Salinity Effects on Anionic AEC Surfactant with n-Decane: IFT, Phase Behavior, Solubilization, Microemulsion Viscosity

n-Decane is a common model oil for isolating salinity effects in surfactant systems. In anionic AEC formulations, salinity influences IFT reduction, microemulsion topology, solubilization capacity, and flow behavior. The objectives of this study were to evaluate the effect of NaCl salinity (0–32,000 ppm) on IFT, Winsor transitions, solubilization ratio, and microemulsion viscosity in n-decane/AEC systems, and to determine working concentrations using a CMC test derived from IFT. In this study, the method used was to determine the CMC from the breakpoint of the IFT curve versus the log concentration of AEC in the reference brine. Working concentrations of 2 percent w/w. were selected above the CMC. IFT was measured with a spinning-drop tensiometer at a controlled temperature. Phase behavior was mapped through a salinity scan to identify Winsor I–III–II. The solubilization ratio was calculated from the volume of the middle phase at equilibrium composition. The viscosity of the microemulsion was characterized using a Brookfield DV3T (C40 spindle) with a stepwise shear protocol. In n-decane model systems, salinity governs interfacial properties and phase structure that, in turn, modulate rheology. CMC-by-IFT selection of working dose and tuning salinity near HLD 0 provide a reliable framework for AEC formulation design and for calibrating process parameters ahead of controlled core-flood studies.