Biosurfactant/surfactant mixing properties at the air–water interface: comparing rhamnolipids and sophorolipids mixed with the anionic surfactant sodium dodecyl benzene sulfonate

There is an increasing interest in the use of biosurfactants in the development of more biocompatible and biosustainable surfactant-based products. To optimise performance and mitigate production costs, biosurfactants are commonly mixed with different synthetic surfactants. Understanding in detail their mixing properties at interfaces and in solution is key to the development of optimal formulations. Reported here is a detailed thermodynamic analysis, using the latest developments in the pseudo phase approximation, PPA, of the mixing behaviour at the air-water interface of two glycolipid biosurfactants, rhamnolipids, RL, containing the mono and di-rhamnose isomers R1 and R2, and the sophorolipids, SL, containing the lactonic and acidic isomers LS and AS, with the anionic surfactant sodium dodecyl benzene sulfonate, LAS. The analysis uses the previously reported adsorption data, from neutron reflectivity measurements, NR, for the associated binary and ternary mixtures. The different rhamnolipid and sophorolipid biosurfactant structures and their relative surface activities have a profound effect on their mixing properties at the air-water interface with the anionic surfactant LAS, due predominantly to the steric constraints of the different molecular structures. This results in different synergistic excess free energies of mixing and different optimal compositions.