Rhamnolipids Sustain Unchanged Surface Activities during Decomposition in Alkaline Solutions

Biosurfactant rhamnolipids (RLs) have gained global interests owing to their fully green properties, potentially wide applications in diverse fields, as well as high stabilities under various harsh conditions. Nevertheless, we doubted the reputed stability of RLs in considering their natural structure of carbohydrate heads and lipid tails. This study, for the first time, systematically investigated the stability of RLs at varying temperatures and pH. As found, the concentration of RLs in an aqueous solution was significantly reduced when the pH was over 11 at room temperature, and this was much more severe with the increase in temperature and preservation time. According to the high-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis, degradation yielded other RL congeners, 3-hydroxy fatty acids, rhamnose, methyl furfural, and organic acids. The newly generated RL congeners and fatty acids still possessed equivalent surface activities in reducing the surface tension of the aqueous solution, well explaining the previously claimed high stability of RLs. The finding will be greatly valued for commercially developing the industrial applications of RLs and other biosurfactants.