Two-dimensional structures and order of nano-objects on the surface of water: Synchrotron x-ray scattering studies

Surface-sensitive synchrotron x-ray scattering techniques have been used to study the microscopic two-dimensional (2D) structures of Langmuir monolayers (LMs) formed by various nano-meter sized macromolecules on water at room temperature. The molecules studied are: sphere-like fullerene-propylamine adduct (C60-PA); rod-like α-helical polypeptide poly(γ-benzyl-L-glutamate) (PBLG); hairy-rod polypeptide poly(γ-4-(hexadecyloxy)benzyl α,L-glutamate) (C16-O-PBLG); and sphere-like gold nanoparticles derivatized with carboxylic acid-terminated alkylthiol chains (AuSC16). PBLG and C16-O-PBLG rods lie down parallel to the interface and align locally with neighboring rods within the monolayer plane.; The observations of broad grazing incidence diffraction (GID) peaks indicate that the C60-PA and AuSC16 monolayers are 2D amorphous solids that are consistent with local hexagonal packing. They possess only short-range positional order with lateral correlations extending over no more than a few nearest-neighbor distances. The large number of electrons contained in the molecules makes the observations of liquid-like 2D x-ray structure factors possible.; The surface pressure versus area isotherm and specular reflectivity (XR) measurements confirm that both the LMs of rod-like PBLG and sphere-like AuSC16 undergo a first-order monolayer/bilayer transition upon lateral compression. In both cases, off-specular diffuse scattering (XOSDS) intensities from compressed monolayers are consistent with the presence of capillary waves on the surface and the lateral homogeneity of the films. However, both PBLG and AuSC16 bilayers exhibit excess off-specular scattering above the predicted capillary contribution, indicating that the bilayers are microscopically inhomogeneous. The results demonstrate the utility of XOSDS techniques for probing inhomogeneities on liquid surfaces.; Due to the additional long alkyl side chains, the monolayers of hairy rod C16-O-PBLG sustain much higher surface pressures than PBLG and the monolayer/bilayer transition is suppressed. In C16-O-PBLG LM, hydrophobic side chains are segregated towards the film/gas interface, while the PBLG cores reside closer to the water/film interface. The two high-angle GID peaks observed at high surface pressures are consistent with a common packing mode of alkyl chains known as the herringbone (HB) packing. One of the peaks is absent at low surface pressures, suggesting that the HB order is first established by the side chains that are confined one-dimensionally between aligned PBLG cores.