Role of Salinity and Ionic Composition on the Kinetic Stability of Water-in-Oil Model Emulsions

The formation of water-in-oil (W/O) emulsions represents a significant economic and operational challenge in the petroleum industry. In this study, the effects of aqueous-phase salinity on the kinetic stability of W/O model emulsions were investigated using a medium-density lubricating oil, the nonionic surfactant Triton X-114, and aqueous phases with varying ionic compositions, including deionized water, tap water, produced water, and saline solutions ranging from 35 to 500 g L^-1, with suspended solids present at concentrations above the saturation limit. The results revealed that stability is nonlinear with respect to salinity. Emulsions with intermediate ionic strength exhibited enhanced performance. In particular, the emulsion prepared with produced water had the smallest droplets and maintained kinetic stability for up to 24 h under gravitational conditions. The emulsion prepared with 35 g L^-1 NaCl also exhibited a significant reduction in droplet size. This behavior is attributed to salting-in effects and ion-dipole interactions that promote denser interfacial packing. In contrast, highly saline systems (saturated solution and saline aqueous suspension), despite exhibiting lower interfacial tension, showed increased flocculation and phase separation because of surfactant dehydration and interfacial film weakening. Thus, it is possible to conclude that an optimal ionic strength range exists that favors interfacial structuring and that the use of model fluids is a viable and safe alternative for simulating complex petroleum interfacial phenomena at the laboratory scale.