From composite molecular structures to a multiplicity of supramolecular aggregates: The role of intermolecular interactions in biosurfactant self-assembly

Microbial biosurfactants are an attractive and promising class of molecules with the potential to replace oil-derived surfactants for the formulation of ecofriendly and biocompatible products. In this review, we discuss literature findings, mostly reported in the last decade, on the self-assembly of the four main classes of microbial biosurfactants, i.e., rhamnolipids, mannosylerythritol lipids, sophorolipids, and surfactin. All of them present composite molecular structures, characterized by different functional groups and ionisable moieties, and a high conformational flexibility. Where possible, we discuss the formation of different aggregate morphologies in terms of biosurfactant molecular structure and variation of environmental parameters. We highlight how the biosurfactant self-assembly is regulated by a complex interplay between various intermolecular interactions, including H-bonds and steric constraints, besides electrostatic and hydrophobic interactions. For this reason, we show that common theoretical approaches to amphiphile self-assembly, such as the critical packing parameter, have limitations in rationalizing and predicting biosurfactant aggregation behaviour.