Crystalline substances, stability

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

This chapter describes some of the properties of solids that affect transport across phases and membranes, with an emphasis on biological membranes. Four aspects are addressed. They include a comparison of crystalline and amorphous forms of the drug, transitions between phases, polymorphism, and hydration. With respect to transport, the major effect of each of these properties is on the apparent solubility, which then affects dissolution and consequently transport. There is often an opposite effect on the stability of the material. Generally, highly crystalline substances are more stable but have lower free energy, solubility, and dissolution characteristics than less crystalline substances. In some situations, this lower solubility and consequent dissolution rate will result in reduced bioavailability. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a... 

Wohler and Streicher,13 however, have prepared the dichloride by ignition of iridium trichloride in chlorine at 770° C., and thus placed its existence beyond all doubt. It is a crystalline substance, brown in colour, and insoluble alike in acids and bases. Its limits of stability axe 763° to 773° C. in an atmosphere of chlorine. Above 773° C. it dissociates into the monochloride and free chlorine. 

In an attempt to stabilize the highly conducting a-Agl phase at lower temperatures, various anionic and cationic substitutions have been tried. The most successftd so far has been the replacement of silver by rubidium in RbAg4l5. This material has the highest ionic conductivity at room temperature of any known crystalline substance (0.27 S cm ) with an activation energy of 0.07 eV. The electronic conductivity of RbAg4l5 is negligibly small ( 10 S cm ). 

Each crystalline substance has a unique structure. Groups of compounds classified as isomorphous have similarities of lattice symmetry, but dimensions, and hence interionic forces, are different. Moreover, a particular substance can adopt alternative structures under changed conditions of temperature, pressure, crystallization conditions, presence of impurities, etc. Ordered packing, with symmetrical intracrystalline forces, appears to confer enhanced stability within the bulk solid so that decomposition processes usually occur at surfaces within a restricted reaction zone. Interfaces can be regarded variously as complex imperfections, zones of destabilizing strain, or (product) sites of catalytic activity. 

The unit cells can be packed into a three-dimensional display of the crystal lattice. The orientation of the molecules is responsible for various properties of the crystalline substance. For example, hydrogen bonding networks may provide high stability, and spaces in the structure may allow easy access of small molecules to provide hydrated or solvated forms. 

The instability of the free Br0nsted acid is recognized in the first reaction by the necessity of the presence of the basic substrate for stability of the catalyst hydrocarbon complex. The water may come from the catalyst where it is a constitutional part, existing as hydroxyl groups. These equations are illustrative only of the stoichiometry of the reactions taking place. It should be realized that these reactions involve crystalline substances and, therefore, rather widespread changes must occur in the microstructure around any one active center. Many aluminum ions around a given point may be involved in this shift. 

The inclusion complexes formed can be isolated as stable crystalline substances. Upon dissolving these complexes, an equilibrium is established between dissociated and associated species, and this is expressed by the complex stability constant Ka- The association of the CD and guest (D) molecules, and the dissociation of the CD/guest complex formed is governed by a thermodynamic equilibrium ... 

These chains are linked at each end by a nitrogen atom. Cryptands, like the crown ethers, can form coordination complexes with ions that can tit into the cavity formed by the open three-dimensional structure, i.e. they can cryptate the ion. Various types of cryptand have been produced having both spherical and cylindrical cavities. The cryptands have the same kind of properties as the crown ethers and the same uses. In general, they form much more strongly bound complexes and can be used to stabilize unusual ionic species. For example, it is possible to produce the negative Na ion in the compound [(2,2,2)-cryptand-Na] Na , which is a gold-coloured crystalline substance stable at room temperature. Cluster ions, such as Pbs ", can be similarly stabilized. 

Materials may exist as amorphous or crystalline substances. Some materials such as cortisone, tetracycline, sulfathiazole, and chloramphenicol palmitate can exist in more than one crystalline form and this property is described as polymorphism. The different crystalline forms which are known as polymorphs exhibit different degrees of stability. The lattice structure of the crystalline substances may be altered by the incorporation of molecules of the solvent from which crystallization occurs. The resultant crystals obtained are called solvates. If the solvent is water, the crystals are said to be hydrated. 

In some cases, where a drug substance is highly insoluble, associated amorphous material may be reasonably morphologically stable. In these cases it may be advantageous to include some associated amorphous material to enhance the bioavailability of the product. For both hydrophobic and hydrophilic materials, the amorphous component associated with a crystalline substance can have a profound effect on the overall performance of the product. Extreme care should be taken to understand and quantify the amorphous material. Reduction of amorphous material in poorly soluble/hydrophobic materials may drastically reduce bioavailability. An increase in amorphous content in a hydrophilic material may cause reduction in chemical stability and transformation to an inappropriate form. 

Hydrated metal sulphates have long been used to study water removal processes, and characteristic kinetic behaviour is conveniently illustrated by reference to these substances. Frost and co-workers [602,603] have investigated the structures, stabilities and adsorption properties of various intermediate amorphous phases, the immediate reaction products which can later undergo reorganization to yield crystalline phase. 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

Glasses typically are metastable substances. Like crystalline solids they exhibit macroscopic form stability, but because of their structures and some of their physical properties they must be considered as liquids with a very high viscosity. Their transition to a thermodynamically more stable structure can only be achieved by extensive atomic movements, but atom mobility is severely hindered by cross-linking. 

Hexamethylcyclotrigermathiane (22) and hexamethylcyclotristannathiane (23) also react easily with Et3P=CHMe, which enabled us to obtain the first betaines with the thiolate center in the germanium (24) and tin (25) series (Scheme 12).60,61 Both betaines are solid finely crystalline white substances, whose solubility and stability in the solid state and in solutions are similar to those of the silicon analog 20n. 

Equation (5.2) also implies that a crystalline solid becomes mechanically unstable when an elastic constant vanishes. Explicitly, for a three-dimensional cubic solid the stability conditions can be expressed in terms of the elastic stiffness coefficients of the substance [9] as... 

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