Catanionics from biosurfactants and regular surfactants: miscibility and structure

Mixtures of anionic and cationic surfactants, often referred to as catanionics, can possess synergistic properties, e.g. lower surface tension compared to their individual components. However, they usually precipitate close to equimolar ratios, and they form vesicles. Surprisingly, we observe that catanionics of the anionic biosurfactants, rhamnolipid or sophorolipid, form micelles instead of vesicles and precipitates. More importantly, the results suggest that specific carbohydrate-carbohydrate interactions between biosurfactant headgroups can overcome electrostatic repulsion and drive nanoscopic phase separation within the aggregates, leading to a heterogeneous internal structure. For this reason, we aim to investigate the limited miscibility and structure of these mixtures using a range of experimental and theoretical methods. We use pulsed field-gradient spin-echo NMR spectroscopy, molecular dynamics simulations, small-angle X-ray scattering and contrast variation by small-angle neutron scattering to study the partially mixed micelles on different length scales and to gain an understanding of the interactions between the surfactants. The peculiar properties of catanionics with biosurfactants compared to typical catanionic systems can be rationalized by the complex, asymmetric, hydrophilic and (non)ionic character of the biosurfactants, exerting strong hydrogen-bonding interactions.