Rheology, characteristics, stability, and pH-responsiveness of biosurfactant-stabilized crude oil/water nanoemulsions

• Stable crude oil/water nanoemulsions stabilized by biosurfactant are formulated. • The formulated nanoemulsions possess favorable characteristics for EOR applications. • At low shear rates, the emulsions showed shearing-thinning behavior. • At high shear rates, the emulsions showed shearing-thickening behavior, • Crude oil is completely recovered from the emulsion in < 1 h upon HCl addition. The successful utilization of nanoemulsions in several applications would require the formulation of emulsions with excellent characteristics. Ideally, the nanoemulsions should be stabilized using bioemulsifiers, which do not negatively impact the environment throughout their cradle-to-grave lifetime. Thus, crude oil-in-water (O/W) nanoemulsions with exceptional properties were prepared in this study using rhamnolipid biosurfactant as a bioemulsifier. The obtained results reveal that rhamnolipid can produce O/W nanoemulsions with an average droplet size as low as 35.0 ± 6.6 nm. The nanoemulsions also have highly negative zeta potential, low interfacial tension, and long-term kinetic stability. Interestingly, almost all the formulated O/W nanoemulsions using different rhamnolipid dosages and oil/water ratios showed three different flow behaviors (i.e., shear-thinning, Newtonian, and shear-thickening at low, medium, and high shear rates, respectively). Additionally, the nanoemulsion formulated using 50/50 crude oil/water volumetric ratio displayed higher apparent viscosity than the crude oil at elevated temperatures (greater than 63 ⁰C). Furthermore, despite that all the formulated nanoemulsions were extremely stable, they can be easily, completely, and quickly (within ≤ 1 h) switched-off if needed via pH-switching. The results presented herein demonstrate the potential of biosurfactants for enhanced oil recovery (EOR) and other oilfield applications.