Spatial heterogeneity in the sol–gel transition of a supramolecular system

Abstract

Heating and then cooling down a dispersion of a peptide amphiphile in water forms hierarchical fibril structures leading to a supramolecular hydrogel. When the gel was physically broken apart by shaking, it transformed into a sol state. After aging it at room temperature for a given time, it returned to the gel state (re-gelation). To obtain a better understanding of such re-gelation processes, we have applied particle tracking to the sol obtained by disrupting the gel, as a function of aging time. The sol was more heterogeneous at the micrometer scale than the initial gel in terms of its viscoelastic properties, and the extent of the heterogeneity in the sol decreased as the re-gelation proceeded. The origin of the heterogeneity could be directly associated with a fibril network confirmed from Fourier-transform infrared spectroscopic, small-angle X-ray scattering and fluorescence microscopic measurements. The particle tracking study using different particle sizes suggested that the characteristic length scale of the heterogeneous network was not larger than 3 μm. This knowledge might be useful for understanding and controlling the gelation, thereby leading to the design and functionalization of soft materials.