Phase behavior and properties of salt-free cationic/anionic surfactant mixtures of oleic acid and stearic acid

Cationic base surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), can be obtained through anion exchange from tetradecyltrimethylammonium bromide (TTABr). Salt-free cationic and anionic (catanionic) surfactant mixtures were studied by mixing TTAOH with oleic acid (OA) or stearic acid (SA) in water. The phase behavior of TTAOH/OA/H₂O is compared with that of TTAOH/SA/H₂O. It was found that the phase behavior of TTAOH/OA/H₂O and TTAOH/SA/H₂O system differs from each other due to the existence of the unsaturated double carbon bond (C=C) in OA. At fixed total surfactant concentration (25 mg/mL) of TTAOH/OA/H₂O system at 25°C, one can observe an isotropic L₁ phase, and a L₁/L_α two-phase region with increasing OA content. The volume of top turbid L_α phase increases while the bottom phase changes gradually from transparently clear to a bit turbid until a single L_α-phase is reached. Finally at high OA concentration, excess OA is separated from the bulk aqueous solutions. TTAOH/SA/H₂O system usually forms white precipitating at 25°C due to the high chain melting temperature of SA. When heated to 60°C, however, the state of samples changes. At fixed total surfactant concentration of 25 mg/mL, an isotropic L₁ phase and a milk-white or bluish L_α-phase are observed with increasing SA concentration. Transparent thin layers which are strongly birefringent form at the tops of some samples within the L_α-phase region. Finally, at high SA concentration, excess SA is separated from the bulk aqueous solutions. In addition to phase behavior study, we also measured the conductivity of TTAOH/OA/H₂O system at 25°C and TTAOH/SA/H₂O system at 60°C, respectively. Surface tension and rheological measurements were also performed on typical samples.