Synergy evaluation between diverse biosurfactants toward the formulation of green oil-in-water nanoemulsions by ultrasonication method

The current study examines the structural compatibility of various combinations of structurally diverse biosurfactants (saponin, a biosurfactant derived from plants, and rhamnolipid and surfactin, biosurfactants derived from microorganisms) in the formulation of stable oil-in-water (O/W) nanoemulsion using the high-energy ultrasonication technique. The oil droplet size analysis revealed that mixed biosurfactants-based nanoemulsion systems at specific compositions displayed comparable stability with single-biosurfactant systems. The rhamnolipid-surfactin (R–S) system formed a stable nanoemulsion at all tested compositions, exhibiting maximal stability at 75:25 (% mol/mol) with a hydrodynamic diameter (HDD) of 168.9 nm, polydispersity index (PDI) of 0.166, and zeta potential value of −77.43 ± 2.11 mV. On the other hand, the other two combinations, rhamnolipid-saponin (R–SP) and surfactin-saponin (S-SP), showed poor kinetic stability. The S-SP exhibited stability only at 75:25 among the tested combinations with HDD, PDI, and zeta potential of 155.7 nm, 0.178, and −49.07 mV, respectively. The plant-derived saponin, whose structure is entirely different from that of microbial-derived surfactin and rhamnolipid biosurfactant, played a dominant role in determining the stability of surfactin-saponin and rhamnolipid-saponin nanoemulsion systems. Also, antimicrobial and scavenging investigations of stable nanoemulsions revealed that two-biosurfactant systems showed comparable efficacies to single biosurfactant nanoemulsions. The results of the current study strongly imply that the optimal synergy between mixed-biosurfactants at the oil-water interface leading to stable nanoemulsion is primarily dictated by the type and composition of biosurfactants used in the formulation.