Metastable, definition

Whilst programmed eooling (i.e. operation at eonstant nueleation rate within the metastable zone) inereases the mean produet erystal size ef. natural eooling, is it the optimum in produeing the largest possible erystals The problem is to find the maximum of the integral of erystal growth over the bateh time. Thus beeause bateh operation is by definition transient, a funetional has to be maximized over time rather than just a funetion at some point in time. Jones (1972, 1974) addressed this problem by applieation of a partieular result in... 

Compound 6 crystallizes from cyclohexane as colorless needles which have no definite melting point there is a change of color to yellow at 128-134 C and the compound then melts sharply at 187-189 r C. When the colorless form is kept for a long time or recrystallized from pyridine or dimethyl sulfoxide it is changed into the yellow modification of mp 187-189 C recrystallization from cyclohexane reverses the process. It has been suggested that the yellow stable form has structure 6A and that the colorless metastable compound is the tautomer 2-methyl-l//-pyrido[2,3-6][l, 4]diazepin-4(5//)-one (6B). There is evidence from 1H NMR spectroscopy that the isomeric pyridodiazepin-2-one, yellow crystals, mp 195—197 " C, exists as an inseparable mixture of the tautomers 4-methyl-l//-pyrido[2,3-6][l,4]diazepin-2(3//)-one (7 A) and 4-methyl-l H-pyrido[2,3-6][l, 4Jdiazepin-2(5//)-one (7B) in the ratio 1 3. 

Unfortunately, even for low molecular weight material it is difficult to obtain clear experimental evidence for a roughening transition [71]. This is mainly due to the fact that during growth the interface generally assumes a metastable shape and relaxation times are long and increase with crystal size. Therefore we certainly cannot expect a definitive answer for macromolecules. We shall therefore just make several comments which hopefully will be of use when reading the literature. 

There are other soUd states which sometimes confuse the measurement and definition of solubiUty. The dmg may crystaUize as a hydrate, i.e. under inclusion of water molecules. If the hydrate form is more stable than the pure form it may be difficult to measure the intrinsic solubility of the drug at all. Often drugs tend to precipitate in an amorphous form, often under the inclusion of impurities. As with metastable polymorphs, such amorphous precipitates may lead to erroneously high solubility measurements. CommerciaUy, drugs are often crystallized in salt form, e.g. as the hydrochloride salt, a cation with a chloride anion. In these co-crystallized salts, a much lower solubility than the intrinsic solubility will typi-... 

If the system is kept at fixed temperature, its thermal fluctuations will explore the configurations around X and may eventually reach the activated configuration, from where it descends to Y. The system thus has a definite probability per unit time, R, of escaping from its metastable state near X. The rate of transfer increases with temperature. 

The possibility to have metastable hadronic stars, together with the feasible existence of two distinct families of compact stars, demands an extension of the concept of maximum mass of a neutron star with respect to the classical one introduced by Oppenheimer Volkoff (1939). Since metastable HS with a short mean-life time are very unlikely to be observed, the extended concept of maximum mass must be introduced in view of the comparison with the values of the mass of compact stars deduced from direct astrophysical observation. Having in mind this operational definition, we call limiting mass of a compact star, and denote it as Mum, the physical quantity defined in the following way ... 

Since the large majority of metalloid clusters E Rthis section is divided in two parts, including the few clusters for In hitherto known. For many of the metalloid clusters discussed in this section (for a definition of metalloid cf. Section 2.3.1, Introduction) the technique of cryochemistry is essential, i.e., trapping of a high-temperature species together with an excess of a suitable solvent in order to obtain a metastable solution. Detailed descriptions and discussions of this technique have been presented recently [7-12],... 

Note 4 If a mixture is thermodynamically metastable, it will demix if suitably nucleated (see Definition 2.5). If a mixture is thermodynamically unstable, it will demix by spinodal decomposition or by nucleation and growth if suitably nucleated, provided there is minimal kinetic hindrance. 

Note 3 A miscible system can be thermodynamically stable or metastable (see Note 4 in Definition 1.2). 

Note In a metastable region of a phase diagram (see Definition 1.2), phase separation is initiated only by nucleation. 

Curve defining the region of composition and temperature in a phase diagram for a binary mixture across which a transition occurs from miscibility of the components to conditions where single-phase mixtures are metastable or unstable (see Note 4 in Definition 1.2). 

This requires a methodology for characterising a large range of metastable solutions and compoimds which, by definition are difficult, if not impossible, to access experimentally. The available methods involve various levels of compromise between simplicity and accuracy and can be categorised by the choice of atomic properties used in the process. 

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