Effect of Oleic Acid on the Dynamic Interfacial Tensions of Surfactant Solutions

The dynamic interfacial tensions (IFTs) of three different types of surfactant solutions, cationic surfactant hexadecyl trimethylammonium (CTAB), anionic surfactant sodium dodecyl benzenesulfonate (SDBS), and non-ionic surfactant Triton X-100 (TX-100), against model oil containing oleic acid (OA) have been investigated by a spinning drop interfacial tensiometer. The influences of the OA concentration and addition of alkali have been studied. The experimental results show that OA added to the oil phase has a significant effect on interfacial behaviors of surfactant solutions. An obvious synergistic effect on reducing IFT can be observed for low-concentration cationic surfactant CTAB solution because the tight mixed adsorption films will be formed through electrostatic attraction between CTAB and OA molecules. However, the synergistic effect will disappear at a high surfactant concentration by the formation of mixed micelles. For the anionic surfactant SDBS system, the mixed adsorption of SDBS and OA molecules leads to a limited decrease of equilibrium IFT as a result of electrostatic repulsion. The hydrophilic group EO chain of TX-100 extends into the aqueous phase and the interfacial film becomes tight with an increasing bulk concentration. As a result, OA shows little effect on the equilibrium IFTs at a high TX-100 concentration. The alkali in the aqueous phase will react with OA in the oil and produce anionic surfactant sodium oleate (SO), which destroys the synergistic effect between CTAB and OA molecules. With an increase of the OA concentration, the dynamic characteristic of IFT has been enhanced and the transient IFTs decrease obviously because more SO molecules have been produced.