Surfactant/biosurfactant mixing: Adsorption of saponin/nonionic surfactant mixtures at the air-water interface

Saponins are naturally occurring biosurfactants present in a wide range of plant species. They are highly surface active glycosides, and are used to stabilise foams and emulsions in foods, beverages and cosmetics. They have great potential for an even wider range of applications, especially when mixed with different synthetic surfactants. Understanding those mixing properties are key to the exploitation of saponins in that wider range of potential applications. The surface adsorption properties of the saponin, escin, with two conventional nonionic surfactants, polyethylene glycol surfactants, have been studied at the air-water interface using neutron reflectivity, NR, and surface tension, ST. Although the saponin and polyethylene glycol, C_nEO_m, surfactants are both nonionic the disparity in the relative surface activities and packing constraints result in non-ideal mixing. Comparison with the predictions of the pseudo phase approximation requires the inclusion of the quadratic, cubic and quartic terms in the expansion of the excess free energy of mixing to explain the variations in the surface composition. For escin/pentaethylene glycol monododecyl ether, C₁₂EO_5, the interaction is attractive and close to ideal. For escin/octaethylene glycol monododecyl ether, C₁₂EO₈, it is repulsive and close to the criteria for demixing. The differences in mixing behaviour are attributed to greater packing constraints imposed by the larger ethylene oxide headgroup of the C₁₂EO₈ compared to C₁₂EO₅.