Crystallization mixed-crystal formation

This procedure can be applied to most P2P mixes but is especially effective on the methods to follow. However, in super clean methods, such as the PdCl2 below, where lots of isosafrole is produced, the iso byproduct can interfere with crystal formation. Someone-Who-ls-Not-Strike once found that when an appreciable amount of isosafrole was formed to the detriment of MD-P2P, the oil screwed up the crystal matrix disallowing it to form. Confused, the chemist tried to rescue the uncrystallized oil from the aqueous solution by extracting out the oil to try other things. But when the solvent hit the solution, the P2P crystallized out. Go figure The... 

Alkali-mctal-mercury compounds decompose rapidly in O2 or moisture and must be prepared in melt atmospheres or under vacuum. To prepare NaHg, NaHgj and Na,Hg2, known amounts of Hg and Na are flushed with N2. The reaction is exothermic, and mixing is carried out slowly, homogeneity being achieved by shaking. An excess of Na or Hg is necessary for crystal formation. ... 

Cesium salts of these compounds crystallize in the /3-K2SO4 form with statistical orientation of the anions. Therefore, one observes no splitting of the degenerate vibrations (see Table 23) similar to anions with C3 S5mimetry such as KCrOsF and KOsOaN (164). The structures of these compounds are discussed in Ref. (160). Due to the method of preparation, the solubilities and the nature of the structure, i. e. the possibility of mixed crystal formation, the r-values are not exactly integers (165). 

Table 3 Liquid Crystal Formation in Mixed systems of...

It is evident that telluric acid is notably different from what might be expected by analogy with sulphuric and selenic acids. One is hardly surprised, therefore, that isomorphism is exceedingly rare between the tellurates and the sulphates or selenates.3 The tellurates do not form alums, they do not as a rule form mixed crystals with the sulphates or selenates, the only fairly satisfactory case of mixed-crystal formation being with rubidium hydrogen sulphate and rubidium hydrogen tellurate. 

The mixed crystal formation in ethylene/vinyl alcohol copolymers was previously quoted 3). The examination of the X-ray data shows a different structure from both that of polyethylene and of polyvinyl-alcohol. The repeat along the chain axis is 2.5 A, indicating a planar... 

We will first quote the case of mixed crystal formation among polymers containing different isotopic species, such as isotactic polypropylene and polydeuteropropylene (52). In such a case, not only the resulting crystallinity is of the same order of magnitude as that of the separate pure polymers, but the X-ray diffraction effects are virtually identical, due to the same van der Waals radii of hydrogen and deuterium. 

Calcium oxalate is found to be a good carrier of rare earth and thorium activities through mixed crystal formation [3921 396]. The... 

Mixtures of poly(vinylidene fluoride) with poly (methyl methacrylate) and with poly (ethyl methacrylate) form compatible blends. As evidence of compatibility, single glass transition temperatures are observed for the mixtures, and transparency is observed over a broad range of composition. These criteria, in combination, are acceptable evidence for true molecular intermixing (1, 19). These systems are particularly interesting in view of Bohns (1) review, in which he concludes that a compatible mixture of one crystalline polymer with any other polymer is unlikely except in the remotely possible case of mixed crystal formation. In the present case, the crystalline PVdF is effectively dissolved into the amorphous methacrylate polymer melt, and the dissolved, now amorphous, PVdF behaves as a plasticizer for the glassy methacrylate polymers. 

When TAGs in the liquid state are mixed, no changes in heat or volume are observed (Walstra et al., 1994). However, ideal behavior is not observed in the solid phase of milk fat (Timms, 1984 Walstra et al., 1994). As a result, the melting curve of milk fat does not equal the sum of its component TAGs (Walstra et al., 1994). Mulder (1953) proposed the theory of mixed crystal formation to explain the complex crystallization behavior of milk fat. Mixed or compound crystals contain more than one molecular species (Rossell, 1967 Mulder and Walstra, 1974). Mixed crystals form in natural fats, like milk fat, which are complex mixtures of TAGs (Mulder, 1953 Sherbon 1974 Walstra and van Beresteyn, 1975b Timms, 1980 ... 

Although the a-polymorph is meta-stable, it can have a relatively long lifetime in milk fat at low temperatures compared to other fats (Walstra et al., 1994). The a-crystal may be stabilized by the formation of compound crystals in milk fat (Walstra et al., 1999). A consequence of both polymorphism and mixed crystal formation in milk fat is that the material is rarely at equilibrium (Walstra et al., 1994). 

Coprecipitation — The -> precipitation of a normally soluble substance that is carried down during the precipitate formation of the desired substance. The coprecipitation of a substance arises from processes such as - adsorption, -r mixed-crystal formation, - occlusion and/or mechanical entrapment [i]. 

An awareness of crystal packing characteristics and polymorphism helps one to understand incompatibility problems of different fats. Crystal formation has specific demands, and individual crystals in mixed systems each consists of only one species ofTAG. However, surfactants and other molecules can act as impurities and interrupt crystal growth. Different TAG are considered compatible when they co-crystallize as separate crystals under the same conditions without the formation of a eutectic. 

In the application to natural minerals some complications occur, however, through multiple mixed crystal formation. Thus, in particular, a part of the Si4+ in the oxygen tetrahedra is often arbitrarily replaced by Al3+ but there is often aluminium as well which does not belong to the tetrahedra of the skeleton. Furthermore a part of the oxygen ions is sometimes present as OH, sometimes isomorphously replaced by F, which is of the same size. The OH and F never form part of the skeleton of oxygen tetrahedra in fact this would be in contradiction with the above formula for zA, which can only be one in this case. In older chemical analyses and the empirical formulae based on them the OH ions were, however, often incorrectly reckoned as water of crystallization. The earlier division into... 

Solvents are important ingredients of emulsifiable concentrates and of solution formulations. When the formulation is to be used on crops, it is critical that the solvent be nonphytotoxic. The solvent must have a high level of solvent power if an EC is being formulated. Because most toxicants are insoluble in water, the solvent must also be water insoluble. Otherwise, when the EC is added to water in the spray tank, the solvent will mix with the water and leave the toxicant behind as a crystalline precipitate. The carrying power of the solvent, i.e., the amount of pesticide it will hold in solution, is important in the storage stability of formulations. If near its saturation point at ordinary temperatures, it may exceed this at low temperature with the result that solvent and pesticide may separate, causing crystal formation and phase separation (Terriere, 1982). 

Although protein microheterogeneity usually disrupts crystal formation, it can occasionally promote crystallization. For example, crystals of Escherichia coli single-stranded binding protein contain a 1 1 mixture of intact and proteolyzed protein (Ollis et ai, 1983). While crystallization experiments were initially conducted with intact protein, crystals grew only when enough molecules to form the mixed crystals had been cleaved by contaminating proteases. 

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