Liquid crystals and phase equilibria binary bile salt-water systems

The phase behavior of several binary sodium bile salt-water systems is inve stigated over the entire concentration range, with emphasis on concentrated regions beyond the isotropic solution phase. The studied bile acid salts c omprise the free salt sodium deoxycholate (SDC), the taurine conjugates sod ium taurocholate (STC), sodium taurodeoxycholate (STDC), and sodium tauroch enodeoxycholate (STCDC) and the glycine conjugate sodium glycodeoxycholate (SGDC). A combination of classical techniques is used, including phase diag ram determination, polarizing microscopy, H-2 NMR, and small-angle X-ray sc attering (SAXS). The aggregation behavior in the isotropic micellar solutio ns of STC and STDC is also investigated by pulsed-field gradient NMR self-d iffusion. The optical textures and the data from SAXS and H-2 NMR clearly p oint to the formation of hexagonal liquid crystals, possibly of the reverse type, beyond the micellar solution for all the bile salts. Several unusual kinetic effects, such as very slow equilibration times and the formation o f transient spherulitic crystals in biphasic regions, are observed. The pha se diagrams and structural data are qualitatively discussed in terms of the molecular structure and solubility of the different salts. The formation o f lyotropic liquid crystals by bile salts, which has remained unknown for d ecades, is clearly demonstrated in this work.