Effect of a bile salt on the aggregation behavior of a double-chained cationic surfactant - the cationic-rich dilute region of the didodecyldimethylammonium bromide-sodium taurodeoxycholate-water system

The phase behavior and aggregate structure of the didodecyldimethylammonium bromide (DDAB)-sodium taurodeoxycholate—water mixed surfactant system have been investigated in the dilute (above 95 wt% water) cationic-rich area, at 25 °C. A combination of techniques has been used, including polarizing and video-enhanced light microscopy, cryogenic transmission electron microscopy and water self-diffusion NMR. Between the dilute lamellar phase (~4-30 wt% DDAB) and the very narrow isotropic solution (below 0.5 wt% DDAB), the lamellar dispersions formed by DDAB contain different types of vesicle structures (vesicles, microtubules and multilamellar structures). On addition of bile salt to the DDAB dispersions, strong electrostatic headgroup interactions and geometric packing effects (owing to the unusual molecular structure of the anionic surfactant) are present. The vesicle aggregates, however, are able to incorporate an amount of bile salt up to roughly 20 mol%. Further addition induces a macroscopic phase separation, with the formation of a strong white dispersion. The light and electron micrographs show that the vesicles are spherical and integral, undergoing an increase in size when the bile salt concentration is varied between 0 and 10 mol%. At higher bile salt concentration, smaller vesicles, tubular structures and submicron-sized dispersion droplets are observed. Water self-diffusion NMR measurements give further information regarding vesicle size and poly-dispersity.