A new physicochemical characterization of sodium taurodeoxycholate/water system

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 (L(1)) and presents a sharp structural transition from spherical micelles to an entangled network upon increasing the concentration. Below 10 degrees C, the L(1) 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 (H(1)). Specifically, the hexagonal mixture with 45 wt% STDC presents a low thermal stability (T < 35 degrees C) and slow kinetics at the liquid-hexagonal transition (45 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 (C(12)E(6)), a nonionic surfactant forming a direct hexagonal phase in a similar interval of compositions and temperatures of STDC.