Divalent cation specificity at carboxylic acid interfaces: pH-driven transitions to ultralow interfacial tension

Controlling oil/water interfacial tension (IFT) through cation selection is central for emulsification and demulsification processes. While divalent cations are known to modulate interfacial properties of carboxylic acid surfactants, the molecular mechanisms underlying cation-specific effects remain poorly understood. Here, we systematically investigate how electronic structure determines interfacial behavior by comparing selected group 2 cations (Mg^{2 +}, Ca²⁺, Sr²⁺, Ba²⁺) with s outer electronic configuration, which are abundant in natural waters, versus Zn²⁺, a transition metal with d -orbital involvement. Using IFT measurements combined with Frumkin–Davies adsorption modeling, we demonstrate that those group 2 cations lower IFT primarily through electrostatic screening, with minimal perturbation of interfacial film organization. In contrast, Zn²⁺ exhibits dramatically enhanced IFT reduction through specific carboxylate complexation. Critically, pH adjustment enables the selected group 2 cations to access interfacial complexation, driving the system from stable interfaces at low pH to ultralow IFT and spontaneous emulsification at high pH values. These results establish that electronic structure, not ionic strength alone, determines cation-specific interfacial behavior, with profound implications for interfacial engineering and emulsification control in applied environments.