11698
J. Phys. Chem. 1996, 100, 11698-11703
XAFS Studies of Solution-Phase Complexes of Cesium with Dibenzo-18-crown-6 Ethers K. M. Kemner,*,† D. B. Hunter,‡ W. T. Elam,† and P. M. Bertsch‡ U.S. NaVal Research Laboratory, Code 6685, 4555 OVerlook AVenue, Washington, D.C. 20375, and DiVision of Biogeochemistry, SaVannah RiVer Ecology Laboratory, UniVersity of Georgia, P. O. Drawer E, Aiken, South Carolina 29802 ReceiVed: February 15, 1996; In Final Form: April 19, 1996X
X-ray absorption fine structure (XAFS) measurements have been made at the Cs LIII absorption edge on (1) 0.04 M acetonitrile solutions with respect to both CsBr and dibenzo-18-crown-6 ether (Cs-D18C6), (2) solids produced by drying the aforementioned Cs-D18C6 solution complex, and (3) crystalline CsBr, CsCl, and CsF powders that were used as standards. XAFS measurements have also been made at the Br K absorption edge on the same 0.04 M solution mentioned above. Due to the many difficulties associated with obtaining high-quality XAFS data on these systems, a custom-manufactured 5-mil-thick Sc foil was used in conjunction with a Soller slit assembly to improve the XAFS signal-to-noise ratio by almost a factor of 6. XAFS analyses of the Cs-D18C6 solution show the presence of a 1:1 Cs-D18C6 complex with a Br contact ion, consistent with previous references derived from 133Cs nuclear magnetic resonance investigations. For interpretation of the Cs LIII edge data, the choice of a Br counterion reduces the error in determining the presence of a single counterion among the lighter backscattering O and C atoms of the crown ether complex. Observation of the normalized XAFS data for the dried Cs-crown ether solution shows that the Cs atom’s local environment changes from a 1:1 Cs-D18C6 complex to a predominantly crystalline CsBr solid phase. This recrystallization during drying illustrates the importance of in situ methods to characterize chemical speciation in solution. The ability of XAFS to directly probe the ternary Cs-D18C6-Br contact ion pair opens many exciting avenues for improving extraction methodologies.
I. Introduction There is an increasing awareness for the need to understand the chemical speciation and interactions of elements in many fields of research, particularly the development of separation technologies (procedures) for contaminant elements via design of reagents having high specific interactions with those elements. 137Cesium is a common radiocontaminant in nuclear waste. An understanding of its structural binding properties with ligands is important in designing highly specific chelates that could be used in waste-treatment processes. Theoretical studies are being conducted to provide a framework for the rational design of new ligands in separation processes at hazardous waste storage facilities.1 There are few spectroscopic methods to experimentally probe the fundamental ion-ligand chemistry for cation specificity to the macrocycle. X-ray absorption fine structure (XAFS) spectroscopy is potentially well suited to the study of these highly specific interactions since it is capable of very detailed molecular structure determinations in chemical systems of limited bonding environments. As one of the heaviest of the alkali metals, Cs possesses some unique and subtle differences in chemical behavior compared to the lighter alkali metals. The large atomic radius coupled with low charge-to-surface ratio results in the Cs+ ion being poorly solvated with a low hydration energy. This low hydration energy results in an ion having an effectively smaller hydrated radius than Li+, Na+, K+, or Rb+.2 Conversely, in solvents less polar than water, Cs+ is more likely to form contact ion pairs with anions than the lighter alkali metals, as evidenced by extremely large chemical shifts in nuclear magnetic resonance spectra.3 * Present address: Environmental Research Division, Argonne National Laboratory, Argonne, IL 60439. † U.S. Naval Research Laboratory. ‡ University of Georgia. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(96)00460-1 CCC: $12.00
In the field of developing macrocyclic compounds with high binding efficiencies for specific ions, macrocyclic polyether (crown ether) compounds have received much study.4 The size of the intramolecular hole in the crown ether can discriminate between different cations based on binding affinity. This high binding affinity or macrocyclic effect is also dependent on the physical properties of the cation being investigated (concentrations, solvation, number of donor atoms, size, etc.).4-6 Previous studies on the complexation of 133Cs+ with 18-crown-6 ether by potentiometric, calorimetric, and NMR techniques provide a useful data base for an XAFS analysis of solution complexation of Cs in 18-crown-6 ether. 4-9 Crown ethers also represent an experimental system suitable for XAFS structural studies by presenting well-defined first- and second-shell environments about the bound Cs. There are, however, many difficulties associated with obtaining high-quality XAFS data on these systems. Firstly, since these experiments are performed in the solution phase at relatively dilute Cs concentrations, there is a very large background signal due to the inelastic scattering from the acetonitrile. Also, solution-phase studies introduce very large XAFS Debye-Waller factors coupled with the typical, quicklydampened, k-dependent backscattering amplitudes of the oxygen and carbon shells. Secondly, the branching ratio of Auger electron to fluorescent X-ray emission favors Auger electron production near the Cs LIII absorption edge energy.10 Thirdly, the radial distances of the oxygen and carbon backscattering shells are such that their backscattering frequencies destructively interfere with each other throughout a large portion of the XAFS data. Finally, because of the presence of the Cs LII absorption edge, the range of useful data is limited to only 350 eV above the LIII absorption edge. However, the ability of XAFS to accurately elucidate the structural environment of the crown ether about a Cs ion will permit an extension of this form of analysis to many other © 1996 American Chemical Society
Complexes of Cesium with Dibenzo-18-crown-6 Ethers systems in which the Cs is selectively bound within a cage of electronegative oxygen donors. Such examples would include humic and other organic substances, clay minerals, and ionselective exchangers used in nuclear waste treatment. Previous XAFS studies of ions complexed to crown ethers in the solid phase have been performed.11,12 Results from these studies indicated a split oxygen coordination shell surrounding the Cs ion and did not indicate the formation of a counterion contact pair, as had been suggested in a previous NMR study.4,7-9 To address this issue, we have performed XAFS studies at the Cs LIII edge on (1) an acetonitrile solution of 0.04 M CsBr and dibenzo-18-crown-6 ether, (2) solids produced by drying the aforementioned solution, and (3) crystalline CsBr, CsCl, and CsF powders that were used as standards. To our knowledge, this is the first Cs XAFS study undertaken in solution phase. XAFS measurements have also been made at the Br K absorption edge on the same 0.04 M solution mentioned above. XAFS analyses of CsBr dissolved in acetonitrile solutions of dibenzo-18-crown-6 (D18C6) ether show the presence of a 1:1 Cs-dibenzo-18-crown-6 ether complex with the Br counterion still in contact with the Cs atom. For interpretation of the Cs LIII edge data, the choice of a heavy backscattering Br anion rather than F or Cl reduces the error in determining the presence of a single backscattering counterion from the signal arising from 6 oxygen atoms and 12 carbon atoms of the D18C6 ether. XAFS analyses of the dried solution indicate that the Cs ions’ local environment changes from a crown ether complex to a predominantly crystalline CsBr environment removed from the crown ether. Potential errors in other XAFS analyses arising from inattention to the counterion will be discussed. II. Experimental Section Cs-dibenzo-18-crown-6 ether solutions were prepared by dissolving reagent-grade CsBr and dibenzo-18-crown-6 (D18C6) ether in acetonitrile so that each were 0.04 M. Previous nuclear magnetic resonance (NMR) data have demonstrated that a Cs: crown ether ratio of 1:1 exists at this molar ratio.3 Similarly, stability constants have shown the ratio of Cs+ in solution to Cs in a crown ether environment to be 1:10 000.7 A 2-cm3 volume of the solution was transferred into a 5-µm (0.2-mil) thick polypropylene bag. XAFS experiments at the Cs LIII edge were performed on beamline X23B13 at the National Synchrotron Light Source with an electron beam energy of 2.54 GeV and stored currents between 130 and 250 mA. (The optical and X-ray properties of this beamline are presented in ref 13.) Measurements on the abovementioned solution were made at room temperature in the fluorescence mode utilizing a filter-slit combination.14,15 Transmission measurements were also made on 99.7% ultradry (
