Hierarchical self-assembly of surfactant/polysaccharide complexes studied by small angle X-ray scattering
MetadataShow full item record
Aqueous solutions of β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS) are known to spontaneously form concentration-dependent assemblies with complex multiscale structure. In particular, supramolecular multilayered tubular aggregates exhibit striking morphological similarities with peptide nanotubes, bacterial protein shells, or multiwalled carbon nanotubes that hint at the existence of some similarities in their formation mechanism. Since the spatial scales involved in this mechanism range from about a nanometer (the size of SDS@2β-CD complexes) through a micron (microtube diameter), small-angle X-ray scattering (SAXS) is an ideal experimental technique that allows one to track changes in the structural organization on different length scales. According to the results of recent SAXS experiments, temperatureinduced microtube assembly/disassembly follows the inward growth mechanism proposed earlier. The outermost tube radius is highly sensitive to the temperature, while SDS@2β-CD complex concentration insignificantly affects this quantity. As temperature increases towards the melting point, the number of walls inside a microtube decreases, and the microtube swells. As a result of the interplay between the bending energy and bond formation, the temperature dependence of the outermost radius of microtubes sheds light on the energetics of the self-assembly, allowing us to estimate the energies of H-bonds involved in this process. On the contrary, the system demonstrates a different, two-level response to the applied moderate hydrostatic pressure (100-200 bar). The first, fast process (~0.3 s) involves the shrinking ofmicrotubes without any significant changes in the number of cylinders inside or the distance between them. In the second slower process (~tens s), inner layers of microtubes disintegrate as less energetically favourable. Opposite to the temperature static studies, this disassembly process is irreversible. After pressure is released, the structure does not return to the initial state presumably being stuck in a local minimum on the energy landscape.