Microcrystal

Microcrystals of SrS04 of 30 A diameter have a solubility product at 25°C which is 6.4 times that for large crystals. Calculate the surface tension of the SrS04-H20 interface. Equating surface tension and surface energy, calculate the increase in heat of solution of this SrS04 powder in joules per mole. 

Allen G L, Gille W W and Jesser W A 1980 The melting temperature of microcrystals embedded in a matrix Acta Metall. 28 1695... 

To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

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). 

Primitive silver halide microcrystals free of counterions... 

Fig. 5. Silver haUde grain morphologies for (a) cubic, precipitated in an environment having a silver ion concentration, [Ag" ], of ca 2.5 x 10 mol/L (b) octahedral, ca 6.0 x 10 mol/L and (c) tabular microcrystals, ca 1.0 x 10 ° mol/L. A cross section of a tabular grain revealing double parallel twin planes...

Chemical Sensitization. After the photographic microcrystals are precipitated but before they are coated on a support, the crystals are treated to enhance their sensitivity to light. Chemical sensitization is a process which improves that abiUty of the emulsion grains to use the absorbed photons, independent of the wavelength. Various methods of post-precipitation chemical sensitization have been developed to reduce the number of photons required to produce a developable latent-image center. 

Spectral Sensitization. The intrinsic absorption, and therefore the intrinsic photographic sensitivity, of silver bromide and silver iodobromide microcrystals falls off rapidly for wavelengths greater than 500 nm (see Fig. 2). In fact, silver chloride crystals have almost no sensitivity in the visible... 

Hardness. The Knoop indentation hardness of vitreous sihca is in the range of 473—593 kg/mm and the diamond pyramidal (Vickers) hardness is in the range of 600—750 kg/mm (1 4). The Vickers hardness for fused quartz decreases with increasing temperature but suddenly decreases at approximately 70°C. In addition, a small positive discontinuity occurs at 570°C, which may result from a memory of quartz stmcture (165). A maximum at 570°C is attributed to the presence of small amounts of quartz microcrystals (166). Scanning electron microscopic (sem) examination of the indentation area indicates that deformation is mainly from material compaction. There is htfle evidence of shear flow (167). 

Over 250,000 metric tons of microcrystaUine cellulose have been sold siace its commercialisation ia 1962 and demand continues to iacrease. Its utihty has led to development of other coUoidal polymer microcrystals (see Colloids). For example, polyamides and polyesters from recycled materials can be biodegraded to give microcrystals having a size of 30 nm (37). 

Mechanism of action of nanosized (0.005 - 0.02 p.m) powders of ferromagnetics on biological systems is based on effect of magnetic fields created by ferromagnetic microcrystal assemblies and on specific action of every metal added which determined the field of practical application. 

Progress in deducing more structural details of these fibers has instead been achieved using NMR, electron microscopy and electron diffraction. These studies reveal that the fibers contain small microcrystals of ordered regions of the polypeptide chains interspersed in a matrix of less ordered or disordered regions of the chains (Eigure 14.9). The microcrystals comprise about 30% of the protein in the fibers, are arranged in p sheets, are 70 to 100 nanometers in size, and contain trace amounts of calcium ions. It is not yet established if the p sheets are planar or twisted as proposed for the amyloid fibril discussed in the previous section. 

An elegant NMR experiment by the group of Lynn Jelinski at Cornell University has established that at least part of the microcrystals is built up from the polyalanine repeats in the protein chains. These experiments, which were made on C-enriched proteins produced by feeding the spiders C-labeled alanine, showed that there were two populations of alanine side chains, one ordered and oriented perpendicular to the fiber axis and a second less ordered. Jelinski s interpretation is that parts of the polyalanine sequences are incorporated as p strands in the microcrystals with an orientation parallel to the fiber axis. Whether or not the Gly-Gly-X repeats also form P strands in the microcrystals remains an open question. 

Silk fibers, which have incredible strength, comprise well-ordered microcrystals of P-sheets that make up about 30% of the protein mass, interspersed in a matrix of polypeptide chains without order. The p strands of the sheets are oriented parallel to the fiber axis. 

In general, the R factor is between 0.15 and 0.20 for a well-determined protein structure. The residual difference rarely is due to large errors in the model of the protein molecule, but rather it is an inevitable consequence of errors and imperfections in the data. These derive from various sources, including slight variations in conformation of the protein molecules and inaccurate corrections both for the presence of solvent and for differences in the orientation of the microcrystals from which the crystal is built. This means that the final model represents an average of molecules that are slightly different both in conformation and orientation, and not surprisingly the model never corresponds precisely to the actual crystal. 

Jezequel, P.-H., 2001. The concept of scaleable reactor in the precipitation of silver halide photographic microcrystals. Chemical Engineering Science, 56(7), 2399-2408. 

Electrochemical methods may also be used in the synthesis of chalcogen-nitrogen compounds. For example, the electrochemical reduction of salts of the [SsNs]" cation (Section 5.3.9) in SO2 or CH2CI2 at low temperatures produces microcrystals of the superconducting polymer (SN). ... 

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