KAMIL WOJCIECHOWSKI1,2
1. Faculty of Chemistry, Warsaw University of Technology, Warsaw, Poland
2. SaponLabs Ltd., Warsaw, Poland

Abstract

The saponin-rich aqueous extracts of the horse chestnut, cowherb and soapwort are able to reduce surface tension down to ca. 42 mN/m, forming highly elastic adsorbed layers with the surface compression elasticity, E’, exceeding 200 mN/m, and reasonable foamability. Thus, despite a lower ability to reduce surface tension, the saponin-rich extracts could be regarded as interesting alternatives to synthetic surfactants. The article compares the effect of the three extracts on model lipid monolayers mimicking the skin (keratinocytes cell membrane and intercellular lipids of the stratum corneum) and the human sebum. In contrast to the commonly employed synthetic surfactants (ethoxylated sodium laureth sulfate and cocamidopropyl betaine), all tested plant extracts not only preserved the model monolayers, but even strengthened their mechanical properties. The study shows that saponin-rich extracts could be a potentially milder alternative to the common synthetic surfactants currently employed in cosmetics. One should bear in mind, however, that when a maximum detergency is sought, the synthetic surfactants are still the better choice.

INTRODUCTION
The major role of surfactants in cleansing formulations is the removal of dirt accumulated on the human skin and hair. However, strong detergents may at the same time negatively affect the skin’s protective lipidic layers. Hence the need to search for milder natural surfactants from sustainable resources, e.g. from plant extracts. Saponins, belonging to the group of glycosides, where a non-polar aglycon (triterpenoid or steroid) is linked to 1-3 polar glycon (sugar) groups, are a good example of plant biosurfactants. Besides simply reducing surface/interfacial tension (similarly to the low molecular weight synthetic surfactants, LMWSS), they can also form highly surface-elastic adsorbed layers, outperforming several surface-active proteins (1–4). Their widespread presence in our daily diet (e.g. in soybeans, chickpeas, oat, asparagus, fenugreek, garlic, sugar beets, potatoes, tomatoes, onions, or even tea) (5) significantly alleviates the consumers concerns. On the other hand, many saponins exert biological activity, often related to their affinity to cholesterol and other lipids (6–9), making them suitable candidates for model membrane ...