Solvent Occlusion

Additional work on the fundamental mechanisms of occlusion formation under various crystallization conditions is needed. The interactions of mixing and addition rates with rates of nucleation and growth, and with interfacial stability need further elueidation. 

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Microscopy is used to identify crystal morphology and size, and to assess physical form issues like agglomeration and solvent occlusion. It can be used to observe polymorphic transformations in real time with the addition of a hot stage fitting. 

Shiu K-B, Lee H-C, Lee G-H, Wang Y (2002) Synthesis, structures, and solvent-occlusion properties of a molecular loop and a molecular square using tetracarbonyl- and diphosphine-ligated diruthenium(I) as building blocks and dicarboxylates as linkers. Organometallics 21 4013 1016... 

The silver-silver bromide electrodes were of the thermal type, made by heating a paste composed of 90 wt % silver oxide and 10% silver bromate, with a little water, to 600°C on a helix of platinum wire. In order to avoid solvent occlusion, the electrodes were not pretreated before use in the cells. After each run, the electrodes were cleaned and remade before immersion in a solution of a different composition. 

The ultimate particle size distribution (PSD) from crystallization is dictated by the balance between nucleation and crystal growth rates. The processes indicated above, because of very high supersaturation, often result in rapid nucleation of too many particles and smaller than desired final product. The rapid nucleation and growth occurring may also result in impurity and/or solvent occlusion. 

Solvent occlusion is shown to be directly related to the ratio of nucleation rate over crystal growth rate. To minimize the occlusion, fine seed, extended addition time, and better mixing were found to be effective. 

The most common sample forms used for the measurement of electrical properties are pressed pellets, thin films, and single crystals. Electrical measurements on pellets are often difficult to interpret because of the presence of polycry staUinity, grain boundaries, large surface area, and unknown amounts of adsorbed gases. Films are useful in some instances, but in many cases also cause uncertainties because of poly cry staUinity. Electrical properties are most easUy and reUably interpreted with single crystals free from unintentional impurities and solvent occlusions. 

II. A method described by Hardt avoids contamination of the product by ammonium salts and solvent occlusions and makes purification by sublimation unnecessary. 

Tetrahydrofurfuryl alcohol 2,2,2-Trifluoroethanol solvent, occlusive Wheat (Triticum vulgare) germ oil solvent, occlusive make up Cocoa (Theobroma cacao) butter solvent, occlusive ointments Walnut (Juglans regia) oil solvent, occlusive OTC pharmaceuticals Cocoa (Theobroma cacao) butter solvent, occlusive pharmaceutical topicals Walnut (Juglans regia) oil solvent, occlusive sun preps. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

In these cases, the product is reslurried with pure liquor or fresh solvent if the solubility is not too high, and refiltered. In order to meet the required product impurity level, several such washings may take place in series. See Coulson and Richardson (1991) and Mullin (2001) for design guides and examples calculations. It is noted that impurities retained within liquid occlusions are particularly difficult to remove without first crushing the crystals. 

Bound moisture This is moisture retained within the solid such that it exerts a vapour pressure less than that of free solvent (Figure 4.24). Such solvent may be adsorbed on the surface, retained in capillaries or within cells or occlusions of liquor. The latter can be difficult to remove without resorting to high temperatures, which may damage the crystals. 

Decimal applications of test material in suitable solvent/carrier on shaved skin site for 6 hours under occlusive patch except for the first which is for 24 hours. 

Since most of our observations on the reacting systems were made by means of conductivity measurements it is necessary to remember that in these systems the only factor which increases conductivity is an increase in the concentration of ions, but that a decrease of conductivity could be due to any or all of the following effects increase of size of cation by polymerisation, increase of viscosity of solvent due to polymer, occlusion of ions in precipitated polymer, trapping of polymer between the electrodes. A similar list was given by Matyska in one of the earliest applications of conductivity measurements to a cationic polymerisation, that of isoprene by aluminium bromide in toluene solvent [19]. 

These results conclusively demonstrate that precipitation of polyacrylonitrile as a fine powder and occlusion of growing chains resulting in post-polymerization do not bring about autoacceleration if a highly polar solvent is present in the system. 

Abraham et al. were the first ones to propose saturating commercially available microporous polyolefin separators (e.g., Celgard) with a solution of lithium salt in a photopolymerizable monomer and a nonvolatile electrolyte solvent. The resulting batteries exhibited a low discharge rate capability due to the significant occlusion of the pores with the polymer binder and the low ionic conductivity of this plasticized electrolyte system. Dasgupta and Ja-cobs patented several variants of the process for the fabrication of bonded-electrode lithium-ion batteries, in which a microporous separator and electrode were coated with a liquid electrolyte solution, such as ethylene—propylenediene (EPDM) copolymer, and then bonded under elevated temperature and pressure conditions. This method required that the whole cell assembling process be carried out under scrupulously anhydrous conditions, which made it very difficult and expensive. 

A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer influence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 

There is no general rule for the prevention or avoidance of occlusion. In some cases, a change of solvent/precipitant system may help to achieve this goal. Raising the drying temperature is also beneficial. 

Liquid-liquid partitioning constitutes tlie most common form of solvent extraction. It is a relatively simple, rapid, and flexible procedure that is readily applicable to all types of matrices and a wide range of analytes ranging from fairly polar to nonpolar compounds (54). Despite the fact that they are simple and rapid, liquid-liquid extractions may result in highly selective isolation (55, 56). However, they also necessitate use of toxic and inflammable solvents, favor formation of emulsions, may cause sample losses by occlusions or adsorption onto glass surfaces, and are often laborious and costly. 

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