Solubility crystal size dependence

The zinc complex is prepared in a manner analogous to that of the manganese complex. The yield is lower due to the greater solubility of the product. The crystal size depends on the amount of water and acid present in the solution, but may be up to 1 mm long. 

Griseofulvin, a BCS class II drug (Fig. 2), is a well-known example whose poor aqueous solubility causes low and erratic oral bioavailability. As shown below, griseofulvin has a hydrophobic molecular structure, and is practically insoluble in water. Its oral absorption is highly variable, ranging from 25% to 100%, depending on the crystal size. Ultramicrosize griseofulvin preparations were shown to have 100% oral absorption (12). 

The supersaturation is expressed by S = C/C0, with Cthe amount dissolved and C0 the normal solubility (kg crystais/kg water). The mean growth velocity is that at one face of the crystal the length increase is G = 2v (m/sec). Data are for crystals in the size range 0.5-1.0 mm in the presence of other crystals. The asterisk denotes that the growth rate probably is size-dependent. 

The salt crystallizes in violet-red to carmine-red prisms, depending on the crystal size. The solubility at room temperature is one part of the salt in fifteen parts of water. The aqueous solution has a basic reaction to litmus. Dilute acids form the diaquotetrammine salt, whereas concentrated acids form the diacido- or monoacidoaquotetram-mine complex. Treatment of the salt with dilute acid, then with an excess of hot dilute ammonia, forms the aquopen-tamminecobalt(III) series of salts. The salt is stable indefinitely, samples 40 years old having yielded satisfactory synthetic results. 

How would you expect the solubility of a solid to depend on crystal size What happens as a newly formed precipitate ages in contact with its supernatant liquid ... 

Potassium tetranitrodiamminecobaltate(III) is a lustrous yellow to brown solid, the exact color depending on the crystal size. It is only very slightly soluble in cold water the solubility at 100° is about 5 g./lOO ml. of water. Prolonged contact with water at temperatures above 50 to 60° causes decomposition. Treatment with excess 10% aqueous oxahc acid yields potassium dinitrooxalatodiamminecobal-tate(III). Silver or mercury (I) nitrate causes the precipitation of the corresponding sparingly soluble metal salts of the anion. The free acid is also relatively stable. Aqueous ethylenediamine displaces nitrite from the complex to give the nonelectrolyte trinitro (ethylenediamine) amminecobalt-(III). ... 

In Section 7.8 we showed that the size dependence of the solubility has also a thermodynamic base. In Figure 7.24a we demonstrated the influence of molar surface on solubility of CuO and Cu(OH)2 at pH = 7. This figure suggested that Cu(OH)2(s) becomes more stable than CuO(s) for very finely divided CuO crystals. In precipitating Cu(II), Cu(OH)2 may be precipitated incipiently d is very small) but CuO becomes more stable than Cu(OH)2(s) upon growth of the crystals, and an inversion of Cu(OH)2(s) into the more stable phase becomes possible (Schindler, 1967). 

Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse that their size-dependent physical properties can be studied in detail [6j. It is also possible to organize these monodisperse nanoparticles via slow evaporation to yield superlattices [11-13]. Superlattices of nanocrystals can rightly be described as a new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals are crystals of amorphous silica spheres [15]. 

Oxidative processes are localized in amorphous interlayers, in interfibrillar regions and others. Crystallinity and crystals sizes increase at initial stages of oxidation [303] it also means that oxidation is localized in amorphous part. Destructive decay of passing macromolecules in amorphous interlayers release them and facilitates folding of chains into crystals. Destruction and amorphicity of crystals takes place only at deep stages of oxidation. Solubility of oxygen in polymer depends not only on polymer crystallinity but on microstructure of amorphous or defect sections. 

It is noteworthy that the Gibbs-Freundlich-Ostwald equation, describing the size dependence of the solubility c(r) of drops or solid crystals is similar to eq.(1.15) ... 

This process continues until the crystal reaches a thickness of several thousand A. Crystal growth does not stop even after equilibrium conversion (polymer-monomer equilibrium) is reached. This is explained in terms of the Ostwald ripening. Solubility of particles depends on their size thus smaller particles tend to redissolve (by depropagation) in favour of further growth of already larger ones. This is illustrated by the data shown in Table 7.7. 

There is some correlation between the hardness of the crystal and the chance that its shows a particle size dependent solubility. In general, hard precipitates, like barium sulfate, show particle size-dependent solubility, whereas soft precipitates like silver chloride do not. 

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