Self-assembly and liquid crystalline phases of the biosurfactant di-rhamnolipid

The growing demand for sustainable surfactants has led to increased interest in biosurfactants such as rhamnolipids. We studied the self-assembly of the biosurfactant di-rhamnolipid ( ) in starting at the cmc up to the Lyotropic Liquid Crystal (LLC) regime. Measuring the phase behaviour at two different pH values, 6.0 ( pH ≈ pKa ) and 8.0 ( pH ≫ pKa ), one sees how the charge of the surfactant influences the properties. At a pH ≈ pKa , behaves similar to nonionic alkyl polyglucoside surfactants, and forms micelles over a large concentration range and a lamellar L α phase at high concentrations. In contrast, at a pH ≫ pKa the surfactant's carboxyl group is deprotonated, leading to ionic behaviour and the formation of globular micelles at low concentrations and a hexagonal H 1 phase at higher concentrations respectively. The transition between the micellar and the H 1 phase occurs over a narrow concentration range, suggesting a sharp structural reorganisation. Our findings highlight the strong influence of the pH on the self-assembly of di-rhamnolipid over a broad concentration range. • Full binary phase diagram of the biosurfactant di-rhamnolipid. • pH-dependent formation of L α and H 1 lyotropic liyquid crystal mesophases. • SANS and SAXS study at high surfactant concentrations.