Self-assembly in saponin mixtures: Escin/tea, tea/glycyrrhizic acid, and escin/glycyrrhizic acid mixtures

Abstract

Saponins are plant based biosurfactants that are obtained from a wide variety of plant species. They are surface active glycosides with a hydrophobic group, which is commonly a triterpenoid or steroidal group, and saccharide hydrophilic units. Their structures are markedly different from conventional synthetic surfactants, and present interesting challenges in understanding their optimal packing in self-assembled structures such as micelles. Furthermore the wide variety of molecular structures available provide interesting opportunities to optimise the packing in mixed systems. In the current literature there is limited information available on saponin self-assembly, and especially in saponin mixtures. The general view is that globular to more elongated micelle structures form at relatively low concentrations. Here small angle neutron scattering, SANS, is used to investigate and quantify the structure of escin, tea and glycyrrhizic acid micelles and particularly of escin/tea, tea/glycyrrhizic acid and escin/glycyrrhizic acid mixtures at relatively low concentrations. The focus is principally on how the different saccharide headgroup structures affect the self-assembly. Tea saponins form relatively small globular micelles, whereas escin and glycyrrhizic acid form larger more elongated globular micelle structures. The micelle structure and changes observed reflect the packing constraints associated with the aglycone triterpenoid hydrophobic section and the differences in the saccharide headgroups. In the escin/tea and glycyrrhizic acid/tea mixtures the greater intrinsic curvature associated with the tea saponin dominates the micelle structure over much of the composition range explored. For the escin/glycyrrhizic acid mixture the micelle size and aggregation number go through a minimum at an approximately equimolar composition, and this implies an intrinsic difficulty in efficient packing of the two different saponin structures, which results in an increase in the preferred curvature