Coacervation of Cationic Gemini Surfactant with N-Benzoylglutamic Acid in Aqueous Solution

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) with pH-sensitive N-benzoylglutamic acid (H2Bzglu) has been investigated by potentiometric pH-titration, turbidity titration, dynamic light scattering (DLS), isothermal titration calorimetry (ITC), TEM, (1)H NMR, and light microscopy. Phase boundaries of the 12-6-12/H2Bzglu mixture were obtained over the pH range from 2 to 9 and in the H2Bzglu concentration range from 30.0 to 50.0 mM at pH 4.5. When the H2Bzglu concentration is beyond 30.0 mM, the 12-6-12/H2Bzglu mixed solution undergoes the phase transitions from soluble aggregate, to precipitate, coacervate, and soluble aggregate again as pH increases. The results indicate that coacervation occurs at extremely low 12-6-12 concentration and lasts over a wide surfactant range, and can be enhanced or suppressed by changing pH, 12-6-12/H2Bzglu molar ratio and H2Bzglu concentration. The coacervates present a disorderly connected lay structure. Coacervation only takes place at pH 4-5, where the aggregates are nearly charge neutralized, and a minimum H2Bzglu concentration of 30.0 mM is required for coacervation. In this pH range, H2Bzglu mainly exist as HBzglu(-). The investigations on intermolecular interactions indicate that the aggregation of 12-6-12 is greatly promoted by the strong electrostatic and hydrophobic interactions with the HBzglu(-) molecules, and the interaction also promotes the formation of dimers, trimers, and tetramers of HBzglu(-) through hydrogen bonds. The double chains of 12-6-12 and the HBzglu(-) oligomers can play the bridging roles connecting aggregates. These factors endow the mixed system with a very high efficiency in generating coacervation.