Phase equilibria of the mixed didodecyldimethylammonium bromide–sodium taurodeoxycholate–water system with a large solution region

The phase behavior over the entire concentration range for the system didodecyldimethylammonium bromide (DDAB)–sodium taurodeoxycholate (STDC)–water, at 25 °C, has been investigated, with emphasis on the DDAB-rich part. Polarizing microscopy, SAXS, 2H NMR and 1H self-diffusion NMR have been used in combination as probing techniques for phase behavior and microstructure. The system forms four major phases, all deriving from the respective binary surfactant systems. The two lamellar phases originating from the binary DDAB–water axis (DI and DII, at 3–30 and 83–91 wt.% DDAB, respectively) are only able to incorporate small amounts of STDC. The DII phase solubilizes a comparatively higher amount of bile salt (up to ca. 6 wt.%), while the DI phase takes up less than 0.25 wt.%. From the STDC–water axis, a solution phase and a “hexagonal-like” liquid crystalline phase are derived, at 0–26 and 37–60 wt.% of STDC, respectively. Heterogeneous regions are also indicated on the basis of NMR and SAXS data. The most striking feature is the large extension of the isotropic solution phase, which originates from the water corner and curves toward the DDAB-rich side of the phase diagram. Even though at the upper limit of the solution phase the amount of water is reduced to 10 wt.%, the measured water and DDAB self-diffusion coefficients exclude the possibility of reverse-type structures.