Influence of Ionic Specificity on the Stability and Properties of Heptol-in-Water Emulsions

Emulsions are colloidal systems composed of two immiscible liquid phases whose stability depends on surfactants adsorbed at the interface. In electrolyte-containing systems, the droplet size and ion-specific effects are key determinants of dispersion stability. Here, oil-in-water model emulsions representative of petroleum systems were prepared using oily phases composed of heptane, toluene, and their mixtures (heptol) at volume to investigate the influence of ionic specificity on the emulsion stability and physicochemical properties. Aqueous phases containing NaCl, NaBr, Na2SO4, KCl, CaCl2, and MgCl2 at ionic strengths equivalent to seawater were combined with a nonionic surfactant (Span 80). Macroscopic stability, viscosity, density, zeta potential, and droplet size evolution were monitored over two months. Electrolyte addition increased the viscosity and aqueous phase density, enhancing macroscopic stability, particularly in the presence of multivalent salts. The time-averaged droplet radius (rGti) increased with ionic radius in both sodium and chloride series, following rGt_CaCl2 > rGt_MgCl2 > rGt_KCl > rGt_NaCl and rGt_Na2SO4 > rGt_NaBr > rGt_NaCl, while viscosity was inversely related to droplet growth. The observed stability trends deviate from protein-based Hofmeister ordering, reflecting system-dependent ion-specific interfacial effects in petroleum emulsions. These results provide quantitative guidance for controlling emulsion stability in crude oil processing and enhanced oil recovery