Amorphous solids techniques

The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

Several new thermal analytical techniques are potentially valuable for the study of second-order transitions in the characterization of amorphous solids and for the accurate determination of glass transition temperatures. These modem techniques can detect and characterize glass transitions and other second-order transitions that are not detectable by conventional thermal analytical techniques such as DSC, TGA, or TMA. 

In amorphous solids there is a considerable disorder and it is impossible to give a description of their structure comparable to that applicable to crystals. In a crystal indeed the identification of all the atoms in the unit cell, at least in principle, is possible with a precise determination of their coordinates. For a glass, only a statistical description may be obtained to this end different experimental techniques are useful and often complementary to each other. Especially important are the methods based on diffraction experiments only these will be briefly mentioned here. The diffraction pattern of an amorphous alloy does not show sharp diffraction peaks as for crystalline materials but only a few broadened peaks. Much more limited information can thus be extracted and only a statistical description of the structure may be obtained. The so-called radial distribution function is defined as ... 

NMR and EPR techniques provide unique information on the microscopic properties of solids, such as symmetry of atomic sites, covalent character of bonds, strength of exchange interactions, and rates of atomic and molecular motion. The recent developments of nuclear double resonance, the Overhauser effect, and ENDOR will allow further elucidation of these properties. Since the catalytic characteristics of solids are presumably related to the detailed electronic and geometric structure of solids, a correlation between the results of magnetic resonance studies and cata lytic properties can occur. The limitation of NMR lies in the fact that only certain nuclei are suitable for study in polycrystalline or amorphous solids while EPR is limited in that only paramagnetic species may be observed. These limitations, however, are counter-balanced by the wealth of information that can be obtained when the techniques are applicable. 

The vapour deposition method is widely used to obtain amorphous solids. In this technique, atoms, molecules or ions of the substance (in dilute vapour phase) are deposited on to a substrate maintained at a low temperature. Most vapour-deposited amorphous materials crystallize on heating, but some of them exhibit an intervening second-order transition (akin to the glass transition). Amorphous solid water and methanol show such transitions. The structural features of vapour-deposited amorphous solids are comparable to those of glasses of the same materials prepared by melt-quenching. 

In investigations of structures of amorphous solids, MAS NMR spectroscopy has been used effectively (see Chapter 2). Structures of borates, silicates and phosphates have been advantageously studied by this technique. An important advance in this area... 

It is necessary to note that in addition to the problems associated with the theoretical analysis of electronic conduction in disordered systems, experimental techniques of established success in the study of crystalline semiconductors have proven to yield ambiguous results when applied to amorphous solids. 

Seldom in the study of heterogeneous catalysis does it prove possible to (I) specify precisely the concentration and nature of the active sites, (2) test whether these sites are of comparable strength and are distributed in a spatially and chemically well-defined manner, and (3) explore the structural and mechanistic features of the system using a wide range of complementary techniques, many of them in situ. Even rarer are situations in which both the access to the active sites and the shape of the reactants may be systematically and subtly varied, so that one is able to compare the performance of the active site in a crystalline environment with an essentially identical one embedded in an amorphous solid. 

Solid-state NMR spectroscopy is a powerful technique for the determination of structure in amorphous solids and for species in solution. Extensive studies of NMR shieldings of electropositive or cationic atoms, such as Si, have established that shieldings are strongly influenced by the composition of the 1st coordination sphere and also show significant, consistent and easily measurable trends related to the identify and geometry of 2nd coordination sphere atoms. Atoms in the 3rd and more distant coordinate spheres generally have little effect upon the shielding. 

After removal of the asphaltene fraction, further fractionation of petroleum is also possible by variation of the hydrocarbon solvent. For example, liquehed gases, such as propane and butane, precipitate as much as 50% by weight of the residuum or bitumen. The precipitate is a black, tacky, semisolid material, in contrast to the pentane-precipitated asphaltenes, which are usually brown, amorphous solids. Treatment of the propane precipitate with pentane then yields the insoluble brown, amorphous asphaltenes and soluble, near-black, semisolid resins, which are, as near as can be determined, equivalent to the resins isolated by adsorption techniques. 

Following the first isolation of YTX by Murata et al. (1987), characterization was carried out by NMR as well as other techniques, and the purified toxin was found as an amorphous solid, +3.01° (c 0.45, CH3OH), maximum absorption in UV spectra was found to be 230 nm (in CH3OH) ( 10600). The same authors found absorption bands in IR (KBr) at 3400, 1240, 1220, 1070 and 820 cm, and this is the only YTX this has been reported for. Later additions to the YTX family have mainly been characterised by NMR and (LC-)MS/MS, UV absorption data being reported for some. 

MD Simulations of Glasses. Increasing use is being made of MD techniques to generate structural models for amorphous solids. The procedure used is simple a molten system is generated and the simulation is then rapidly cooled to below... 

The deposited-film technique is particularly useful for polymers, resins, and amorphous solids. The samples are dissolved in a reasonably volatile solvent, the solution poured onto a suitable window, and the solvent evaporated by gentle heating or vacuum treatment. 

Using EXAFS it is possible to obtain information on each type of atom separately in a complex solid. Crystallinity is not essential and the technique has been applied to disordered materials such as gases, liquids, solutions, biological systems, amorphous solids, and complicated alloys and compounds. Serious limitations of the technique for studying highly disordered systems, however, have been emphasized recently. ... 

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