Solutes at Interfaces Electronic Spectroscopy

Much more detailed information can be obtained from molecular dynamics and Monte Carlo simulations. This includes the solute orientational profile, which can be expressed using the orientational probability distribution P cp z). If d is a vector fixed in the molecular frame of a solute molecule, the probability distribution of the angle (p between this vector and the normal to the interface is calculated easily using computer simulations as a function of the solute location z. For relatively large dye molecules with a slow reorientation time, convergence can be slow, so it is important to verify that the computed F(0 z) is uniform for z values in the bulk region. Only a few molecular dynamics studies have been reported, with results that generally show an orientational preference with broad distributions. 

Experimental and theoretical studies of visible and UV spectra of solute molecules in the condensed phase have a very long history. These techniques 

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A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

The relevance of Auger electron spectroscopy to the solution of various theoretical and practical problems arising from the presence of sulfur at metallic surfaces and interfaces is discussed the relations between phenomenological data such as reaction rates or activation energies and surface characterization with respect to the crystallographic structure and the composition of the surface are reviewed and commented upon. 

Various planar membrane models have been developed, either for fundamental studies or for translational applications monolayers at the air-water interface, freestanding films in solution, solid supported membranes, and membranes on a porous solid support. Planar biomimetic membranes based on amphiphilic block copolymers are important artificial systems often used to mimic natural membranes. Their advantages, compared to artificial lipid membranes, are their improved stability and the possibility of chemically tailoring their structures. The simplest model of such a planar membrane is a monolayer at the air-water interface, formed when amphiphilic molecules are spread on water. As cell membrane models, it is more common to use free-standing membranes in which both sides of the membrane are accessible to water or buffer, and thus a bilayer is formed. The disadvantage of these two membrane models is the lack of stability, which can be overcome by the development of a solid supported membrane model. Characterization of such planar membranes can be challenging and several techniques, such as AFM, quartz crystal microbalance (QCM), infrared (IR) spectroscopy, confocal laser scan microscopy (CLSM), electrophoretic mobility, surface plasmon resonance (SPR), contact angle, ellipsometry, electrochemical impedance spectroscopy (EIS), patch clamp, or X-ray electron spectroscopy (XPS) have been used to characterize their... 

Adsorption can be characterized by the amount of the surfactant adsorbed as a function of equilibrium concentration. The quantitative amount of the adsorbed surfactant is usually determined by analyzing the liquid phase by spectroscopic, calorimetric, and electrophoretic methods [5,6]. The physical and chemical state of the adsorbed layer can be examined by spectroscopy, electron diffraction, x-ray diffraction, electron spectroscopy for chemical analysis (ESCA), interferometry, and electron microscopy. The orientation of the adsorbed surfactant molecules depends on the nature of the adsorbent and the surfactant, the interaction between the adsorbent and the surfactant, the lateral interaction between the adsorbed surfactant molecules, and the concentration of the surfactant in solution. The adsorbed layer may be homogeneous or consist of coadsorbed surfactants of different composition. Solvent molecules may also be adsorbed at the solid interface. 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

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