X-ray and quasi-elastic light-scattering studies of sodium deoxycholate

Sodium and rubidium deoxycholate (NaDC and RbDC, respectively) fibers have been drawn near the gelation point by lowering the pH of aqueous micellar solutions. Their X-ray diffraction patterns show a very close resemblance and can be interpreted by means of similar unit cell parameters and helical structures, formed by trimers arranged in 8/1 helices. Because the structures of the resulting fibers are connected with those observed in crystals, the helix of the RbDC crystal has been chosen to construct the 8/1 helix of the fibers. Calculations of interatomic distances support the 8/1 helix that can be used as a structural model of the NaDC micellar aggregates, especially near the gelation point. The lowering of pH within a narrow range in NaDC aqueous solutions causes a remarkable increase of the apparent hydrodynamic radius (R-h) and of the average scattered intensity. The intensity strongly changes within 2 degrees C around the temperature of the sol-gel transition. A bimodal R-h distribution, corresponding to very small and very big aggregates, is observed. The dominant mechanism causing the decay of the intensity autocorrelation function seems to be the translational motion of the center of mass of the aggregates. The absence of an appreciable rotational contribution is ascribed to the formation of roughly isotropic aggregates with spheroidal shape. It is proposed that the isotropic big aggregate is formed by helices randomly coordinated around a solvated hydrogen ion or a cluster containing hydrogen ions and water molecules, and that a network of cross-links, due to polar forces, connects the big aggregates and is responsible for the gel formation.