Phase eutectic

FIG. 22-2 Simple eutectic-phase diagram at constant pressure. (Zief and Wilcox, Fractional Solidification, i>c/. 1, Marcel Dekker, New York, 1967, p. 24.)... 

R A. Monnard et al. from the laboratory of D. W. Deamer also worked on ice/eutectic phases at 255 K. They studied the influence of solutions of inorganic ions (such as Na+, CD, Mg2+, Ca2+ and Fe2+) both on the formation of vesicles and on non-enzymatic polymerization of activated RNA monomers (Monnard et al., 2002). 

It is also interesting to note that electroless plating of silver on carbon fiber surface improves the wettability between molten aluminum and PAN based carbon fibers using the liquid infiltration technique in vacuum (Warrier et al., 1993). This is attributed to the formation of an eutectic phase between silver and aluminum, and the silver coating of the fibers during processing. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

Naturally, the fixed composition phase transformations treated in this section can be accompanied by local fluctuations in the composition field. Because of the similarity of Fig. 17.3 to a binary eutectic phase diagram, it is apparent that composition plays a similar role to other order parameters, such as molar volume. Before treating the composition order parameter explicitly for a binary alloy, a preliminary distinction between types of order parameters can be obtained. Order parameters such as composition and molar volume are derived from extensive variables any kinetic equations that apply for them must account for any conservation principles that apply to the extensive variable. Order parameters such as the atomic displacement 77 in a piezoelectric transition, or spin in a magnetic transition, are not subject to any conservation principles. Fundamental differences between conserved and nonconserved order parameters are treated in Sections 17.2 and 18.3. 

Figure 10. Schematic of an A-AB simple eutectic phase diagram.

Interpretation of the eutectoid phase diagram is generally the same as that of the eutectic phase diagram. 

The polymerization of activated nucleotide derivatives has been also performed under eutectic phases in ice, a special microenvironment providing both concentration of reagents and preservation of newly formed oligomers. For example, Kanavarioti and Monnard reported the polymerization of... 

Figure 11.6 shows an example of the phase diagram for a reactive system, in which a compound C is formed from components A and B. An isothermal cut and the polythermal projection are also shown. Such a phase diagram can be obtained via a reaction invariant projection of a higher-dimensional simple eutectic phase diagram. AS and BS are binary nonreactive eutectics, since their presence is not affected by the reaction, while ACSb and BCSa are ternary reactive eutectics. Similar... 

Industrially, milk fat is commonly added to cocoa butter to produce milk chocolate. However, there is a thermodynamic incompatibihty between the miUc fat and cocoa butter solids which results in eutectic phase behavior. This eutectic formation leads to a chocolate with a decreased hardness (Timms 1980 Bama et al. 1992 Bystrom and Hartel 1994 Reddy et al. 1996). Extensive work has been carried out on the addition of milk fat to cocoa butter because of its abihty to reduce the incidence of bloom formation in chocolate (Hartel 1996 Kleinert 1961 Dimick et al. 1993). Chocolate bloom is evident as a white-gray layer on top of the chocolate surface and eventually the chocolate will acquire a crumbly texture. 

Mabrey S, Sturtevant JM. Investigation of phase-transitions of lipids and lipid mixtures by high sensitivity differential scanning calorimetry. Proc. Natl. Acad. Sci. U.S.A. 1976 73 3862-3866. Lin HN, Huang CH. Eutectic Phase-behavior of l-stearoyl-2-cap-rylphosphatidylcholine and dimyristoylphosphatidylcholine mixtures. Biochim. Biophys. Acta 1988 946 178-184. 

A eutectic phase is formed between TAGs whose molecular shapes are largely different. 

Rossell (85) suggested that a eutectic phase with a limited region of solid solution was formed for the stable p form yet, the solid solution phase was formed in the... 

A eutectic phase was formed in the most stable p form. 

Stacking gap. In contrast, p polymorph has all-trawi-hydrocarbon chains and these rigid chains cannot adjust themselves to their circumstance. Therefore, p polymorph shows a eutectic phase. As for the LLL/PPP and LLL/SSS mixtures, eutectic phases occur for aU polymorphs. Because of large differences in carbon numbers for fatty acid chains between LLL and PPP (An = 4), and between LLL and SSS (An = 6), there are very large methyl-end stacking gaps in these crystals. Therefore, the increased entropy of methyl-end stacking becomes predominant and phase separation must be favored thermodynamically for all polymorphs. 

The polymorphic nature of the multicomponent TAG systems is related to phase behavior that is affected by molecular interactions among the component TAGs. The fat crystals in a miscible phase may exhibit simple polymorphic properties. By contrast, the immiscile eutectic phase may show complicated polymorphic properties as a superposition of the polymorphic forms of the component TAGs. Furthermore, if the molecular compound is formed by specific TAG components, the polymorphic behavior becomes complicated, as shown for the case of POP-OPO (see Section 5.2). Therefore, knowing the phase behavior of the principal TAG components is a prerequisite for precise understanding of the polymorphism of natural fats. 

The phase behavior of the mixed TAG system is influenced by polymorphism. For example, a miscible phase is formed in a and p polymorphs, but it transforms into a eutectic phase in p, as revealed in the SSS-PPP mixture. Then, the polymorphic occurrence is largely affected by cooling rate and temperature fluctuation, and it is therefore necessary to observe the polymorphic properties of the natural fats by varying the rate of coohng or by fluctuating the temperature (so-called tempering). 

The determination of purity by means of DSC is based on the assumption that impurities depress the melting point of a pure material according to the eutectic phase diagram behavior. 

In the case of the surface exposure of the metallic coating layer it is eutectic phase that acts exclusively as the cathodic surface, whereas in the case of cut-edge exposure the coupled steel acts cathodically with respect to zinc. In aerated solutions at near-neutral pH, these represent the sites for the supporting oxygen reduction reaction, equation 2. 

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