Oil–Water Emulsion Behavior Under Variable Brine Salinity and pH Conditions

Abstract

Abstract. The independent effects of aqueous-phase salinity and acidity on oil–water dispersions remain insufficiently understood, as most prior investigations have emphasized mixed electrolytes containing monovalent and divalent salts to mimic crude-oil brines. As a result, the specific contribution of individual ionic species to emulsion stability has not been clearly established. The present study systematically examines the influence of sodium chloride and hydrochloric acid on the stability of emulsifier-free oil-in-water (O/W) and water-in-oil (W/O) emulsions. Emulsions were generated by mixing Exxsol™ D110 mineral oil with distilled water (DW) at 2500 rpm for 5 min. The oil-to-water density and viscosity ratios were maintained at 0.805 and 0.330, respectively. Sodium chloride concentrations in the aqueous phase were varied from 1 to 60 g L−1, while acidity was adjusted with 1.0 M HCl to achieve pH values between 4.80 and 1.88. Four dispersed-phase volume fractions (25%, 50%, 75%, and 90%) were investigated, and stability was assessed over time scales ranging from less than 1 min to approximately 4 h. Results demonstrate that elevated salinity and acidity substantially reduce the stability of O/W emulsions, particularly at higher water fractions (75% and 90%), where lifetimes decreased from several hours to only minutes. By contrast, W/O emulsions at 25% and 50% water fractions exhibited negligible sensitivity to aqueous-phase modifications, displaying consistently short stability times. These findings contribute to a more fundamental understanding of electrolyte-driven destabilization mechanisms in simple dispersions and establish a baseline for interpreting salt and acid effects in complex emulsion systems.