(418h) Interaction and Interfacial Dynamics of Oil-Soluble Fatty Acid and Water-Soluble Cationic Surfactant at the Oil/Water Interface

Interfacial tension synergism, the condition whereby a particular degree of interfacial tension reduction is achieved using a lower total concentration of a surfactant mixture than is required for either individual surfactant acting alone, is commonly investigated for mixtures of water-soluble components. This investigation considers systems where an oil-soluble surfactant and a water-soluble surfactant access the interface from opposite sides. Using an oil-soluble fatty acid that may ionize at the oil/water interface and a quaternary ammonium water-soluble cationic surfactant with a pH-independent charge, the propensity to interfacial tension synergism was tuned via the aqueous phase pH. Pendant drop interfacial tension and dilatational rheology studies were conducted to determine the relationship between interfacial tension synergism conditions and interfacial dynamics that control the storage and loss components of the interfacial dilatational modulus.

The aqueous pH controls the degree of ionization of the fatty acid at the interface. Ionization of the carboxyl group at the interface significantly enhances the interfacial activity and alters the relaxation properties of the oil-soluble fatty acid in the absence of the water-soluble surfactant. Fatty acid ionization also strengthens its interactions with the water-soluble surfactant when it is present. In this work, palmitic acid (PA) and tetradecyltrimethylammonium bromide (TTAB) were used as the oil-soluble fatty acid surfactant and the water-soluble cationic surfactant respectively. PA was dissolved in tetradecane, while TTAB was dissolved in phosphate buffer at either pH 3 or pH 7. Interfacial tension isotherms and dilatational rheological behaviors were measured for the following interfacial conditions: (1) PA in tetradecane vs. aqueous buffer, (2) tetradecane vs. TTAB in aqueous buffer and (3) PA in tetradecane vs. TTAB in aqueous buffer. Dilatational moduli were measured at selected interfacial tensions via the oscillating drop method at frequencies between 0.05 and 1 Hz.

The pH value had a controlling effect on the systems involving PA, both in the presence and absence of TTAB, but it had no significant effect on the interfacial behavior of TTAB in the absence of PA. At pH 7, PA exhibited greater interfacial tension reduction acting alone than it did at pH 3, and the PA/TTAB system showed a strong interfacial tension synergism across a range of interfacial tensions. In contrast, the PA/TTAB showed no interfacial tension synergism at any interfacial tension. While the PA and TTAB alone systems exhibited no or small moduli at both pH values, the PA/TTAB system exhibited a significantly higher storage modulus at pH 7 than at pH 3, indicating that the electrostatic complexation of ionized PA and TTAB at the oil/water interface inhibited surfactant desorption from the interface.