Revealing Two-Dimensional Self-Assembled Structures on Liquid Surfaces Using X-Ray Scattering Methods

Liquid surfaces offer intrinsic two-dimensional (2D) confined spaces with angstroms-level surface roughness, ease of 2D motion, and convenient liquid transport of chemicals to the interfaces. These surfaces provide an ideal platform for the interfacial self-assembly of 2D soft materials, including biomolecules, surfactants, polymers, and nanoparticles (NPs). Interface-assembled 2D soft materials are flexible, free-standing, and potentially tunable in lateral spacing and thickness, exhibiting distinct electronic, optical, mechanical, and catalytic functions. In-situ approaches are essential to monitor and quantify the role of the liquid surfaces in the self-assembly process of 2D soft materials. The Open Platform Liquid Surfaces (OPLS) endstation at the Soft Matter Interfaces (SMI) beamline at National Synchrotron Light Source II is well-suited for probing molecular and nanoscale structures at vapor/liquid and liquid/liquid interfaces. At OPLS, we provide a suite of surface-sensitive X-ray techniques: X-ray reflectivity (XRR) for determining the surface-normal electron density profile, and grazing-incidence small-angle and wide-angle X-ray scattering (GISAXS/GIWAXS) for examining the lateral nanoscale/molecular structures. Here, we present investigations of interfacial structures in two self-assembly systems, i.e., fatty acids with two terminal methyl groups (19-methyleicosanoic acid, 19-MEA) and nanoparticles grafted with charged-end polyethylene glycol (PEG-NPs). Our in-situ results reveal that interfacial 19-MEA molecules self-assemble into 2D hexagonal lattices of nano-objects resembling squished hemispheres, with a lateral lattice constant that decreases with increasing surface pressures. For charged PEG-NPs, we observed the formation of distinct checkerboard square lattices across various pH levels and molar ratios of the constituents. Theoretical models to explain the self-assembly behaviors in these systems are being developed and will be discussed.