Crystallization eutectic point

The influence of micellar solubilization on co-crystal eutectic points is presented in Table 11.2 for carbamazepine-salicylic acid (CBZ-SLC) in aqueous media. Addition of surfactant reverses the co-former to drug molar concentration ratios at the eutectic. In the absence of surfactant, [SLC]e > [CBZ]e , indicating that the co-crystal requires excess co-former to be at equilibrium with pure drug. This situation is reversed in 35 mM (1%) SLS solution, where [CBZ]eu > [SLC]eu demonstrating that there is a CSC at SLS concentrations below 1 %. 

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

These materials are essentially combustion improvers and tend to have fairly simple formulations (e.g., 3% copper chloride, 7% manganese chloride, 90% ammonium chloride). They are designed to change the crystalline structure within the clinker crystal lattice and raise the clinker eutectic point, thus minimizing the formation of noncombustible clinker, residual ash, and other deposits. Feed rates are approxiimately 0.5 to 2.0 lb per bone-dry ton. 

C. Graphite ordering increases with temperature. Formation of microcrystalline graphite and millimeter-size graphite crystals occurs above the eutectic point in the Fe/Fe3C system. 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at 80 °C it is customary, particularly for navy blues, to use a mixture of two or more components of similar hue. If these behave independently, each will give its saturation solubility in the fibre. In practice, certain mixtures of dyes with closely related structures are 20-50% less soluble in cellulose acetate than predicted from the sum of their individual solubilities [87]. Dyes of this kind form mixed crystals in which the components are able to replace one another in the crystal lattice. The melting point depends on composition, varying gradually between those of the components, and the mixed crystals exhibit lower solubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilar molecular shape do not form mixed crystals, the melting point curve of the mixture shows a eutectic point and they behave additively in mixtures with respect to solubility in water and in the fibre. 

A primary role of crystallization is to purify the desired product and exclude impurities. Such impurities are frequently related in chemical structure to the desired product, through the mechanisms of competitive reaction and decomposition. Where the impurities are similar in structure it is likely that their interactions with the solvent in the liquid phase will also be similar. In this instance the selectivity of crystallization is mainly attributed to the difference between the respective pure solid phases. The ideal solubility equation can be applied to such systems [5, 8] on a solvent free basis to predict the eutectic composition of the product and its related impurities. The eutectic point is a crystallization boundary and fixes the available yield for a single crystallization step. 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Figure 7.9A shows the NaAlSi04-Si02 (nepheline-silica) system, after Schairer and Bowen (1956). Let us first examine the Si02-rich side of the join. At P = 1 bar, the pure component Si02 crystallizes in the cristobalite form (Cr) at r = 1713 °C (cf figure 2.6). At P = 1470 °C, there is a phase transition to tridymite (Tr), which does not appreciably affect the form of the liquidus curve, which reaches the eutectic point at P = 1062 °C. 

Transition Region Considerations. The conductance of a binary system can be approached from the values of conductivity of the pure electrolyte one follows the variation of conductance as one adds water or other second component to the pure electrolyte. The same approach is useful for other electrochemical properties as well the e.m. f. and the anodic behaviour of light, active metals, for instance. The structure of water in this "transition region" (TR), and therefore its reactions, can be expected to be quite different from its structure and reactions, in dilute aqueous solutions. (The same is true in relation to other non-conducting solvents.) The molecular structure of any liquid can be assumed to be close to that of the crystals from which it is derived. The narrower is the temperature gap between the liquid and the solidus curve, the closer are the structures of liquid and solid. In the composition regions between the pure water and a eutectic point the structure of the liquid is basically like that of water between eutectic and the pure salt or its hydrates the structure is basically that of these compounds. At the eutectic point, the conductance-isotherm runs through a maximum and the viscosity-isotherm breaks. Examples are shown in (125). 

In the liquid state sulfur and selenium are known to mix in all proportions. The provisional phase diagram shows an eutectic point at 40 mol-% of selenium (m.p. 105 °C). Mixtures with lower selenium content should show freezing points between 105 and 118 °C while those with higher selenium content are expected to have their freezing points at considerably higher temperatures. In practice equilibrium crystallization of the melt is hindered by supercooling and therefore only the melting points can be studied. 

The third and the most common type is complex phase transformations, including the following (i) some components in a phase combine to form a new phase (e.g., H2O exsolution from a magma to drive a volcanic eruption the precipitation of calcite from an aqueous solution, Ca + + COf calcite the condensation of corundum from solar nebular gas and the crystallization of olivine from a basaltic magma), (ii) one phase decomposes into several phases (e.g., spinodal decomposition, or albite jadeite + quartz), (iii) several phases combine into one phase (e.g., melting at the eutectic point, or jadeite +... 

Figure 4-34 is a phase diagram for the system titanite-anorthite. Suppose a crystal of titanite is initially in contact with a crystal of anorthite. The two are heated to 1350°C. Either phase by itself would not melt. But because the temperature is higher than the eutectic point of the two phases, at the interface there is melting. As melting proceeds, a thin melt layer would form between the two crystals. The melting of the two phases continues and the rate may be controlled by different factors. The rate would depend on the controls, as outlined below. 

By a similar usage in petrology, a euleeltc is a discrete mixture of two or more minerals, in definite proportions, which have simultaneously crystallized front the mutual solution of their constituents The eutectic point is the lowest temperature at any given pressure at which the above physical-chemical process may lake pluce, The eutectic ratio is the ratio hy weight of two minerals dial originate bv the above process. 

Point E is called the eutectic point or eutectic. In the field S(A) + L the system contains a homogeneous liquid mixture and crystals of the solid A. 

Fig. 1.13. Temperature as a function of the concentration of water-glycerine mixture at which phase transformations occur (Figure 14 from [1.10]) Definitions by Luyet AE, formation of small crystals or mo lecular groups E, eutectic point EB, formation of clusters R, eruptive recrystallization G, glass transition...

It is known that the melting characteristic of a crystal is correlated with its solubility to solvents. That is, the solubility of a compound showing a high melting point and nicely crystallized solid is relatively small whereas it is the largest at the eutectic point where the melting point is lowest. 

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