Sub-CMC solubilization of n-alkanes by rhamnolipid biosurfactant: the Influence of rhamnolipid molecular structure

Solubilization of n-alkanes by dirhamnolipid (diRL) biosurfactant at sub-critical micelle concentrations (sub-CMC) was studied and the results were compared to that for monorhamnolipid (monoRL) obtained in our prior study. The results show that the apparent solubility of the four alkanes (decane, dodecane, tetradecane and hexadecane) increases linearly with the increase of diRL concentration at diRL concentrations below CMC. The solubilization potential of diRL indicated by molar solubilization ratio (MSR) is higher at sub-CMC than at hyper-CMC concentrations (0.43 vs 0.03, 7.91 vs 0.02, 5.16 vs 1.98 and 3.71 vs 1.26 for decane, dodecane, tetradecane and hexadecane, respectively). The maximum access at the surface of the alkane-rhamnolipid aggregates (Гmax) is smaller for diRL than for monoRL (2.47 vs 3.09, 2.35 vs 2.86, 2.26 vs 3.60, and 2.01 vs 4.09 mol/m2 for decane, dodecane, tetradecane and hexadecane, respectively). In addition, more significant impact of surface access change on the size of the aggregates were observed for diRL, indicating that the difference in the structure of polar group between diRL and monoRL (one more rhamnose ring in diRL polar group) has influence on alkane-solubilization behavior. Due to the smaller access at the surface of the aggregates, diRL can produce larger number of aggregates than monoRL in the solution and thus show higher solubilization efficiency of dodecane, tetradecane and hexadecane. The findings of the study add to the knowledge of solubilization of hydrophobic organic compounds by surfactants and contribute to application of rhamnolipid biosurfactant in the area of remediation.