Characterization of Crystalline Products

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

The so-called hyphenated techniques , incorporating thermal methods as one of the combined analytical techniques are sure to play an increasing role in the identification and characterization of crystalline forms of pharmaceutical substances. The combination of TGA with FTIR allows the simultaneous quantitative analysis of weight changes during thermal processes with the IR identification of the decomposition products (e.g. solvent) resulting from those processes (Materazzi 1997). For substances with low volatility, the FTIR analysis may be replaced with mass spectroscopy (Materazzi 1998). 

For the class of compounds discussed here, the self-assembly process always takes place in solution, where the components have sufficient mobility. For the characterization of the product the method of choice is X-ray crystallography, which requires the isolation of a crystalline solid. However, a total reliance on crystal structure determination poses several problems. The structural information available from single crystal diffraction is very complete, but it is quite often impossible to grow suitable crystals, and even when it is possible, disordered solvent and counter ions can give considerable problems in the resolution and refinement of the structure. A more fundamental question is whether the crystallization process, itself a form of self-assembly, has not resulted in a structural change. As an example we may quote the complex [Cu2(mimpy)2] which exists as isolated double helical units in solution, but which crystallises in columns with strong stacking interactions between individual units [5]. 

The structure and morphology of the membrane also influence its permeability. XRD has been widely applied for the characterization of crystalline structure, especially for confirmation of alloy formation in multicomponent materials and identification of bulk corrosion products. SEM and atomic force microscopy are routinely applied to the characterization of membrane topography and are helpful for identification of failure mechanisms. More advanced methods, including... 

The dealumination of faujasite,mazzite and offretite with ammonia hexaflu-orosilicate and the characterization of the products with various techniques have been reported [192]. The maximum level of dealiunination, which could be achieved without loss of X-ray crystallinity, corresponded to 50% for faujasite and 30% for mazzite and offretite. The dealiunination capability was foimd to depend on the texture of the crystals, which may have indicated that the process was diffusion-controlled. 

The complete characterization of a particulate material requires development of a functional relationship between crystal size and population or mass. The functional relationship may assume an analytical form (7), but more frequentiy it is necessary to work with data that do not fit such expressions. As such detail may be cumbersome or unavailable for a crystalline product, the material may be more simply (and less completely) described in terms of a single crystal size and a spread of the distribution about that specified dimension. 

The development of the internal orientation in formation in the fiber of a specific directional system, arranged relative to the fiber axis, of structural elements takes place as a result of fiber stretching in the production process. The orientation system of structural elements being formed is characterized by a rotational symmetry of the spatial location of structural elements in relation to the fiber axis. Depending on the type of structural elements being taken into account, we can speak of crystalline, amorphous, or overall orientation. The first case has to do with the orientation of crystallites, the second—with the orientation of segments of molecules occurring in the noncrystalline material, and the third—with all kinds of structural constitutive elements. 

Sulfoxides form adducts with Lewis acids like SbCls and SnCU [80]. In the case of SsO the crystalline products SsO-SbCh [81] and (SsO)2-SnCl4 [82] have been prepared and characterized by single crystal X-ray diffraction analyses. Reaction of SsO with SbCls in CS2 at 20 °C and subsequent cooling of the solution to -50 °C resulted in orange orthorhombic crystals of SsO-SbCls in 71% yield. These molecules are of Cg symmetry see Fig.6 [81]. 

The redox behavior of the SeSO -Zn-EDTA system has been discussed on the basis of Pourbaix and solubility diagrams [11], Different complexes and substrates have been employed in order to optimize the electrodeposited thin films. By the selenosulfate method it is generally possible to grow ZnSe with an almost stoichiometric composition however, issues of low faradaic efficiency as well as crystallinity and compactiveness of the product, remain to be solved. Interestingly, in most reports of photoelectrochemically characterized ZnSe electrodeposits, the semiconductor film was found to be p-type under all preparation conditions (ZnSe is normally n-type unless deliberately doped p-type). 

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