Effect of oil/water ratio on rheological behavior, droplet size, zeta potential, long-term stability, and acid-induced demulsification of crude oil/water nanoemulsions

Abstract

Nanoemulsions are interesting nano-fluids with a number of potential industrial applications. However, the effective utilization of these colloidal systems will not be achieved without studying their characteristics (e.g., droplet size, interfacial tension, zeta potential), flow behavior (e.g., rheology), and long-term stability. In this work, crude oil-in-water (O/W) nanoemulsions have been formulated and stabilized by sodium dodecylbenzenesulfonate (SDBS). The volume of the dispersed crude oil phase has been varied in order to probe the oil content effect on the nanoemulsion properties and stability. The obtained results revealed that the interfacial tension of the nanoemulsions decreases significantly as the dispersed phase volume fraction decreases. Similarly, average droplet size and zeta potential also decrease with decreasing the crude oil content, reaching 37 nm and −42 mV, respectively, at 5 vol% crude oil. Unprecedented and interesting flow behavior consisting of 3 regions (shear-thinning/Newtonian/shear-thickening) has been observed for the nanoemulsions with a crude oil content ≤20 vol%. All the formulated nanoemulsions displayed a long-term stability with 0% demulsification. Despite the extreme stability and the pH-irresponsive nature of SDBS, a complete oil separation might be achieved via the addition of a small quantity of dilute HCl. The demulsification extent, however, decreases with decreasing the crude oil content. • Extremely stable O/W nanoemulsions containing different oil 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.