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 mic ellar aggregates of bile acid rubidium salts. In a previous study at t he Rb K-edge it was found that the Rb+ ions of the rubidium deoxychola te (RbDC) micellar aggregates have the same or a very similar coordina tion as that of the helical structure in the crystal, which is remarka bly 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), rub idium glycodeoxycholate (RbGDC), and rubidium glycocholate (RbGC). Cry stals and micellar solution spectra are compared with a theoretical si gnal calculated for a coordination model of a Rb+ ion surrounded by wa ter molecules by employing the Rb-O radial distribution function obtai ned from molecular dynamics simulations and refined by using the EXAFS spectrum of a RbOH aqueous solution. This strategy has been successfu lly checked by analyzing the EXAFS spectrum of a RbDC aqueous micellar solution. The crystal spectra of RbTDC and RbGC agree with the Rb+ co ordination observed in their known crystal structures, whereas that of RbGDC indicate that the Rb+ coordination is liquidlike in the corresp onding crystal of unknown structure. The spectra of the aqueous soluti ons, containing micellar aggregates of small size, show that the Rb+ i ons are exposed to or dipped into the solvent and allow some structura l models for the micellar aggregates to be discarded.