Solutions metastable

As-polymerized PVDC is not in its most stable state annealing and recrystaUization can raise the temperature at which it dissolves (78). Low crystallinity polymers dissolve at a lower temperature, forming metastable solutions. However, on standing at the dissolving temperature, they gel or become turbid, indicating recrystaUization into a more stable form. 

Some guidelines have been provided for defining the metastable region. If the seed crystals dissolve when added to the metastable solution, this implies that saturation conditions have not been reached. If the addition of the seed leads to the formation of an oil dispersion, it may be concluded that supersaturation has been realized (Anderson, 2000). 

A metastable solution of xylene/diethyl ether reacts in the temperature range -78 to 30°C with solid LiN(SiMe3)3 to give black crystals identified as [Al7 N(SiMe3)2 6F [Li(OEt2)3]+. The central Al atom of the anion 1 (Fig. 3) is surrounded by a distorted octahedron of six further Al atoms, each of which is bonded to one NR2 ligand. 

SBF is a solution that has inorganic ion concentrations similar to those of human blood plasma but does not contain any cells or protein. A brief summary of SBF, introduced by Cho et al. [17], follows. The ion concentrations of SBF are given in Table 11.1 [17]. The pH of SBF is typically adjusted to 7.25 or 7.40 at 36.5 °C. This fluid is a metastable solution containing calcium and phosphate ions supersaturated with respect to hydroxyapatite. SBF is prepared by successively dissolving the reagent-grade chemicals in ultra-pure water in the order given in Table 11.2 [17]. Each new chemical is added after the previous one has completely dissolved. The temperature... 

This equation defines the internal surfaces in the system. The model has been studied in the mean held approximation (minimization of the functional) [21-23,117] and in the computer simulations [77,117,118], The stable phases in the model are oil-rich phase, water-rich phase, microemulsion, and ordered lamellar phase. However, as was shown in Refs. 21-23 there is an infinite number of metastable solutions of the minimizahon procedure ... 

In tracing a reaction path, likewise, we may find a mineral in the calculation results that is unlikely to form in a real system. Quartz, for example, would be likely to precipitate too slowly to be observed in a laboratory experiment conducted at room temperature. A model can be instructed to seek metastable solutions by not considering (suppressing, in modeling parlance) certain minerals in the calculation, as would be necessary to model such an experiment. 

The clusters described so far have in common, that the number of metal atoms is less or equal than the number of substituents. However, there is a still growing number of both neutral and anionic clusters in which the number of metal atoms is larger than the number of substituents. As a consequence, these metal-rich clusters contain naked metal centers which are only bonded to other metal centers. Schnockel referred these ones to as metalloid clusters. Several metalloid A1 and Ga clusters were prepared by standard salt elimination reactions using metastable solutions of metal subhalides MX (M = Al, Ga X = Cl, Br) as well as solutions of Gal. Since the metal subhalides were found to play the key role for the successful synthesis of this particular class of compounds, they will be discussed first. (For excellent review articles see Refs 273 and 274.)... 

Metastable solutions of GaX (X = C1, Br) as well as conventionally prepared Gal react with lithium or sodium organometallics in standard salt metathesis reactions with formation of M2R4 (Section 3.07.4.1) as well as neutral and anionic clusters of the type MnRm]x m > n) (chapter 4.1). Moreover, metalloid clusters [MnRm]x (m < n), which feature different types of metallic core structures, have been obtained. Their formation strongly depends on the reaction conditions, in particular the reaction temperature, and the (donor) solvent. 

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],... 

A method for the synthesis of Al and Ga clusters has been described by Schnepf and Schnockel (2002) which is based on the preparation of the gaseous monohalides subsequently isolated in metastable solutions at — 78°C. The halogen atoms are substituted by bulky groups and, in a parallel disproportionation reaction, large Al (or Ga) clusters are formed. 

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. 

The action of the complex is compared to that of the acidic PL phosphatidyl serine, phosphatidyl inositol and phosphatidic acid. These PL first remove calcium, and a small amount of phosphate from the metastable solution, forming a material similar to the complex isolated from bone, and then form HA. The rate of HA proliferation, once phosphatidyl serine and phosphatidyl inositol are converted to Ca-PL-P04 complexes, is the same as the rate observed for comparable weights of the complex isolated from bone110 ... 

However, methane-diphosphonate could not prevent the growth of apatite crystals in vitro on prepared sinews of rats tail out of a metastable solution with calcium and phosphate ions. On the contrary, the precipitated crystalline particles were bigger and better crystallized than those from control solutions. This is in surprising contrast to most of the information from the literature. No other calcium phosphate minerals besides apatite have been found by X-ray diffraction, whereas under comparable conditions brushite and octacalcium phosphate grow on collagenous sinews549. ... 

Riboflavin, a yellow solid, has a low solubility of 100 mg /1 at 25°C. Three crystalline forms are known. One of these, the "readily soluble form," is ten times more soluble than the others and can be used to prepare metastable solutions of higher concentration. One crystalline form is platelike and occurs naturally in the tapetum (Box 13-C) of the nocturnal lemur. 

The metastable (supersaturated) zone between lines AB and CD where spontaneous crystallization is improbable. However, if a crystal seed were placed in such a metastable solution, growth would occur on the seed. 

Because of the low reactivity of the surface of CNTs, fluorination was taken into consideration as one of the first sidewall functionalization reactions [27]. Fluorine as the most electronegative element in its elemental form is a powerful oxidizer. Mickelson et al. reported in 1998 extensive controlled and nondestructive sidewall fluorination of SWCNTs (Fig. 1.3) [28]. The functionalized F-SWCNTs dissolved well in alcohol and gave long-living metastable solutions [29]. 

Facile routes for the convenient syntheses of similar complexes of the aluminum(II) subhalides do not exist at all. These compounds [X2(L)A1-A1(L)X2 (X = Cl, Br, I L = NMe2SiMe3, OEt2, PEt3, MeOPh)] were obtained in low yields by the treatment of metastable solutions of aluminum monohalides A1X with the corresponding donor substances similar to the reactions described in Eq. (3).33 35 A12C14 was supposed to be formed from elemental A1 and A1C13 at 120 °C in a solvent, however, it was not isolated in a pure form and was characterized on the basis of some not really specific reactions.36 In view of the results obtained recently with respect to the synthesis and stability of aluminum subhalides,33 35 it seems to be rather implausible that indeed considerable concentrations of a subhalide had formed. Quantum-chemical calculations... 

In the saturator process (see Figure 12.7), neutralization and crystallization are carried out in the same vessel. The sulfuric acid is delivered to the suction side and the ammonia to the pressure side of the forced circulation pump. Crystallization of the metastable solution gives particle sizes generally between 0.5 and 3 mm. The salt is continuously discharged at the lower end of the saturator. The salt is separated in centrifuges, dried, and cooled. The mother liquor is returned to the saturator. Impurities in the sulfuric acid can adversely affect crystallization. Small quantities of phosphoric acid, urea, or inorganic salts are added to promote crystal growth295. 

S. Nakano, A. Miyashita, and H. Nohira, Metastable solution structures of spirobenzoselenazoli-nobenzopyrans and their negative photochromic properties, Chem. Lett., 1993, 13-16. 

In Figure 2F-1 the composition where d2( G)/d 22 s equal to zero, or at the inflection point on the Gibbs energy surface, is defined as the spinodal composition. This corresponds to the boundary between an unstable solution and a metastable solution. If the necessary amount of free energy is supplied to the metastable system, the solution will phase separate into two phases with binodal compositions unstable system will always phase separate into the two phases. The temperature where the two points of inflection on the energy surface merge into a single point is defined as the critical solution temperature. 

The key to calculating the activation energy associated with nucle-ation from supersaturated (metastable) solutions is Gibbs formula (G3) for the work of forming a new phase within a homogeneous fluid ... 

Potassium thiocyanate, like others in the series, a soluble and hygroscopic ionic salt, finds use in photographic developers. KNCSe may be similarly prepared. " The TeCN anion is only isolable with very large organic cations, and so no solid alkali tellurocyanate is known. Metastable solutions of M+TeCN may, however, be prepared. 

Metastable solutions of the monohalides AIX and GaX (X = Cl, Br, I) have been prepared using this technique, and oligomeric species (MX) -Em containing a variety of donors (E) have been crystallised. The solutions disproportionate to the trihalide and the metal (equation 2) when warmed to temperatures in the range -40 to 4-50 °C, depending on the halide, the donor, and the concentration. Species with oxidation states both higher and lower than +1 (i.e. on the path to both disproportionation products) have been isolated. The reduced species (0 < Nox < 1) are discussed in the section on metalloid clusters, and the monohahdes and more oxidized species (1 < Nox < 3) are discussed here. A sonochemical synthesis of a subvalent galhum species, possibly Gal, has also been developed. ... 

The equilibrium constant of reaction (1), K = [Cu ][Cu ]/[Cu ], is of the order of 10 thus, only vanishingly small concentrations of aquo-copper(I) species can exist at equilibrium. However, in the absence of catalysts for the disproportionation—such as glass surfaces, mercury, red copper(I) oxide (7), or alkali (311)—equilibrium is only slowly attained. Metastable solutions of aquocopper(I) complexes may be generated by reducing copper(II) salts with europium(II) (113), chromium(II), vanadium(II) (113, 274), or tin(II) chloride in acid solution (264). The employment of chromium(II) as reducing agent is best (113), since in most other cases further reduction to copper metal is competitive with the initial reduction (274). 

Figure 5. Polyamide Pip-10, stirred polymerization reactions with the polymer in metastable solution...

Figure 7.2. Solubility and saturation. A schematic solubility diagram showing concentration ranges versus pH for supersaturated, metastable, saturated, and undersaturated solutions. A supersaturated solution in the labile concentration range forms a precipitate spontaneously a metastable solution may form no precipitate over a relatively long period. Often an active form of the precipitate, usually a very fine crystalline solid phase with a disordered lattice, is formed from oversaturated solutions. Such an active precipitate may persist in metastable equilibrium with the solution it is more soluble than the stable solid phase and may slowly convert into the stable phase.

---