Internal-salt-regulated stability of brine-in-oil emulsions: interfacial mechanisms and bulk-property effects

Water-in-oil (W/O) emulsions are widely encountered in petroleum production, separation processes, and functional emulsion formulations. In practical systems, the dispersed aqueous phase commonly contains dissolved salts, whereas their influence on W/O emulsion stability is still not fully understood. This review therefore summarizes the effects of internal salts on W/O emulsion stability from a mechanism-oriented perspective. Internal salts can interact with interfacially active species and thereby regulate their interfacial adsorption and organization, which is reflected in changes in interfacial tension and interfacial viscoelasticity. They can also modify electrostatic interactions between droplets, altering attractive or repulsive forces during droplet approach and coalescence. In addition, salt-induced changes in density, emulsion viscosity, and solubility can affect gravitational sedimentation, droplet migration, and collision kinetics, while limited solubility may cause crystallization and disturb emulsion homogeneity. Finally, current challenges and future perspectives are discussed, including coupled characterization, realistic-condition testing, and predictive formulation design. This review provides a framework for linking salt composition, interfacial mechanisms, bulk-property effects, and W/O emulsion stability.