Mixtures simple eutectic systems

In the case of a simple eutectic system shown in Fig. 20-2, a pure solid phase is obtainecf by cooling if the composition of the feed mixture is not at the eutectic composition. If liquid composition is eutectic, then separate crystals of both species will form. In practice it is difficult to attain perfect separation of one component by crystallization of a eutectic mixture. The solid phase will always contain trace amounts of impurity because of incomplete solid-liquid separation, slight solubility of the impurity in the solid phase, or volumetric inclusions. It is difficult to generalize on which of these mechanisms is the major cause of contamination because of analytical difficulties in the ultrahigh-purity range. 

FIGURE 18.4 Phase diagram of a simple eutectic system. AttemperaturesbelowcurveXYorYZ, either solid A [drug] or solid B [carrier] solidi es rstfrom the molten mixture, respectively. At eutectic composition Y both drug and carrier solidify simultaneously as a mixture of nely divided crystalline components. 

Mixtures of components that exhibit solid solution behaviour cannot be separated in a single step as can, for example, simple eutectic systems. Multistage or fractional precipitation schemes must therefore be employed (section 7.1). The distribution of an impurity between the solid (i.e. solid solution) and liquid phases may be represented by the Chlopin (1925) equation ... 

Common UCST behaviour has also been observed for mixtures consisting of two ionic liquids with the same anion and a different cation.Less common lower eritical solution temperature (LCST) behaviour has been observed for binary ionic liquid mixtures with benzene,chloroalkanes " and polymers. Interestingly, the LCST behaviour of benzene with an imidazo-lium-based ionic liquid changed via hour-glass to UCST behaviour when the imidazolium alkyl chain length was increased.Moreover, imidazolium-based ionic liquids were found to form clathrate structures with benzene at low temperatures, whereas the solid-liquid equilibria (SLE) of other organics or water with ionic liquids are simple eutectic systems. 

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). 

Figure 16.2. Some phase diagrams, (a) The water end of the system potassium chloride and water, (b) The water end of the system sodium chloride and water, (c) The water end of the system magnesium sulfate and water the heptahydrate goes to the mono at 150°C, and to anhydrous at 200°C. (d) /3-methylnaphthalene and /S-chloronaphthalene form solid solutions, (e) Mixtures of formamide and pyridine form a simple eutectic, (f) These mixtures form binary eutectics at the indicated temperatures and a ternary eutectic at mol fractions 0.392 dibenzyl, 0.338 diphenyl, and 0.27 naphthalene.

Investigations of condensed phase equilibria in several systems involv-ing UFe and other components are recorded. The binary systems, BFs-UFe (9, 13), CIFb-UFo (2i), HF-UFe (J5), BrFs-UFe (4), BrFo-UFe (4), and Br2-UFe (5), have been studied. The ternary system ClFs-HF-UFe (16) has also been studied. In all of these cases, the binary mixtures show the formation of a simple eutectic without solid solubility. 

The phase diagram of a simple two-component eutectic system contains four planes (Figure 3.20). The plane L represents the region of presence of the homogeneous solution of the components A and B. The plane A + L is the region of coexistence of the crystals of the solid phase A and the melt saturated with the component A, the plane B -h L is the region of coexistence of the crystals of the solid phase B and the melt saturated with the component B. Finally, the plane A + B is the region of coexistence of both the solid phases A and B, it is thus their solid, mechanical mixture. 

In the case of a mixture shown on Figure 3.32 by the figurative point X3, the situation is similar as in a simple eutectic ternary system. Starting with the crystallization of component C, the composition of the melt moves up to the point 5, where the component A4B begins to crystallize. At the ensuing cooling, the composition of the melt moves on the boundary line Pt—Ct up to the ternary eutectic point, where also component B crystallizes until the whole system solidifies. 

If we have a mixture AX-AY-BY, the composition of which is in Figure 3.35 shown by the figurative point Xi, the crystallization path at its cooling is completely similar as in the case of a simple ternary eutectic system. The component BY begins to crystallize first, the composition of the melt moves towards point Mi, where also component AX starts to crystallize. At the ensuing cooling, both the components fall out from the melt simultaneously and the composition of the melt moves on the boundary line etj — et2 from point Mi up to the eutectic point etj, where also component AY starts to crystallize and where also the whole system will solidify. 

White solids previously reported in the pyridine-POClg system are probably mixtures of pyridine hydrochloride and py,HOPOCl2 that are due to the presence of moisture, and a re-examination of the system shows the presence of a simple eutectic at 76.4% pyridine and —55.6 C. With PCI3, the compound 2py,PCl3 (m.p. —42 C) is formed, which gives a crystalline precipitate 2py,PCl3,H20 on treatment with moisture. Chloroformamidines (53) result... 

The systems KCl H2O and (NH4)2S04-H20 are good examples of aqueous salt solutions that exhibit simple eutectic formation. In aqueous systems the eutectic mixture is sometimes referred to as a cryohydrate, and the eutectic point a cryohydric point . 

Other more complex systems may be encountered, however, including minimum melting solid solutions, eutectics with compound formation, etc., but in a comprehensive survey of binary organic mixtures, Matsuoka (1991) estimated that over 50% exhibited simple eutectic behaviour, about 25% formed inter-molecular compounds and about 10% formed solid solutions of one kind or another. Interestingly, fewer than 2% formed simple solid solutions Figure 8.15b). 

Multistage crystallization in systems with complete miscibility of the components in the liquid and solid states and with systems which form mixed crystals, leads to an almost complete separation. In systems forming an eutectic mixture, simple separation is only possible up to the eutectic composition. 

More than half of the true binary organic mixture systems in the literature exhibit simple eutectic behavior (Matsuoka, 1991) (see Figure 1(A)), while about 10% of binary solid systems form solid solutions (Matsuoka, 1991) (see Figure 1(B)), in which the atoms or molecules of one of the components occupy sites in the crystal lattice of the other component... 

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