Thermal and phase behavior of didodecyldimethylammonium bromide aqueous dispersions

Abstract

Abstract This work aims to delineate the single isotropic vesicle phase (Ves) in the binary didodecyldimethylammonium bromide (DDAB)/water system, limited by the critical vesicle concentrations, CVC 1 ≈ 0.05 mM (2.3 × 10 −3 wt%) and CVC 2 ≈ 0.7 mM (3.1 × 10 −2 wt%), as the onset of unilamellar vesicle formation and of two-phase separation into Ves + Lam (lamellar) phases, respectively. Isothermal titration calorimetry (ITC), turbidity, and crossed polaroids observations indicate that below CVC 1 the dispersion is dominated by free monomers or micelles, but rich in uni- and multilamellar vesicles between CVC 1 and CVC 2 , with CVC 2 = 0.21 mM (9.5 × 10 −3 wt%) being the onset of multilamellar vesicle formation. Above CVC 2 , the volume of the Lam phase increases, while the volume for the Ves phase decreases to vanish around CVC 3 ≈ 21 mM (0.95 wt%). Differential scanning calorimetry (DSC) data show that the gel-to-liquid crystalline transition at T m ≈ 16 °C is highly cooperative (Δ T 1/2 ≈ 0.3 °C), and the melting enthalpy (Δ H m ) increases with DDAB concentration. Because of the remarkably slow liquid crystalline-to-gel kinetics, a cooling transition around T m ′ ≈ 9.5°C is reported here for the first time, we ascribe to the liquid crystalline-to-gel transition (thermal hysteresis Δ T m ≈ 6.5 °C). Vesicle and lamellar structure formation is supported also by hydrodynamic diameter ( D H ) data, being 100–140 and 800–1200 nm, respectively, at 25 °C. The zeta potential ( ζ ) increases with DDAB concentration but does not change much with temperature, indicating no pronounced structural change when temperature varies around T m .