Interfacial and Solution Aggregation Behavior of a Series of Bioinspired Rhamnolipid Congeners Rha-C14‑Cx (x = 6, 8, 10, 12, 14)

Rhamnolipids are glycolipids produced by microorganisms with outstanding surfactant properties. They are a class of biosurfactants that are potential candidates for biodegradable and nontoxic replacements of current specialty synthetic surfactants. Building on our previous efforts in developing an efficient and practical chemical methodology to synthesize rhamnolipids allows us to now explore the tunability of rhamnolipid properties. Here, we explore the impact on solution self-assembly and adsorption at the air/water interface of symmetry of the two lipid tails for diastereomeric mixtures of a series of monorhamnolipids of the generic structure Rha-C14-Cx. Surface activity of the anionic forms of these molecules at pH 8 is described by surface tensiometry. Characteristics of their aggregation behavior in aqueous solutions including hydrodynamic radius, aggregation number, and aggregate morphology are determined using dynamic light scattering and time-resolved fluorescence quenching spectroscopy. The solution aggregation behavior of this series is found to unexpectedly vary in a nonmonotonic fashion. This is explained by molecular structural attributes of each series member that result in differences in the respective intermolecular interactions of various parts of these surfactants.