Coacervation and aggregate transitions of a cationic ammonium gemini surfactant with sodium benzoate in aqueous solution

Abstract

Coacervation in an aqueous solution of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (C₁₂C₆C₁₂Br₂) with sodium benzoate (NaBz) has been investigated at 25 °C by turbidity titration, light microscopy, dynamic light scattering, cryogenic temperature transmission electron microscopy (Cryo-TEM), scanning electron microscopy (SEM), isothermal titration calorimetry, ζ potential and ¹H NMR measurements. There is a critical NaBz concentration of 0.10 M, only above which coacervation can take place. However, if the NaBz concentration is too large, coacervation also becomes difficult. Coacervation takes place at a very low concentration of C₁₂C₆C₁₂Br₂ and exists in a very wide concentration region of C₁₂C₆C₁₂Br₂. The phase behavior in the NaBz concentration from 0.15 to 0.50 M includes spherical micelles, threadlike micelles, coacervation, and precipitation. With increasing NaBz concentration, the phase boundaries of coacervation shift to higher C₁₂C₆C₁₂Br₂ concentration. Moreover, the C₁₂C₆C₁₂Br₂–NaBz aggregates in the coacervate are found to be close to charge neutralized. The Cryo-TEM and SEM images of the coacervate shows a layer–layer stacking structure consisting of a three-dimensional network formed by the assembly of threadlike micelles. Long, dense and almost uncharged threadlike micelles are the precursors of coacervation in the system.