Optimization and Stability of Geraniol NanoemulsionStabilized Synergistically Using Mixed Biosurfactant System of Saponinand Lecithin

The utilization of biosurfactants for optimization and formulation of essential oil (EO) nanoemulsions (NEs) makes them ideal green nanocarriers for diverse agri-food applications. Geraniol oil-in-water (o/w) NEs were formulated and stabilized through ultrasonication, employing a synergistic combination of saponin and soy-lecithin emulsifiers. The investigation also assessed the stability of these NEs under various environmental stressors including storage conditions, salt addition, and pH variations. The results indicate that, for lecithin-stabilized NEs, the formulation with an oil-to-surfactant ratio (OSR) of 2:1 exhibited the smallest size, with hydrodynamic radii (<i>R</i>_H), polydispersity index (PDI), and zeta potential values of 72.8 nm, 13.5%, and −56.9 mV, respectively. In the case of the binary biosurfactant mixture of lecithin and saponin, one finds synergistic interaction on NE formation, and smaller droplets with <i>R</i>_H below 50 nm and a PDI of less than 20% are observed. Evaluation of droplet growth rates over one month for the r(1:1) mixture indicated that Ostwald ripening predominantly contributed to an initial increase in droplet size during the first 2 weeks, followed by coalescence, leading to destabilization of the formulation. Moreover, NEs with lecithin-to-saponin ratios of r(3:1) and r(1:1) demonstrated stability against phase separation for salt concentrations up to 300 mM and within a pH range of 4–10. These findings provide valuable insights into the use of binary mixtures of biosurfactants, demonstrating their synergistic effects and contributing to the development of long-term stable green NEs.