Concentrated Rhamnolipid Formulations: Bridging Chemodiversity to Structure, Flow Behavior, and Functionality

The development of ultraconcentrated biobased formulations is one of the latest frontiers in sustainable product design. This study demonstrates that biosurfactants are well suited to the scope with their natural chemodiversity being a key factor in optimizing formulation structure and function. The composition of a low-cost commercial rhamnolipid sample was investigated by using nuclear magnetic resonance and mass spectrometry. The results revealed a complex mixture of congeners with a predominance of dirhamnolipids and double-tailed species and the presence of long-chain free fatty acids. Polarized optical microscopy, small-angle X-ray scattering, and electron paramagnetic resonance show that this rhamnolipid sample, in aqueous solution, forms ellipsoidal micelles with a highly hydrophobic core, whose dimensions are almost insensitive to concentration (up to 65 wt %) and temperature (up to 50 °C). Analysis of the results hints that a fundamental role in tuning the system behavior is played by the specific rhamnolipid congener composition and by the free fatty acids acting as cosurfactants. At concentrations exceeding 65 wt %, small domains with different supramolecular ordered structures form, suggesting congener segregation. This aggregation behavior explains the preserved low viscosity and good cleaning efficiency. Thus, rhamnolipids are established as valuable candidates for the design of innovative, sustainable formulations.