Influence of pH and Salinity on the Interfacial Properties of 5β-Cholanic Acid and Its Importance in Emulsion Formation

The interfacial behavior of 5β-cholanic acid (CA), a model compound for naphthenic acids, was investigated as a function of pH and salinity to elucidate its role in emulsion formation and stability. Interfacial tension and dilatational interfacial rheology measurements were performed using a pendant drop tensiometer. A kinetic-thermodynamic model incorporating adsorption, diffusion, and concentration-dependent transport was developed to interpret the dynamic interfacial tension data. The diffusion coefficient in the organic phase was found to decrease with increasing CA concentration, and an interfacial pKa of 7.3 was estimated─substantially higher than the reported bulk value─indicating significant interfacial effects on acid–base behavior. It was observed that both NaCl and CaCl2 reduced interfacial tension by promoting CA adsorption, but solid interfacial films were only formed under specific conditions. Calcium ions facilitated interfacial film formation at neutral and basic pH, whereas sodium ions induced film formation only under basic conditions. These solid films were associated with the immediate formation of stable water-in-oil emulsions, which exhibited gel-like properties as confirmed by rheological analysis. Emulsion formation was not directly correlated with interfacial tension or rheological moduli alone, suggesting that interfacial structuring, ion-specific interactions, and pH-driven dissociation play dominant roles.