How an Extra Terminal Group on Fatty Acids Leads to The Formation of Nanostructured Langmuir Films?

A characteristic of fatty acids (FAs) covalently associated with fur and hair is two terminal methyl groups rather than the single methyl group found in simple FAs. When these FAs form Langmuir monolayers, the geometric constraint of the extra methyl group leads to periodic lateral nanostructures, a feature that doesn’t appear in simple FAs. To unravel the nature of these nanostructures on aqueous surfaces we use in-situ surface X-ray scattering techniques directly on Langmuir monolayers. In our experiments, we investigate surface films prepared using 19-methyleicosanoic acid (19MEA) at different surface pressures with X-ray reflectivity (XRR) and grazing-incidence small-angle and wide-angle X-ray scattering (GISAXS/GIWAXS) methods. Our in-situ results reveal that interfacial 19MEA molecules self-assemble into two-dimensional (2D) hexagonal lattices of objects that resemble squished half hemispheres with a lateral lattice constant of 40-50nm. Surprisingly, this length uncovered by the GISAXS measurements is nearly 100 times that of the molecular width. Complementary GIWAXS studies show that the lateral order between the individual hydrocarbon chains is similar to simple FAs. With increasing of surface pressure, the lattice constant shrinks and the surface roughness measured by XRR increases from that found for simple FAs to a value which is 3 times larger. We have also measured the phase diagram of mixtures of 19MEA with arachidic acid, a simple FA. With increasing of the arachidic acid composition, we have found a cross-over from a hexagonal to a 1D nanostructure. A real-space model and a theoretical framework have been developed to explain the self-assembly behavior of the FAs that have two terminal methyl groups. This combined experimental and theoretical work provides a basis for understanding the unusual nanostructures which form in these simple molecular systems.