Formulating with Biosurfactants - The Physical Chemistry behind the Competition between Protons and Water Hardness.

Glycolipid biosurfactants, especially Rhamnolipids, have gained considerable interest in recent years because of their exceptional combination of excellent solubilization and outstanding mildness against aquatic organisms, skin, proteins, enzymes, (poly)cations and plastics. Because of this unusual combination of properties, it should not be too surprising that they behave differently both when formulating as well as during applications, as compared to standard anionic surfactants such as SDS or SLES. Considering the packing parameter concept of surfactant aggregation, the glycolipid biosurfactants are quite hydrophilic. Therefore, there have been attempts to combine them with rather hydrophobic co-surfactants such as fatty alcohol ethoxylates with a low degree of ethoxylation. As it turned out, the choice of co-surfactants also has an influence on the acidity (pKa) of the biosurfactants and hence on the degree of protonation at formulation pH. Apart from adding co-surfactants, the other option to reduce the hydrophilicity of glycolipids is to alter the chemical structure of the biosurfactant's sugar headgroup, e.g. by removing one or even both of the sugar groups. Such changes in molecular architecture have important consequences in terms of accessibility of the carboxylate group, which in turn influences not only the pKa of the carboxylate group, but also the interaction with other cations than protons, such as water hardness (i.e. Ca2+). These are important effects which need to be taken into account when formulating with biosurfactants. [ABSTRACT FROM AUTHOR]