Solution and liquid crystalline microstructures in sodium taurodeoxycholate/D2O mixtures

Nuclear magnetic resonance (NMR) and rheology were used as two useful and complementary techniques to characterize the solution and liquid crystalline microstructures of the sodium taurodeoxycholate (NaTDC)/ D2O system. NaTDC is very soluble in water. At room temperature NaTDC/D2O mixtures form a low-viscosity isotropic micellar solution up to a concentration of ca. 12 wt % NaTDC. It follows a micellar solution region, 12-25 wt % NaTDC, that behaves as a Newtonian viscous fluid with a well-defined viscosity value. At higher concentration ( > 25 wt % NaTDC), a region with very high viscosity and viscoelastic behavior exists. At room temperature and at ca. 37 wt % NaTDC, the NaTDC/D2O system exhibits a phase transition into a birefringent phase. In this region, extending to ca. 65 wt %, the samples exhibit a gel-like behavior and behave as a shear thinning fluid. NMR and rheological data indicated that micellar solution is composed of a region with spherical micelles and of another with cylindrical aggregates, starting with 12 wt % NaTDC. The liquid crystalline phase, which disappears to ca. 36 °C, is a direct hexagonal phase as revealed by water self-diffusion NMR and the calculated diffusional obstruction factors. Dynamic viscoelastic measurements showed that micellar solutions (>25 wt % NaTDC) and the liquid crystalline phase behave as a "weak gel". A possible relationship between the rheological responses and the phase microstructures using a cooperative-flow model was discussed.