Microstructure studies on biosurfactant-rhamnolipid/n-butanol/water/n-heptane microemulsion system

A microemulsion system, containing a natural biosurfactant, is characterized in a rhamnolipid/n-butanol/water/n-heptane system. The surface tension of rhamnolipid solution has been studied in detail, and an especial post-CMC has been found. Based on the surface tension measurements, it can be concluded that the phase behavior and microstructure of this microemulsion are rational to the conformational changes of rhamnolipid molecules at the interface of oil/water. For a given biosurfactant-to-cosurfactant-to-oil ratio, an aqueous dilution line is studied. At 298 K, along this dilution path, a structural transition sequence: water drops → bicontinuous microstructure → oil drops is inferred by conductance method, which is also confirmed by other two methods as dynamic light-scattering (DLS) and freeze-fracture transmission electron (FFTE). The droplet dimension, the polydispersity, and the diffusion coefficient are found to be distinctly different between the w/o and o/w samples. Droplets in w/o microemulsion are smaller and more uniform than those in o/w one.