Effect of molecular structure of catanionic surfactant mixtures on their interfacial properties

Abstract Catanionic surfactant mixtures have shown advantages in many applicable aspects for strong synergetic behavior. In the present work, the adsorption behaviors of catanionic mixtures at oil-aqueous interface have been studied by means of interfacial tension measurements and interfacial dilational rheological experiments. Because of the electrostatic attraction, catanionic mixtures are tightly packed: at low concentration, catanionic surfactant mixtures pack as co-surfactant like Gemini, while at high concentration (≥CMC), catanionic mixtures generate closely-packed network adsorption layers at oil/aqueous interfaces, which are strongly viscoelastic. The interfacial properties indicate the ratio of anionic and cationic surfactant at interface is independent with the ratio in the solution but depends on the molecular structure of surfactants. The interaction between anionic and cationic follows the order: SDS-C12TAB > SDDS-C12TAB > DAS-C12TAB. Therefore the adsorption layer of SDS and C12TAB packs more closely at the interface and possesses higher elasticity. As to the hydrophobic groups, the increase of alkyl chain length enhances the hydrophobic force, but makes the molecular motion, orientation and rearrangement more difficult. These results provide basis for the design of catanionic mixtures to regulate and control the interfacial properties to adapt to the different applications.