Crystal habit, characterization using

In order to bridge the gap between an SHG-active material and one optimized for use in an optoelectronic device, many compounds have been synthesized, characterized, modified and then ultimately rejected during the past decade (i-3). This is not surprising, since the ideal material must fulfil a plethora of stringent requirements (4-7). The most critical condition for an SHG-active material is that it must form noncentrosymmetric structures however, thermal stability, in volatility, transparency, lack of colour, mechanical strength and crystal habit are also crucial properties for materials to be incorporated into practical devices. 

The most general use of the microscope for the study of polymorphism is simply for observing the homogeneity or diversity of a crystalline sample. Variations in size, shape or colour may indicate the presence of polymorphism and the need for further examination (see, for instance Fig. 3.2). As noted in Section 2.4.1, differences in crystal habit are not necessarily indicative of polymorphism. Further physical characterization of the individual crystals may involve measurements such as optical constants (Hartshorne and Smart 1960, 1964 Wahlstrom 1969 McCrone et al. 1978) or interfacial angles (Winchell 1943 Porter and Spiller 1951 Terpstra and Codd 1961). 

It is a pharmacopoeial requirement that suspensions should be redispersible if they settle on storage. However, the pharmacopoeias do not offer a suitable test that can be used to characterize this aspect of the formulation. In an attempt to remedy this situation, Deicke and Stiverkrtip (1999) have devised a mechanical redispersibility tester, which closely simulates the action of human shaking. The crystal habit may also affect the physical stability of the formulation Tiwary and Panpalia (1999) showed that trimethoprim crystals with the largest aspect ratio showed the best sedimentation volume and redispersibility. 

In spite of the difficulty in definitely characterizing alkaloids by definition, they do have a surprising number of physical and chemical properties in common. For the most part, the alkaloids are insoluble or sparingly so in water but form salts (by metathesis or addition) that are usually freely soluble. The free alkaloids are usually soluble in ether or chloroform, or other immiscible solvents, in which, however, the alkaloidal salts are insoluble. This permits the isolation and purification of the alkaloids as well as their quantitative estimation. Most of the alkaloids are crystalline solids, although a few are either amorphous (coniine, nicotine, sparteine) or liquid. It is interesting to note that the liquid alkaloids have no oxygen in their molecules. Alkaloidal salts are invariably crystalline, and their crystal form and habit are often useful in their rapid microscopical identification (Sollmann, 1944). 

An extremely important tool for the characterization of polymorphs and solvates is that of microscopy, since the observable habits of differing crystal structures must necessarily be different and therefore useful for the characterization of such systems. Clearly, visual observation of materials suspected of being... 

An extremely important tool for the characterization of polymorphs and solvates is that of microscopy, since the observable habits of differing crystal structures must necessarily be different and therefore useful for the characterization of such systems [27]. Common sense would dictate that the visual observation of such materials would immediately follow an x-ray crystallographic study, which would in principle make the science of optical crystallography [28-30] an essential aspect of any program of study. A review of crystallography from the pharmaceutical viewpoint is available [31]. 

No experimental study of the antifoam effect of mixing hydrophobed crystals of weU-characterized habit with oils has been reported. Experimental confirmation of the expectation of synergy only under restricted circumstances is therefore not available. However, Frye and Berg [75] have shown evidence of only weak antifoam synergy when hydrophobed ground-glass particles, which possess sharp edges (and are therefore relatively effective antifoams when used alone), are mixed with hexadecane. 

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