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The application of x-ray absorption spectroscopy methods to both materials and life sciences is well appreciated. However, the power of extended x-ray absorption fine structure (EXAFS) spectroscopy as a quantitative structural technique has largely been limited by its application to the microscopically homogeneous systems, in which the local environment around each absorbing atom in the sample is the same. The growing interest in time-resolved EXAFS studies of systems in physics, chemistry, biology, and materials science has reintroduced the requirement for an analytical tool to probe heterogeneous mixtures in situ. While long being recognized as a premiere technique for this role, EXAFS studies of mixtures have been particularly difficult due to the strong model dependence and correlations between parameters in the fit. To circumvent these drawbacks, we introduce two new techniques in EXAFS analysis: the principal component analysis and the residual phase analysis. Using a test case of a heterogeneous mi...

A hydrophobic nanostructured composite has been created through the addition of an organic monolayer to the external surface of a smectite clay. This film inhibits the free exchange of water in to and out of the interlayer located between the aluminosilicate sheets of the clay. The effect of the presence of such a monolayer, which is formed from octadecyltrimethoxysilane, on iron(III) cations exchanged into the clay interlayer has been examined using X-ray absorption spectroscopy. Mild hydrothermal conditions have been used to increase the rate of potential reactions involving the ferric ions. The addition of the organic coating results in partial reduction of the iron to the ferrous state, a process that is furthered by hydrothermal treatment of the hydrophobic clay. In contrast, the iron in the simple ion-exchanged clay aggregates into small iron−oxygen clusters but is not reduced. These results are discussed in the context of the reduction potential of iron(III) and the susceptibility of that iron to h...

The use of principal component (factor) analysis for the interpretation of EXAFS spectra is described. The components derived from EXAFS spectra share mathematical properties with the original spectra. As a result, the abstract components can be analyzed using standard EXAFS methodology to yield bond distances and other coordination parameters. The number of components that must be analyzed is usually less than the number of original spectra. The method is demonstrated using a series of spectra from aqueous solutions of uranyl ions.

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

Long-chain alkyltrichlorosilanes, C13Si (CH2)nX, adsorb from solution onto silicon-silicon dioxide (Si/SiO2) substrates and form ordered alkylsiloxane monolayer films. These films were characterized by wettability, ellipsometry, and XPS. Except for very short chains (n = 0,1,2), the wetting of these monolayers was approximately independent of chain length. The presence of small amounts of water was necessary for the formation of these films. The alkylsiloxane monolayers were stable in common organic solvents, water, and acid, but were destroyed by prolonged exposure to base. Simple reactions on vinyl-terminated monolayers generated alcohol-, carboxylic acid-, and bromine- terminated films whose contact angles were lower than the starting monolayers, but whose lengths were largely unchanged. Measurements of the contact angle of acid-terminated interfaces as a function of pH indicated that ionization of soluble carboxylic acids. Monolayers containing mixtures of methyl- and carboxyl-functionalities exhibited wetting properties that mirrored the composition of the interface.

The use of sulfur-containing organic monolayer films improves adhesion between gold and silicon dioxide. The structures of these monolayers were analyzed using contact angle, ellipsometry, and XPS. The zone of adhesive failure was at or near the gold-monolayer interface.

The thicknesses of C10-C18 alkylsiloxane monolayers on silicon-silicon dioxide substrates have been measured with ellipsometry and low-angle X-ray reflection. Although, for any given sample, thicknesses measured by the two methods agree to within experimental error, ellipsometric measurements are systematically larger by approximately 2 {angstrom}. This difference may result from variations in the sensitivity of the two techniques to the structure of the interface between silicon dioxide and the alkylsiloxane monolayer. The x-ray reflectivity measurements provide evidence that these organic monolayers do not build up as island structures and demonstrate that the approximate area projected by each alkyl group in the plane of the monolayer is {approximately} 21 {plus minus}3 {angstrom}{sup 2}. Preliminary studies indicate that this technique can be used to follow the changes in the structure of a monolayer which result from chemical transformations. The influence of damage that is induced by x-ray radiation on these measurements is discussed.