X-ray absorption study on the solvated copper(II) ion: transition from a solvated solid to the dissolved state

X-ray absorption spectroscopy has often been used to examine the local environment around an absorbing cation in both the solid and the liquid state. In order to examine the properties of a transition-metal ion under conditions in which the degree of solvation can be regulated, the authors have placed cupric ions within a smectite clay. Smectite clays consist of negatively charged sheets of aluminosilicates which are separated by an interlayer whose thickness changes upon absorption of solvent. In the native clay used for these experiments, bentonite, the calcium was replaced with copper by stirring the clay in a 0.1 M aqueous solution of CuCl[sub 2]. For the experiments described here, Cu K-edge spectra were acquired at the National Synchrotron Light Source and Stanford Synchrotron Radiation Laboratory in both transmission and fluorescence modes. Four samples of Cu-bentonite powder, dried from water, methanol, ethanol, and ethylene glycol, were examined, as well as slurries of these samples in their respective solvents. The normalized X-ray absorption near edge (XANES) spectra are obtained for copper ion in the Cu-bentonite dried from methanol and for the Cu-clay as a slurry in the same solvent. The spectrum of the dry material clearly shows the electronic 1s-4p transitionmore » as a resolved shoulder on the absorption edge at 8986 eV. This transition, which is characteristic of a copper ion with square planar coordination, has broadened and almost disappeared in the slurry. The observed change suggests that as the solvent penetrates into the clay, some of the solvent molecules enter the coordination sphere of the copper(II) ion. The addition of two axial oxygen ligands shifts the coordination geometry of the Cu(II) from a square planar configuration to a distorted octahedron. There is an obvious change in the EXAFS data between the dry clay and the slurry which cannot be readily discerned by examination of the radial distributions alone. 23 refs., 2 figs., 1 tab.« less