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Surfactant formulations must demonstrate safe and reliable preservation against microbial contamination over the entire product lifespan. To address evolving market demands, formulators increasingly turn to multifunctional ingredients as elements of a preservation strategy based on the hurdle technology approach. Multifunctionals such as medium-chain terminal diols (MCTDs) and chelating agents help overcome challenges associated with organic acid and aromatic alcohol preservatives. The physicochemical interactions between surfactants and multifunctionals must be carefully considered from the onset of formulation design to ensure that product performance and preservation requirements are achieved. So-called “drop-in” additions of multifunctionals to existing surfactant formulations should be avoided; instead, a holistic approach that considers the multifunctional ingredients as components of the surfactant system should be employed.

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.