Rhamnolipids are glycolipid biosurfactants mainly synthesized by bacteria such as Pseudomonas species. They are composed of one or two L-rhamnose sugar residues linked glycosidically to β-hydroxyalkanoyl-β-hydroxyalkanoate lipid chains, allowing remarkable surface activity. Rhamnolipids are capable of reducing surface tension to approximately 25 mN/m and exhibit strong emulsifying stability. With more than 60 identified molecular congeners, rhamnolipids represent one of the most extensively studied microbial biosurfactant families.

Chemical Structure

Rhamnolipids typically consist of mono-rhamnolipid and di-rhamnolipid forms, characterized by rhamnose-based hydrophilic heads linked to 3-hydroxyfatty acid dimers. The most common configuration involves C10–C10 fatty acid chains, although chain lengths may vary from C8 to C14 and may include saturated or branched structures. For example, dirhamnolipid Rha-Rha-C10-C10 has a molecular weight of approximately 650.8 g/mol (C32H58O13).

The molecular architecture is defined by α-1,2 glycosidic bonds and ester linkages, generating amphiphilic behavior and partial zwitterionic characteristics. Mono-rhamnolipids generally exhibit higher critical micelle concentration (CMC) values, whereas di-rhamnolipids demonstrate stronger interfacial activity, with reported CMC ranges between 5 and 400 mg/L.

Physical Properties

Rhamnolipids are typically presented as colorless to amber viscous liquids with molecular masses ranging from 300 to 700 Da. They show high water solubility exceeding 100 g/L and maintain structural and functional stability across a broad pH range (pH 4–12). Their critical micelle concentration is generally between 10 and 50 mg/L, outperforming non-ionic surfactants such as Tween 80 in surface activity. These biosurfactants are biodegradable, demonstrating more than 60% degradation in 28 days according to OECD 301B guidelines.