Abstract
Dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM) tests demonstrated aggregate formation for dirhamnolipid biosurfactant (diRL) at concentrations lower than surface-tension-based critical micelle concentration (CMCst). An increase of diRL concentration and solution pH results in a decrease of the aggregate size at diRL concentrations below CMCst, whereas it has no influence at diRL concentrations above CMCst. The cryo-TEM micrographs show spherical morphology of the aggregates, and the logarithm of aggregate size follows Gaussian distribution. The aggregates are negatively charged. The zeta potential of the aggregates decreases with an increase of diRL concentration to CMCst, and stabilizes at diRL concentrations higher than CMCst. An increase of the solution pH causes a decrease of the zeta potential. A transitional state assumption is raised for the interpretation of the diRL aggregation behavior. The results demonstrate formation of aggregates at significantly low diRL concentrations, which is of importance for the cost-effective application of rhamnolipid biosurfactants.