EXAFS STUDY OF MICELLAR AGGREGATES OF BILE-ACID RUBIDIUM SALTS

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been employed to characterize the local environment of the Rb+ ions in micellar aggregates of bile acid rubidium salts. In a previous study at the Rb K-edge it was found that the Rb+ ions of the rubidium deoxycholate (RbDC) micellar aggregates have the same or a very similar coordination as that of the helical structure in the crystal, which is remarkably different from the coordination of the aqueous solvated Rb+ ions. In the present paper we report the EXAFS data analysis of crystals and aqueous micellar solutions of rubidium taurodeoxycholate (RbTDC), rubidium glycodeoxycholate (RbGDC), and rubidium glycocholate (RbGC). Crystals and micellar solution spectra are compared with a theoretical signal calculated for a coordination model of a Rb+ ion surrounded by water molecules by employing the Rb-0 radial distribution function obtained from molecular dynamics simulations and refined by using the EXAFS spectrum of a RbOH aqueous solution. This strategy has been successfully checked by analyzing the EXAFS spectrum of a RbDC aqueous micellar solution. The crystal spectra of RbTDC and RbGC agree with the Rb+ coordination observed in their known crystal structures, whereas that of RbGDC indicate that the Rb' coordination is liquidlike in the corresponding crystal of unknown structure. The spectra of the aqueous solutions, containing micellar aggregates of small size, show that the Rb+ ions are exposed to or dipped into the solvent and allow some structural models for the micellar aggregates to be discarded.