The role of monovalent ions on water-in-crude oil emulsion stability

The formation of stable water-in-crude oil (W/O) emulsions presents significant operational challenges in the petroleum industry. The role of monovalent ions in stabilizing these emulsions, which are primarily stabilized by asphaltenes and resins, remains poorly understood. This study systematically investigates the effect of monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺) and anions (Cl⁻, Br⁻, I⁻) on the stability and rheological properties of W/O emulsions prepared with a crude oil containing a high concentration of natural surfactants. Our results demonstrate a clear ion-specific effect, consistent with the Hofmeister series. Specifically, the smallest cation, Li⁺, and the largest anion, I⁻, were the most effective at destabilizing the emulsions, leading to rapid phase separation. Rheological measurements further revealed a contrasting effect: LiCl systematically reduced emulsion viscosity, while NaCl and CsCl increased it. Because microscopic analysis showed that droplet size remained largely unchanged, the macroscopic destabilization is attributed primarily to molecular influences. Based on these observations, we propose a molecular hypothesis where highly hydrated Li⁺ ions disrupt the interfacial hydrogen-bonding network of the asphaltene film, while less-hydrated Na⁺ and Cs⁺ ions promote structural reinforcement through cationic clustering. While direct spectroscopic validation remains a subject for future work, these findings provide fundamental insights into monovalent ion specificity, offering a rational basis for optimizing ’smart water’ formulations and demulsification processes.