Coacervation of Cationic Gemini Surfactant with <i>N</i>‑Benzoylglutamic Acid in Aqueous Solution

Abstract

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis­(dodecyldimethylammonium bromide) (12–6–12) with pH-sensitive <i>N</i>-benzoylglutamic acid (H₂Bzglu) has been investigated by potentiometric pH-titration, turbidity titration, dynamic light scattering (DLS), isothermal titration calorimetry (ITC), TEM, ¹H 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