Heterogeneous eutectic melting

An interesting example of scale-up of peptide synthesis in such low-melting point mixtures derived from eutectic melts has been described [70]. Neat combination of the pure substrates in the complete absence of water/solvent (adjuvant) provided simple heterogeneous systems consisting of the eutectic melts plus an excess of solid substrate (Figure 12.5). 

In this case the freezing-point curve is of the simple type (Fig. 33). Such curves have been obtained in the case of a number of pairs of metals, e.g, zinc—cadmium, zinc—aluminium, copper—silver (Heycock and Neville), tin— zinc, bismuth—lead (Gautier), and in other cases. From molten mixtures represented by one branch of the freezing-point curve one of the metals will be deposited while from mixtures represented by the other branch, the other metal will separate out. At the eutectic point the molten mass will solidify to a heterogeneous mixture of the two metals, forming what is known as the eutectic alloy, Such an alloy, therefore, will melt at a definite temperature lower than the melting-point of either of the pure metals. 

Now what happens if a melt with a eutectic composition of x is cooled A Uquid with this composition will solidify as a whole at a single definite temperature (like a pure substance). This means that none of the components have separated out before. A heterogeneous mixture has been formed of simultaneously precipitated a and p mixed crystals that also exhibits a total composition of Jt (for simplicity, the lever rule is not drawn here). In contrast to all the others, the eutectic mixture does not need to be cooled slowly in order to have conditions for equilibrium and one obtains a uniform and very fine-grained structure (microcrystals). 

If the melt has eutectic composition from the start (in our example, Xb = 0.6), it will continuously cool imtil it reaches the eutectic freezing temperature. When the mixture s temperature falls below this temperature, a simultaneous precipitation of A and B takes place imtil the entire sample has solidified. Correspondingly, the temperature remains crmstant over a Irmger period of time compared to other mixtures. A micrograph shows a heterogeneous mixture of A and B microcrystals of approximately the same size. 

Briefly, crystallization is carried out by cooling from an aqueous emulsion. Impurities, in the form of eutectic mixtures, remain in the emulsion, from which they may be recovered by further cooling. The organic substance should be (a) practically insoluble in water and (b) able to melt and solidify in a heterogeneous aqueous medium, and remain stable. 

For the liquid phase to appear, the formation of the local sites with a temperature higher than the eutectic temperature is not necessary. The formation of such local sites is reasonably explained from the standpoint of the heterogeneous melting under consideration. The calculations (Fig. 20) performed using relationship show that even at temperatures of 400-450°C, which are substantially lower than the eutectic temperature (577°C), a 8-10 nm thick liquid layer should exist on the aluminum surface. 

Heterogeneous alloys include tin-lead solder mercury amalgams (once used for dental fillings) and Bi/Cd, which melts over a range of temperatures, except for one composition (eutectic), which melts at a fixed temperature like a pure compound. 

---