Crystallization general

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Prolonged Action Parenterals Injections. Intramuscular injections have been developed to achieve prolonged therapeutic effects. This can be accompHshed by suspension of dmg particles in oils or flowable gels, from which the dmg slowly diffuses. Aqueous suspensions can also provide such therapeutic response. In these cases, the soHd dmg crystals generally are quite water insoluble and of a controlled particle size and crystallized form. 

Crystallization generally involves the evaporation and subsequent cooling of a solution to the point of supersaturation, whereupon the formation of crystals takes place. Modern technology often focus on the control of crystal size, since product demands frequently are rigorous in this regard. The process of crystallization is often conducted in evaporators. As in the evaporation of salt and in the recovery of salt and glycerin in soap manufacturing, salt separators are used to remove crystallized materials as rapidly as it settles. 

Calamitic liquid crystals generally contain two or more rings, at least one flexible chain and often a small, polar function in some cases, small, lateral groups (normally fluorine) are attached. Some will contain metal... 

Solution crystallization, general separation heuristics for, 22 319-320 Solution-diffusion (SD) models, 21 639-640, 660... 

Emsley, J.W. Liquid crystals general considerations, vol. 4, Wiley, Chichester 1996. 

Feher G, Kam Z. 1985. Nudeation and growth of protein crystals general principles and assays. Methods Enzymol 114 77-112. 

It is also of practical interests to control hydrate decomposition. The early studies were intended to understand the mechanism of hydrate decomposition when plugging in pipeline was encountered. The hydrate crystals generally decompose by de-pressurization (Kelkar et al., 1998, Peters et al., 2000 Hong et al., 2006). Thermal stimulation has also been considered in order to provide strategies for methane recovery from the natural hydrate by thermal stimulation (Selim and Sloan, 1989 Ji et al., 2001 Hong et al., 2003 Hong and Pooladi-Darvish, 2005). Hydrate decomposition studies may also be applicable to gas storage in hydrates. 

Forty parts of the decahydrated salt are boiled in a flask with 8 to 10 parts of water until everything is dissolved, and no monohydrated salt is formed. Close the flask with a cork fitted with two glass tubes. Pour in alcohol as in the preparation of the a-salt, and keep the fiask closed till the two liquids have mixed. Rectangular crystal-plates of the / -salt are formed—the growth of the crystals generally begins near the zone of contact of the two liquids. The crystals are left in contact with the alcohol six to eight days after the salt liquor has been drawn off. 

Diffusion of relatively small atoms that normally occupy interstitial sites in the solvent crystal generally occurs by the interstitial mechanism. For example, hydrogen atoms are small and migrate interstitially through most crystalline materials. Carbon is small compared to Fe and occupies the interstitial sites in b.c.c. Fe illustrated in Fig. 8.8 and migrates between neighboring interstitial sites. 

For amorphous compounds stored isothermally, the time for crystallization generally decreases with an increase in temperature, due to an increase in viscosity (Saleki-Gerhardt and ZograL, 1994) A mixture of two amorphous compounds will exhibit a single glass transition temperature intermediate between the two original components (Hancock and ZograL, 1994). If the densities of the... 

Nucleation and growth processes such as glass crystallization generally follow the Johnson-Mehl-Avrami (JMA) model ... 

Approximately one part of hydrated lime for every two parts of liquor is added slowly over a one-hour period while the temperature is raised to about 95°C. The precipitated calcium citrate is filtered on a vacuum filter, and the filtrate, free of citrate, is discarded as waste. The calcium citrate cake is moved to acidulation tanks, where it is acidified with dilute sulfuric acid. It is then filtered, and the citric acid mother liquor is decolorized by a charcoal treatment. The purified liquor is concentrated in a vacuum evaporator, run into a crystallizer where, upon cooling, citric acid crystallizes, generally in the form of the monohydrate. The resulting acid is of USP grade. 

The most commonly used methods for isolation and purification of phospholes are crystallization, generally performed at low temperature, and column chromatography on alumina or silica gel. A wide variety of solvents have been used as eluents, including protic solvents such as methanol. 

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