Phase-composition diagrams

Plots of melting point against optical purity are commonly referred to as phase-composition diagrams. The direct proportionality of melting point with heat of fusion has also been employed to construct similar plots based on thermal analysis data. The classification of racemic modifications into three different types with regard to their crystal packing (32) can be made based on the overall shape of these plots as follows ... 

Racemic mixture. The phase-composition diagram exhibits a minimum thus, the molecules having identical absolute configurations have the greatest affinity and, in effect, a chiral molecule regards its enantiomer as an impurity. 

Racemic compound or racemate. The phase-composition diagram exhibits a maximum thus, the molecules of opposite configuration have the greatest affinity. This is the most commonly encountered type of behavior. 

Racemic solid solution. The phase-composition diagram has a zero slope across the whole composition range thus, there is little difference in the affinity among molecules of either configuration (i.e., no chiral discrimination). 

Phase-composition diagrams may also be plotted for mixtures of optically pure samples that are structurally similar, but not identical. Such plots have been employed frequently as an empirical correlation to absolute configuration, which is referred to as the method of quasiracemates. The details, variations, limitations, and numerous examples of this method have been presented by others (33). We merely note here that if a phase-composition diagram of the racemic compound type were obtained for a particular pair of enantiomers having known configuration, and if the phase-composition diagram of mix-... 

Figure 8. Pressure-equilibrium phase composition diagram for isobutane-carbon dioxide system, calculations using Peng-Robin-son equation of state...

Figure 32. Temperature-equiUbrium phase composition diagram for butanolfl)— water(2) system. Calculations are based on UNIQUAC equation with temperature-independent parameters.

Fig. 15.32 Phase composition diagram of water and carbon dioxide [15, 38] and water and nitrogen [39] as a function of pressure...

Since the infrared spectra in the solid state of the racemates are different from those of the enantiomers for both diastereomers, the racemates in tri-dimensional arrangement are probably racemic compounds. These observations were confirmed by differential scanning calorimetry (DSC), which shows that the racemates melt at higher temperature than the enantiomers, and by the phase-composition diagram of the ma jor diastereomer (Fig. 2). 

Total reflux for a symmetric separation. Note, the term symmetric separation is used here to mean that on a McCabe-Thiele diagram, the liquid phase compositions of the overhead product and bottom product are roughly equidistant from 0.5. 

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

This concept has also been demonstrated at ambient temperature in the case of the Li-Sn-Cd system [47,48]. The composition-de-pendences of the potentials in the two binary systems at ambient temperatures are shown in Fig. 15, and the calculated phase stability diagram for this ternary system is shown in Fig. 16. It was shown that the phase Li4 4Sn, which has fast chemical diffusion for lithium, is stable at the potentials of two of the Li-Cd reconstitution reaction plateaus, and therefore can be used as a matrix phase. 

In each of the composition diagrams in Fig. 14.2, the numbers represent a series of reactions run at a defined composition and temperature. These are isometric sulfur slices through three-dimensional K/P/RE/S quaternary phase diagrams. As just one example of what we have studied. Table 14.1 identifies the compositions at each point and the resulting phase(s). We have rigorously studied how phase formation is dependent upon the compositions of reactions for the rare-earth elements Y, Eu, and La and we have also discovered key structural relationships between the rare-earth elements, indicating a significant dependence on rare-earth and alkali-metal size for sulfides and selenides. 

To differentiate and to be able to determine the differences between the phases that may arise when two compounds are present (or are made to react together), we use what are termed "phase-diagrams" to Illustrate the nature of the interactions between two solid phase compositions. 

Up to this point, we have considered only one solid at a time. However, when two (2) or more solids are present, they can form quite complicated systems which depend upon the nature of each of the solids involved. To differentiate and to be able to determine the differences between the phases that may arise when two compounds are present (or are made to react together), we use what are termed "phase-diagrams to illustrate the nature of the interactions between two solid phase compositions. You will note that some of this material weis presented earlier in Chapter 1. It is presented here again to further emphasize the importance of phase diagrams. 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

The advantage of the window diagram method is that the optimum mobile phase composition can be easily loeated visually or using a computer. 

Isoperibolic calorimetry measurements on the n-butanol/water and n-butoxyethanol/water systems have demonstrated the accuracy and convenience of this technique for measuring consolute phase compositions in amphiphile/water systems. Additional advantages of calorimetry over conventional phase diagram methods are that (1) calorimetry yields other useful thermodynamic parameters, such as excess enthalpies (2) calorimetry can be used for dark and opaque samples and (3) calorimetry does not depend on the bulk separation of conjugate fluids. Together, the present study and studies in the literature encompass all of the classes of compounds of the amphiphile/CO ydrocarbon/water systems that are encountered in... 

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