Self-assembling properties of mono and di-rhamnolipids characterized using small-angle X-ray scattering

Rhamnolipids are glycolipid surfactants composed by a hydrophilic head of either one (mono-RL) or two (di-RL) rhamnose moieties coupled to hydroxyaliphatic chains that can be of different lengths. In spite of their importance in different fields of applications, as bioremediation processes for instance, self-aggregation physico-chemical properties of RLs are not unique. This because a variety of aggregates morphologies (shape and size) can either exist or coexist in aqueous dispersion due to mono-RL:di-RL molar ratio, hydrophobic tails length, pH and the presence of co-surfactants and additives. Recently, a theorethical approach reported the self-assembling morphologies of either pure mono or di-RL in aqueous environment, predicting the formation of spherical to ellipsoidal micelles to worm-like and disk-like aggregates depending on RL concentration and fatty acid chain length. In order to add new information to those previously available, the present work investigated the self-assembling properties of mono-RL-C10-C10 and di-RL-C10-C10 separately in aqueous dispersion by small angle X-Ray scattering (SAXS). A novel approach was applied to the data analysis coupling the scattering length density profiles of the RLs chemical groups and Monte Carlo simulations. Such an approach allowed us to infer about the preferred mono-RL and di-RL conformations that fit better in the self-assembling morphologies. In this way, we show that mono-RL-C10-C10 self-assembles into lamella-like aggregates coexisting with 30 % of multi-lamella aggregates (circa of 5 closed stacked lamella) from a concentration ranging from 10 to 50 mM, with hydrophobic thickness of about 12 Å, a hydrated polar head thickness of 10 Å, and an area per glycolipid of 76 Å