Water-in-oil or oil-in-water emulsions, typically stabilized by indigenous surface-active components, are ubiquitous in many natural and industrial settings. In petroleum engineering, these emulsions are highly undesirable owing to their adverse effects on water–oil separation, transportation, quality of the oil products, etc. The application of an external electric field has demonstrated its potential as a promising method for demulsification, especially considering its advantages of energy efficiency and environmental compatibility. The demulsification ability of an electric field arises from the desorption of surface-active species (e.g., asphaltenes in petroleum) from the water–oil interface, which reduces the rigidity of the interfacial film and facilitates the coalescence of the emulsion droplets. The natural presence of salt ions in the water phase, however, can impact electric field induced demulsification process, and this potential influence is studied in this work by molecular dynamics simulations. We report an interesting phenomenon where the presence of NaCl in the water phase can hinder the ability of an electric field to desorb asphaltenes from a brine–oil interface. In particular, NaCl ions exhibit a higher probability of crystallization in bulk brine as the electric field increases, and this crystallization releases free water to the brine–oil interface which hampers the desorption of a model asphaltene. In contrast, CaCl2 lacks the tendency to crystallize, and as the electric field strengthens a continued increase is observed in the desorption of the model asphaltene. Therefore, demulsification by electric field is potentially more effective for brine–oil systems containing CaCl2 than those containing NaCl. This work highlights the interplay between salt ions and electric field, and how it influences interfacial behaviors of surface-active molecules. The findings could benefit the design and optimization of demulsification technologies in industries such as wastewater management, petroleum processing and other chemical treatments. © 2024 The Author(s)
