The self-assembly of glycyrrhizic acid into nanofibrils

Abstract

The self-assembly of glycyrrhizic acid (GA) in aqueous solutions has significant implications for soft matter science and material design. Here, we describe the self-assembly of GA under well-defined conditions, focusing on measurements of the critical aggregation concentration (CAC) - when GA dimers start stacking into nanofibrils, and of the critical gel concentration (CGC) - when nanofibrils entangle into a viscoelastic network. We examine in detail the structure at low concentrations and its evolution into a viscoelastic hydrogel, addressing the kinetic pathway. We highlight the dependence between GA's concentration and pH changes in solution, and its influence on molecular amphiphilicity and on the self-assembly process. Building upon previous structural models, our findings introduce a refined GA dimer configuration, "tail-to-tail", which reconciles inconsistencies in nanofibril dimensions observed in prior scattering studies. We confirm that GA follows an anisotropic growth mechanism, ruling out classical micellization, and confirm it through the thermo-reversible morphological transition between fibrillar and globular assemblies. The GA nanofibril structure is fully characterized as a core-shell assembly, where neutron and X-ray contrast variations reveal distinct contributions from the hydrophobic core and hydrophilic shell to the nanofibril dimensions. These insights establish a comprehensive framework for understanding GA self-assembly, based on the molecule's amphiphilic and chiral structures, enabling the design of tunable bio-based materials.

Keywords: Critical aggregation concentration; Glycyrrhizic acid; Green surfactants; Helical structure; Nanofibrils; Plant-based surfactants; Self-assembly.