Aggregation of low-concentration dirhamnolipid biosurfactant in electrolyte solution

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 (CMC_st). An increase of diRL concentration and solution pH results in a decrease of the aggregate size at diRL concentrations below CMC_st, whereas it has no influence at diRL concentrations above CMC_st. 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 CMC_st, and stabilizes at diRL concentrations higher than CMC_st. 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.