Self-assembly in dilute mixtures of non-ionic and anionic surfactants and rhamnolipd biosurfactants

[Display omitted] •Self-assembly of ternary and 5-component surfactant mixtures studied.•In ternary mixtures, scattering data modelled as globular interacting micelles.•In ternary mixtures, variation in aggregation number implies micelle composition close to solution composition.•In 5-component mixture addition of rhamnolipid biosurfactants affects evolution in phase behaviour.•For rhamnolipid rich compositions transition from micelle to lamellar/micellar co-existence occurs.•Transition associated with synergistic packing effects. The self-assembly of dilute aqueous solutions of a ternary surfactant mixture and rhamnolipid biosurfactant/surfactant mixtures has been studied by small angle neutron scattering. In the ternary surfactant mixture of octaethylene glycol monododecyl ether, C12E8, sodium dodecyl 6-benzene sulfonate, LAS, and sodium dioxyethylene monododecyl sulfate, SLES, small globular interacting micelles are observed over the entire composition and concentration range studied. The modelling of the scattering data strongly supports the assumption that the micelle compositions are close to the solution compositions. In the 5-component rhamnolipid/surfactant mixture of the mono-rhamnose, R1, di-rhamnose, R2, rhamnolipids with C12E8/LAS/SLES, globular micelles are observed over much of the concentration and composition range studied. However, for solutions relatively rich in rhamnolipid and LAS, lamellar/micellar coexistence is observed. The transition from globular to more planar structures arises from a synergistic packing in the 5 component mixture. It is not observed in the individual components nor in the ternary C12E8/LAS/SLES mixture at these relatively low concentrations. The results provide an insight into how synergistic packing effects can occur in the solution self-assembly of complex multi-component surfactant mixtures, and give rise to an unexpected evolution in the phase behaviour.