Hydrogelling properties of microbial glycolipid biosurfactants

Abstract: Microbial glycolipids constitute a wide range of amphiphiles generally addressed to as microbial biosurfactants. However, the word biosurfactant can be misleading when it comes to describe the stunning, and mostly unexpected, hydrogelling properties of many of these molecules. Hydrogels are largely employed in the food, cosmetic and pharmaceutical industries and biobased low molecular weight hydrogels are certainly promising substitute of more common polymeric gels, more difficult to degrade. In the past few years, more and more microbial glycolipids have displayed a gelling behaviour in water, including cellobioselipids, C18:0 and C16:0 sophorolipids and glucolipids but also bolaform sophorosides (for a broad summary, please refer to Table 12 in Green Chem., 2021, 3842). Interestingly, the structure of the hydrogels is quite variable, spanning from common (and potentially expected) micellar gels to less expected fibrillar gels or even more exotic lamellar gels. More astonishing compared to the broad literature of low-molecular weight gelators, such structural variety is associated to few molecules, all having comparable chemical structures. This presentation will summarize which microbial glycolipids form hydrogels and it will give a short overview of the main hydrogel structures, elastic properties and response to external stimuli. It will also discuss a rare example of sol-gel transition driven by kinetics and associated to the minima of conformational energy of the sugar headgroup.