Effect of salinity on the characteristics, pH-triggered demulsification and rheology of crude oil/water nanoemulsions

• Extremely stable O/W nanoemulsions with different salt contents are formulated. • The nanoemulsions displayed unusual rheological behavior and pH-responsive character. • The formulated nanoemulsions possess practical attractive characteristics. • Emulsions displayed shearing-thinning, Newtonian, and shearing-thickening behaviors. • Crude oil can be completely recovered from the nanoemulsions within 1 h. The interest in utilizing nanoemulsions in a number of industrial applications is growing rapidly because of their superiority over conventional emulsions. Therefore, the formulation of highly stable crude oil-in-water nanoemulsions with different salinity (i.e., NaCl) levels is preseented in this study. Despite the observed extreme emulsion stability, zeta potential of the nanoemulsions decreased with increasing the salt concentration as a result of the charge screening effect induced by NaCl addition. Charge screening effect and, accordingly, the weakening of the electrostatic repulsion range and strength resulted in an increase in the average droplet size and also in a wider size distribution as the salinity level in the nanoemulsions increased. Additionally, the nanoemulsion viscosity also increased with increasing the salt concentration, however, the increase was marginal (except at 20 g/L NaCl). One interesting and unprecedented observation reported herein is the change in the nanoemulsion flow behavior from shear-thinning to Newtonian and then to shear-thickening as the applied shear rate increased. Nonetheless, at a salt concentration of 20 g/L NaCl, the shear-thickening behavior disappeared. Another important finding is that the presence of NaCl made the on-purpose destabilization of these stable nanoemulsions through the addition of NaOH or HCl (i.e., pH-alteration induced demulsification) more effective.