Interfacial Shear Rheology of Oil–Water Interfaces: Investigating Effects of Aging, Salt Type, and Concentration

Understanding the rheological behavior of oil–water interfaces is critical for optimizing processes, such as emulsion stabilization, enhanced oil recovery, and oil–water separation. Using interfacial rheology measurements, we probe the mechanical response of crude oil–water interfaces under various conditions, including aging time, type of electrolyte, ionic strength, and the presence of nanoparticles (NPs) that form Pickering emulsions. Specifically, we examine how the storage (G′) and loss modulus (G″) evolve with time under different conditions. We find that ionic strength plays a dominant role in modulating interfacial viscoelasticity with low ionic strength leading to faster increases in moduli and stronger interfacial structures, regardless of the specific ions present. Contrary to expectations, divalent ions, including Ca2+ and SO42–, did not significantly enhance viscoelasticity compared to monovalent ones. Furthermore, a high ionic strength suppresses the viscoelastic response, possibly due to screening and compression of the electrical double layer. Nanoparticle assembly at the oil–water interface dramatically enhances interfacial viscoelasticity, with the effect being more pronounced when the particles are suspended in brine containing a mixture of monovalent and divalent ions, suggesting that nanoparticles reduce electrostatic repulsion and facilitate stronger interfacial networks. These insights provide a new understanding of interfacial dynamics in crude oil–water systems, offering potential improvements for practical applications such as emulsion control and enhanced oil recovery.