Promoting the adsorption of saponins at the hydrophilic solid-aqueous solution interface by the coadsorption with cationic surfactants

Hypothesis: Saponins are highly surface active glycosides, and are extensively used to stabilise emulsions and foams in beverages, foods, and cosmetics. Derived from a variety of plant species these naturally occurring biosurfactants have wider potential for inclusion in many low carbon and or sustainably sourced products. Although their adsorption at the air-solution and liquid-liquid interfaces has been extensively studied, the nature of their adsorption at solid surfaces is much less clear. The aim of this study was to establish the criteria for and nature of the adsorption of saponins at both hydrophilic and hydrophobic solid surfaces. Experiments: Adsorption at the hydrophilic and hydrophobic solid surfaces was investigated using neutron reflectivity. Measurements were made for the saponins escin, quillaja and glycyrrhizic acid. At the hydrophilic surface measurements were also made for escin / cetyltrimethyl ammonium bromide, C16TAB, mixtures; using deuterium labelling to determine the surface structure and composition. Findings: At a range of solution concentrations, from below to well in excess of the critical micelle concentration, cmc, there was no saponin adsorption evident at either the hydrophilic or hydrophobic surface. This implies an inherent incompatibility between the surface OH- groups at the hydrophilic surface and the saponin sugar groups, and a reluctance for the hydrophobic triterpenoid group of the saponin to interact with the octadecyltrichlorosilane, OTS, hydrophobic solid surface. Above a critical composition or concentration escin / C16TAB mixtures adsorb at the hydrophilic solid surface; with a surface composition which is dominated by the escin, and a structure which reflects the disparity in the molecular arrangement of the two surfactant components. The results provide an important insight into how cooperative adsorption can be utilised to promote adsorption of saponins at the solid- solution interface.