Mechanistic insights into the interfacial activity of saturated fatty acids in oil–brine systems: Effects of ionic environment and pH variations

• Fatty acids reduce IFT by 23–39% compared to pure n-heptane–brine • IFT decreases with shorter-chain acids (e.g., Myristic) down to 19.5 mN/m at pH 10 • IFT of myristic acid for NaCl decreases from 32.96 at pH 4 to 21.83 mN/m at pH 10 • Divalent cations increase IFT via salting-out effects (e.g. Ca²⁺:31.93 vs Na + :30.55) • SO₄²⁻ with higher hydration number (∼3.1) and energy (–1080 kJ/mol) gives lowest IFT Understanding how naturally occurring surface-active components such as fatty acids behave at oil–brine interfaces is vital for improving the design of water-based formulations used in wastewater treatment, emulsification, and enhanced oil recovery. Despite their importance, the combined effects of molecular structure, pH, ion type, and partitioning of fatty acids on interfacial tension (IFT) remain insufficiently understood. This study provides mechanistic insight into how these parameters jointly control the IFT between n -heptane solutions of saturated fatty acids and brines. Myristic, palmitic, and stearic acids were selected to represent varying hydrocarbon chain lengths, and their IFTs were measured using the pendant drop method across different pH and salt compositions. The results indicate that higher pH generally promotes acid ionization, lowering IFT, whereas divalent cations such as Ca²⁺ and Mg²⁺ increase IFT due to salting-out and electrostatic screening effects. Sulfate anions enhance acid retention and interfacial adsorption through hydrogen bonding and acid–base interactions. Shorter-chain acids, particularly myristic acid, consistently display stronger interfacial activity because of faster diffusion and lower self-association at the interface. Overall, these findings highlight the key factors that govern IFT, offering guidance for designing brine–oil systems in enhanced oil recovery, emulsion control, and wastewater treatment applications.