perpendicular to the liquid interface.  Overall, in-plane disorder was much higher for NDs than NEs. The same systems were probed by GISAXS and interfacial tensiometry, the latter establishing that vacuum is not a prerequisite for the interfacial assembly.