Dispersed microcrystals

The first satisfactory photographic film was produced in 1888 when gelatin-dispersed microcrystals of silver haUde were coated on celluloid sheets (23). Within a year George Eastman prepared and marketed toU films on a base produced by dissolving nitrocellulose with camphor and amyl acetate in methanol (qv). 

Details about preparation and characterization of dispersed microcrystals can be found in review chapters [322] and will not be dealt with here. All investigations indicate that the properties of microcrystals differ considerably from those of bulk metals (and from those of adatoms and thin films as well) [328], and that they can also be influenced by the nature and texture of the support. In particular, micro-deposits of precious metals on various inert supports (Ti, Ta, Zr, Nb, glassy carbon etc.) exhibit enhanced electrocatalytic effects as evaluated per metal atom, while the mechanism of H2 evolution remains the same [329], and the enhancement increases as the crystallite size decreases [326, 331] (Fig. 17). However, while this is the case with Rh, Pt, Os and Ir, Pd shows only an insignificant increase, whereas for Ru even a drastic decrease is observed [315, 332]. Thus, the effect of crystal size on the catalytic activity appears to depend on the nature of the catalyst (without any relation with the crystal structure group) [330]. 

Thus, a high-adsorption potential of MA catalysts Influences their Increased selectivity in hydrogenation of different compounds. In their turn. specific adsorption properties of MA catalysts are due to peculiarities of their structure.which can be imagined as a sinter of high-dispersion microcrystals of an active phase that are fixed on particles of an unleached initial aluminide. 

Many compounds, including clay minerals, form needle- or plateshaped crystals. With finely dispersed minerals, the electron diffraction method can give a special kind of diffraction pattern, the texture pattern, which contains a two dimensional distribution of a regularly arranged set of 3D reflections [2], Specimens of fine-grained lamellar or fiber minerals, prepared by sedimentation from suspensions onto supporting surfaces or films, form textures in which the component microcrystals have a preferred orientation. Texture patterns of lamellar crystals tilted with respect to the electron beam are called oblique texture electron diffraction patterns [1]. 

The photographic emulsion is essentially a dispersion of microcrystals of AgBr in an organic substrate called gelatin . The emulsion therefore consists of microscopic grains which have a specific size distribution. This distribution is important in determining the sensitivity and the resolution of the emulsion. 

The preparation of pure mercury is not difficult. Any metal with a standard potential more negative than that of mercury may be oxidized easily (with the exception of nickel, which forms a mercury intermetallic compound) by dispersing mercury into a solution of its salts acidified with HN03 and saturated with oxygen. Metals insoluble in mercury may be also removed this way, although the process may be slow. More effective in this respect is the separation of metal microcrystals by filtration. The elimination from mercury of metals more noble than itself (as well as less noble metals) is accomplished by distillation under reduced pressure. Usually such distillations are repeated several times. Triple-distilled mercury is commonly used for electrochemical purposes. 

While it may be reasonable (at least to a first approximation) to distinguish between resonant atoms in a surface shell and those in the bulk, the application of this model to particle size determination is also complicated by the possible presence of lattice modifications (e.g., expansions or contractions) for small particles. This point has been emphasized by Schroeer (752) and interpreted in terms of an internal pressure. For example, Ktindig et al. (143) found a nearly linear dependence with dispersion of the quadrupole splitting e for a-Fe203 microcrystals. As noted above, this was explained in terms of the shell model. However, if the lattice parameter a of a 5-nm particle is increased over the bulk value by 2% (755), then the increase in the quadrupole splitting Ae with decrease in particle size may be related to a corresponding increase in the lattice parameter A a by (Ae/e)/(Aa/a) 65... 

Studies with adatoms and microcrystals have clearly shown that the electronic structure of these systems are profoundly different from those of the corresponding bulk material. Electrocatalytic activation can be expected only as the structure of the material is substantially changed (cf. Raney metals). The case of DSA has shown how widely the properties of a material can be varied as it is obtained in a poorly crystalline, dispersed, non-stochiometric form. 

Highly dispersed powders can be prepared in several ways. When prepared by combustion of metallic zinc, the resulting very pure ZnO (with 10 m2g-1 specific surface area) is constituted of microcrystals characterized by well-defined elongated prismatic habits (Fig. 14) exposing preferentially unreconstructed (1010) and (1120) faces (394). When prepared by decomposition of ZnC03, the resulting high-surface-area powder ( 50 m2g ) is constituted of very small microcrystals with ill-defined shapes. 

Abstract. Gas interstitial fullerenes was produced by precipitation of C6o from the solution in 1,2 dichlorobenzene saturated by O2, N2, or Ar. The structure and chemical composition of the fullerenes was characterized by X-ray powder diffraction analysis, FTIR spectroscopy, thermal desorption mass spectrometry, differential scanning calorimetric and chemical analysis. The images of fullerene microcrystals were analyzed by SEM equipped with energy dispersive X-ray spectroscopy (EDS) attachment. Thermal desorption mass spectroscopy and EDS analysis confirmed the presence of Ar, N and O in C60 specimens. From the diffraction data it has been shown that fullerite with face centered cubic lattice was formed as a result of precipitation. The lattice parameter a was found to enhance for precipitated fullerene microcrystals (a = 14.19 -14.25 A) in comparison with that for pure C60 (a = 14.15 A) due to the occupation of octahedral interstices by nitrogen, oxygen or argon molecules. The phase transition temperature and enthalpy of transition for the precipitated fullerene microcrystals decreased in comparison with pure Cgo- Low temperature wet procedure described in the paper opens a new possibility to incorporate chemically active molecules like oxygen to the fullerene microcrystals. 

In natural cellulose, the microcrystals are packed tightly in the fiber direction in a compact structure resembling bundles of wooden match sticks placed side by side. Unhinging the interconnecting chains by acid treatment does not destroy this structure. However, the unhinged crystals are now free to be dispersed by mechanical disintegration.. . . We immediately set out to explore this new avenue, developing uses for colloidal dispersions of microcrystalline celluloses, known commercially as Avicel. 

The photochemical fragmentation of 7-methyl-2,2,5-triphenyl-l-oxa-5,6-diaza-spiro-[2,4]-hept-6-en-4-one has been studied." Interest in solid state photochemistry continues to burgeon. The present paper" discusses the problems associated with the proximity of the components of radical pairs or biradicals within the constrictions of the crystalline environment. This problem has been addressed by examining the photochemical reactivity of a series of cyclohexanone derivatives (119) whose solution-phase photochemistry is well known. The irradiations, using X = 350 nm, were carried out on microcrystals dispersed in potassium bromide. The influence of the conformations within the crystals and substitution were studied. The relative yields of the product, the corresponding cyclopentane, are shown beside the appropriate structure." ... 

Carrot extracts (E 160(a)), carrot oil, palm oil, and related plant extract are also available on the market. Their main components are P- and a-carotenes (Formulae 9.1 and 9.2, respectively). Processes for the commercial extraction of carotene from carrots were developed. Purified crystalline products contain 20% a-carotene and 80% P-carotene and may be used for coloring fat-based products as dispersion of microcrystals in oil. 

It has been claimed ( ) that a slight excess of neutralizing agent could be present In the material and could appear as microcrystals dispersed in the sample. However It has not been In evidence otherwise in the bulk samples quantitatively neutralized in the same conditions, using for Instance EXAFS spectroscopy (5). 

Most organic compounds crystallize in the monoclinic or orthorhombic crystal systems, which contain substantial macroscopic anisotropies, and thus the singlecrystal CD technique cannot be applied, although our method of measurement may be useful if the macroscopic anisotropies are not very large. An alternative way to obtain crystalline-specific information is to examine the microcrystalline state. Measurements can be usually made either in nujol mull or KBr disc form, where microcrystals are dispersed randomly either in nujol or in a KBr microcrystalline matrix. The method was developed and applied to inorganic complexes by Mason [34], Bosnich [35], and Kuroda [9,10], and since then its application to metal complexes has been carried out sporadically [36,37]. Recently, the technique has become popular in the field of organic chemistry as well, probably stimulated by our work [38]. An alternative technique recently developed by us is diffuse reflectance CD (DRCD) which will be briefly described in V.B.2. 

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