Rhamnolipid–SLES aqueous mixtures: From the molecular self-aggregation to the functional and ecotoxicological properties

Abstract

Biosurfactants represent an ecosustainable alternative to conventional surfactants, allowing to reduce the environmental impact arising from the increasing production of industrial surfactant-containing formulations. In the present work, the physico-chemical properties of the aqueous solutions of rhamnolipids (Rha) are compared to those of sodium lauryl ether sulfate (SLES), a widely used surfactant in many applications. Moreover, in view of a possible partial replacement, mixtures of these surfactants are analyzed. The surfactant self-aggregation in bulk solution is investigated by surface tension, dynamic light scattering (DLS) and electron paramagnetic resonance (EPR), finding that Rha and SLES form mixed aggregates whose morphology, dimension and internal structure depend on the composition of the mixture. An increase of the Rha content induces the formation of large micelles, with a compact inner core. A nonideal behavior of Rha-SLES mixtures is found, indicating antagonistic interactions between the two surfactants. Contact angle, emulsification and foaming measurements are carried out to investigate the functional behavior of the surfactant aqueous mixtures at the interface with solids, oil and air. Rha improves the wettability of hydrophobic surfaces but decreases the emulsion stability and the foamability. The results of the different experimental techniques are rationalized in a framework based on the molecular architectures, highlighting that the balance between the flexibility of the SLES headgroups and the bulkiness of the Rha ones allow tuning the mixture behavior, both in bulk and at the interfaces. Understanding the relationship between the physico-chemical properties of these mixtures and their functional behavior is strategic to steer the rational exploitation of Rha in green chemical formulations. In this direction, a comparative ecotoxicological assay is finally presented, whose results demonstrate the better profile of Rha compared to SLES.