Thermal and phase behavior of didodecyldimethylammonium bromide aqueous dispersions

• On increasing DDAB concentration, the following phase sequence appears: monomer, vesicle, lamellae.
• DDAB unilamellar vesicles are formed in water only in a very narrow concentration range.
• DDAB multilamellar vesicles co-exist with unilamellar vesicles in this concentration range.
• Phase separation occurs in DDAB/water, with lamellae on top and vesicles on bottom.
• The reverse melting temperature of DDAB/water is around 9.5 °C.

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, CVC1 ≈ 0.05 mM (2.3 × 10−3 wt%) and CVC2 ≈ 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 CVC1 the dispersion is dominated by free monomers or micelles, but rich in uni- and multilamellar vesicles between CVC1 and CVC2, with CVC2 = 0.21 mM (9.5 × 10−3 wt%) being the onset of multilamellar vesicle formation. Above CVC2, the volume of the Lam phase increases, while the volume for the Ves phase decreases to vanish around CVC3 ≈ 21 mM (0.95 wt%). Differential scanning calorimetry (DSC) data show that the gel-to-liquid crystalline transition at Tm ≈ 16 °C is highly cooperative (ΔT1/2 ≈ 0.3 °C), and the melting enthalpy (ΔHm) increases with DDAB concentration. Because of the remarkably slow liquid crystalline-to-gel kinetics, a cooling transition around Tm′ ≈ 9.5°C is reported here for the first time, we ascribe to the liquid crystalline-to-gel transition (thermal hysteresis ΔTm ≈ 6.5 °C). Vesicle and lamellar structure formation is supported also by hydrodynamic diameter (DH) 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 Tm.

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