Influences of temperature, pH and salinity on the surface property and self-assembly of 1:1 salt-free catanionic surfactant

The 1:1 salt-free catanionic surfactant dodecyltrimethylammonium decanoate (DTAD) was synthesized directly by a novel method, and the product was recrystallized to come up to purity received. Surface activity and self-assembly of 1:1 salt-free catanionic surfactant DTAD and the influences of temperature, pH and salinity on which have been studied in the present work. The critical micelle concentration (CMC), surface tension reduction (γ CMC ), surface excess (Γ max ), and mean molecular surface area ( A min ) were determined from plots of surface tension (γ) versus logarithm of surfactant concentration. The results showed that the salt-free catanionic surfactant had much lower CMC, γ CMC and A min values than their corresponding single cationic and anionic surfactants . The Gibbs energy of mixing (△   G E ) was negative, indicating the strong interaction of cationic and anionic parts. The tested temperature and salinity had no significant effect on the surface properties of DTAD. The surface activity enhanced in acidic pH and decreased slightly in alkaline pH . The aggregate structure almost had no change with temperature, but transformed into spheres with addition of inorganic salts. The pH effect was complex, the ability of vesicles formation was improved in strong alkaline condition; vesicles transformed into large disks or large cubes in acidic condition.