Nanodomains and the topography of water: An X-ray revelation of tuneable self-assembly in insoluble films.

Long, straight chain saturated fatty acids form homogeneous, featureless monolayers on a supramolecular length scale at the water-air interface. In contrast, a naturally occurring saturated branched fatty acid, 18-methyl eicosanoic acid (18-MEA) has been observed to form three-dimensional domains of size 20-80 nm, using a combination of Langmuir trough, Atomic Force Microscopy (AFM) images of the deposited monolayers, and Neutron reflectometry (NR) and X-Ray reflectometry (XRR). It is hypothesized that these domains result from the curvature of the water surface induced by the steric constraints of the methyl branch. Accordingly, in this work, we investigate in situ the structure of such films using Grazing Incidence Small Angle X-ray Scattering and Diffraction (GISAXS and GIXD). The branched fatty acids indeed form curved nanodomains as revealed by their two-dimensional scattering pattern whereas straight chain fatty acids form the expected featureless film, with no GISAXS scattering peaks. Mixed monolayers consisting of 18-MEA and eicosanoic acid (EA) display a phase transition in the structure from hexagonally packed at high 18-MEA ratio to structures with one-dimensional translational ordering (aligned stripes) for 50:50 mol% and lower ratios. Moreover, the GIXD patterns of monolayers containing 18-MEA display a peak with curved distribution of intensity, indicating a continuous distribution of collective molecular orientations, consistent with the local curvature of the water surface. Finally, we report on an unusual double peak phenomenon in the GISAXS data that is interpreted as being due to a hexagonal packing of elliptical domains - i.e. with two characteristic dimensions. Synchrotron X-Ray scattering experiments have thus unambiguously confirmed the self-assembly, out of plane, "cobbling" of the water interface by these branched structures.