Phase transformation limited solubility

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

The kinetics for a solvent mediated phase transformation will be governed by the kinetics of dissolution, nucleation, and crystal growth. These rates will depend directly on the solvent and any step may be rate limiting. As discussed in earlier sections of this chapter, the solvent influences the nucleation rate and crystal growth rate via two factors 1) solute solubility and 2) specific solvent-solute interactions. The dissolution rate will also be solvent dependent as pharmaceutical materials generally exhibit a wide range of dissolution rates in different solvents. 

Fluorous biphasic catalysis is another active area in multiphasic homogeneous catalysis. The term fluorous was introduced [90] as the analogue to the term aqueous, to emphasize the fact that one of the phases of a biphase system is richer in fluorocarbons than the other. Fluorous biphase systems can be used in catalytic chemical transformations by immobilizing catalysts in the fluorous phase. A fluorous catalyst system consists of a fluorous phase containing a preferentially fluorous-soluble catalyst and a second product phase, which may be any organic or inorganic solvent with limited solubility in the fluorous phase (Figure 2.8a). 

The experimental studies have shown that due to limited solubility of components in the solid state, PbBi alloy is subject to phase-structural transformations, in the course of which its properties change. It has turned out that it is possible to describe these changes using kinetic relationships that have been determined for such most significant properties as density,... 

Reactions that otherwise would be earried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones with the aid of supercritical fluids, so that inter-phase transport limitations are eliminated. This is realized due to enhanced solubilities of the reaetion components in the supercritical fluids. Typical examples are reactions in water (supercritical water ean solubilize organic compounds), homogeneous catalytic reactions, and reactions of organometallic compounds. Homogenizing one com-poxmd more than the others in a system may also affect relative rates in complex reactions and enhance the selectivity. 

Brittain HG, Bogdanowich SJ, Bugay DE, Devincentis J, Lewen G, Newman AW (1991) Physictil characterization of pharmaceutical solids. Pharm Res 8 963—973 Brouwers J, Brewster ME, Augustijns P (2009) Supersaturating diug delivery systems the answer to solubility-limited oral bioavaUability J Pharm Sci 98 2549-2572 Cardew PT, Davey RJ (1985) The kinetics of solvent-mediated phase transformations. Proc Royal Soc Lond A 398 415-428... 

Immiscible-phase separation Transformation Processes No Fluids (such as gasoline) that are immiscible in water are a significant consideration in near-surface contamination. Deep-well injection is limited to wastestreams that are soluble in water. Well blowout from gaseous carbon dioxide formation is an example of this process that is distinct to the deep-well environment. 

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