(634a) Augmenting the Stability and Properties of Chemical Surfactant-Stabilized Nanoemulsions with Trace Rhamnolipid Biosurfactant Addition

The formulation and stabilization of nanoemulsions have gained significant interest due to their extensive applications across various industries, including food, cosmetics, pharmaceuticals, and oil recovery. However, the traditional reliance on high concentrations of synthetic surfactants raises environmental and economic concerns. In contrast, bio-based surfactants offer a green alternative, yet their application in forming stable nanoemulsions at low concentrations remains underexplored. Rhamnolipids, a class of biosurfactants, have shown promise in forming and stabilizing nanoemulsions at low concentrations, presenting an eco-friendly alternative to synthetic surfactants. Previous research has highlighted the potential of rhamnolipids in various sectors, including enhanced oil recovery and environmental remediation. However, studies focusing on the combined use of rhamnolipids with synthetic surfactants like Tween 40 in nanoemulsions have not been investigated yet. This research explores the synergistic effects of bio-based rhamnolipids and Tween 40 surfactants in stabilizing diesel-in-water nanoemulsions. By exploring a mixed surfactants system, the study seeks to enhance the stability and performance of nanoemulsions, offering a sustainable approach to their formulation. The investigation encompasses comprehensive characterization and rheological assessments to understand the impact of surfactant synergy on emulsion properties.

The concentration of Tween 40 was varied at 0.02, 0.1, 0.5, and 1 wt%, while the dose of rhamnolipids was kept minimal at 0.004 wt%, identified as the minimum concentration that could not form stable emulsions. These strategic formulations resulted in stable nanoemulsions with excellent characteristics, outperforming those prepared with Tween 40 alone. The prepared nanoemulsions were systematically compared to those formulated using individual emulsifier in terms of stability, droplet size, zeta potential, interfacial tension, morphology, and rheological properties. The inclusion of rhamnolipids not only improved emulsion stability and increased zeta potential but also significantly reduced the dynamic interfacial tension, showcasing the enhanced performance of the mixed surfactant system. The rheological assessment demonstrated the synergy between Tween 40 and rhamnolipid biosurfactants, significantly altering the rheological properties of the nanoemulsions. The inclusion of rhamnolipids accentuated the shear-thinning behavior and increased both the storage (G') and loss (G") moduli, suggesting an improved viscoelastic nature. This indicates stronger structural integrity and a dynamic response to oscillatory shear, essential for enhancing emulsion performance across various industrial applications.

Additionally, the nanoemulsions showed decreased viscosity with rising temperatures. This temperature-sensitive behavior highlights the crucial role of precise surfactant selection and concentration in crafting nanoemulsions with optimal flow characteristics. Rhamnolipidss' presence not only lifted the viscosity but also bolstered the thermal stability of the emulsions, showcasing the utility of such biosurfactants in creating resilient nanoemulsions suitable for diverse operational environments. Employing rheological models like Ostwald-de Waele, Bingham, and Herschel-Bulkley provided a thorough insight into the nanoemulsions' behavior under varied conditions.

Our investigation into the synergistic effects of combining bio-based rhamnolipids with the synthetic surfactant Tween 40 has yielded pioneering findings in the stabilization of diesel-in-water nanoemulsions. This study successfully demonstrates that even a minimal total surfactant concentration of just 0.004 wt%—comprising 0.02 wt% each of Tween 40 and rhamnolipids —can achieve superior emulsion stability compared to formulations using individual surfactants.