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 when formulating as well as during applications, as compared to standard surfactants such as SDS or SLES.
Considering the packing parameter concept, the glycolipid biosurfactants which have been discussed up to now 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. The other option to reduce the hydrophilicity of glycolipids is to alter the chemical structure of the sugar headgroup, e.g. 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 interaction with cations, but also the acidity (pKa) of the carboxylate group. As will be shown, also the choice of co-surfactants has an influence on the degree of protonation at formulation pH.
Therefore, supposedly surprising effects encountered when formulating with glycolipids of different chemical structure can all be explained by the laws of physical chemistry. While some consider formulating to be an art, it is mainly science.