On the micellar aggregates of alkali metal salts of deoxycholic acid

The X-ray patterns of lithium (LiDC) and potassium (KDC) deoxycholate fibers, drawn from aqueous micellar solutions, have been interpreted by means of a packing of 8/1 helices formed by trimers. Previously, these helices satisfactorily represented the structure of the sodium (NaDC) and rubidium (RbDC) deoxycholate micellar aggregates. Dielectric measurements show that the trend of the average electric dipole moment mu of a NaDC monomer as a function of temperature and concentration supports a two-structure equilibrium. The high mu values (32-58 D) can be explained by the remarkable hydration of the NaDC micellar aggregates. The mu moderate decrease when the size of the aggregates increases can agree with the presence of small helices but disagrees with the existence of aggregates that are disordered or have a center (or a pseudocenter) of symmetry. Formerly, it was observed that sodium taurodeoxycholate (NaTDC) micellar aggregates, represented by 7/1 helices, formed by trimers, behave similarly. The contribution to the electrical conductivity of NaDC and NaTDC in aqueous solutions containing NaCl tends to zero by increasing the NaCl concentration, denoting strong interactions between Naf ions and anion aggregates. According to the similar 7/1 and 8/1 helices, which have the Naf ions in their inner part, the micellar size and the fraction of Na+ ions trapped inside the helices increase together. The aggregate apparent hydrodynamic radius (RI,) increases by increasing the ionic strength in the order LiDC > NaDC > KDC > RbDC. Fibers drawn from solutions containing two cations at the same concentration show that the affinity for the anionic structure seems to follow the order Li+ > Na+ > K+ > Rb+ at high ionic strength. The Rh values VS the mole fractions of Li+ and Rb+ or Na+ and K+ at lower ionic strength are fitted by straight lines. Probably, the free energy gains, associated with the cation and anion transfer from the bulk solution to the micellar aggregates, are almost equal for the four salts at lower ionic strength.