In this paper, some new physicochemical properties of sodium taurodeoxycholate (STDC)–water system are carefully investigated using shear rheology and NMR diffusometry. Of clear interest to us was the study of structure and/or morphology of the isotropic liquid and the hexagonal phase.

We observed that the isotropic liquid phase (C < 30 wt% STDC) consists of a micellar solution (L₁) and presents a sharp structural transition from spherical micelles to an entangled network upon increasing the concentration. Below 10 °C, the L₁ phase transforms slowly into a firm, gel phase (V) which results stable in the composition interval 15 < C < 30 wt%. The linear viscoelasticity of this phase is interpreted by means of the cooperative-flow theory.

The liquid crystalline phase (36 < C < 65 wt% STDC) is unequivocally characterized as having a direct hexagonal structure (H₁). Specifically, the hexagonal mixture with 45 wt% STDC presents a low thermal stability (T < 35 °C) and slow kinetics at the liquid–hexagonal transition (>5 hours). The linear viscoelasticity of this phase is interpreted by means of the generalized Maxwell model.

All these findings are confirmed by some additional measurements performed on a lyotropic mixture of hexaethylene glycol monododecyl ether (C₁₂E₆), a nonionic surfactant forming a direct hexagonal phase in a similar interval of compositions and temperatures of STDC.