Physical science of the didodecyldimethylammonium bromide-water system: 1. Equilibrium phase behaviour

Surfactant molecules in solvents self-assemble into a large variety of structures depending on their chemical composition, concentration and temperature, summarized in the system's equilibrium phase diagram. However, the occurrence of long-lived metastable states can lead to incomplete or partly incorrect phase diagrams. By applying a set of complementary techniques and recording changes on different length scales, we determine an improved aqueous equilibrium phase diagram of the widely used double-chain surfactant didodecyldimethylammonium bromide (DDAB) over a broad concentration range (ϕ_DDAB = 3-100 wt%). We reveal that DDAB molecules exist as zero-hydrates in the room temperature solid state and decompose above 90 °C: the upper temperature of the phase diagram. Differential scanning calorimetry was used to characterise the transition's heat energy, kinetics and temperature, while the structure of the phases was characterized by small angle X-ray scattering and microscopy. Raman spectroscopy combined with computational techniques provided information regarding the conformational properties of the surfactant molecules. Our results were in good agreement with the literature phase diagram for moderate temperatures and surfactant concentrations. At 16 °C, a transition from a frozen lamellar phase (L_β) to a fluid lamellar phase L_α has previously been suggested across all concentrations (Dubois et al. 1998), with T_m coinciding with the Krafft temperature (T_c) determined in dilute systems. Here, we characterize for the first time the low temperature equilibrium phase for ϕ_DDAB > 3 wt% as a crystalline dispersion, and determine the position and shape of the Krafft eutectic. The equilibrium phase below 14.1 °C is now assigned to a coexistence region of surfactant hydrate crystals and water XW + W. At intermediate temperatures, the crystal hydrates XW melt gradually into the previously reported L_α phase, leaving a narrow coexistence region in the phase diagram XW + L_α. In conclusion, an amended broad equilibrium phase diagram is presented, combining our new results with those previously reported in the literature.