Immiscibility temperature

A number of techniques have been used in the experimental determination of immiscibility boundaries. The method discussed above is often aided by use of clearing studies. If we consider the microstructure of a sample heat treated at a temperature just below the immiscibility boundary, we soon realize that the detection of phase separation may not be very easy. The lever rule predicts that only a very small quantity of the minor phase will be present, which may make detection difficult. A clearing study relies upon pretreatment of our samples at a lower temperature, where the extent of phase separation is much greater and hence more apparent. The pretreated samples are heated to the region of the immiscibility boundary, held for a specified time, and examined for the disappearance of the opalescence. The immiscibility temperature then brackets the temperatures where opalescence remains or disappears. 

Other methods for determination of immiscibility temperatures involve direct detection of phase separation using either x-ray small angle scattering or electron microscopy. Both methods have been successfully used in a number of studies, but are much more instrumentation-intensive than simple visual examination. Methods based on physical property measurements, which have also been used to a limited extent, will be discussed later in this text in chapters dealing with those properties. 

The thermal expansion curve of a phase separated sample containing two glassy phases may show two glass transitions, as is shown in Figure 7.11. This type of curve will be observed if the more viscous phase is continuous, and if the immiscibility temperature for the glass lies above the Tg of the more viscous phase. Curves of this type have been observed for lead borate and barium silicate glasses, where the immiscibility temperature is greater than the Tg of either phase. 

For the melting temperature. T, no distinction has been made between congruent, peritectic, monotcctic or syntectic melting. For some ranges of solid solutions, critical immiscibility temperatures or maxima or minima in the cun es of the melting temperature have been listed and are defined in a footnote. 

The principal point of interest to be discussed in this section is the manner in which the surface tension of a binary system varies with composition. The effects of other variables such as pressure and temperature are similar to those for pure substances, and the more elaborate treatment for two-component systems is not considered here. Also, the case of immiscible liquids is taken up in Section IV-2. 

Steam Distillation. Distillation of a Pair of Immiscible Liquids. Steam distillation is a method for the isolation and purification of substances. It is applicable to liquids which are usually regarded as completely immiscible or to liquids which are miscible to only a very limited extent. In the following discussion it will be assumed that the liquids are completely immiscible. The saturated vapours of such completely immiscible liquids follow Dalton s law of partial pressures (1801), which may be stated when two or more gases or vapoms which do not react chemically with one another are mixed at constant temperature each gas exerts the same pressure as if it alone were present and that... 

Some liquids are practically immiscible e.g., water and mercury), whilst others e.g., water and ethyl alcohol or acetone) mix with one another in all proportions. Many examples are known, however, in which the liquids are partially miscible with one another. If, for example, water be added to ether or if ether be added to water and the mixture shaken, solution will take place up to a certain point beyond this point further addition of water on the one hand, or of ether on the other, will result in the formation of two liquid layers, one consisting of a saturated solution of water in ether and the other a saturated solution of ether in water. Two such mutually saturated solutions in equilibrium at a particular temperature are called conjugate solutions. It must be mentioned that there is no essential theoretical difference between liquids of partial and complete miscibility for, as wdll be shown below, the one may pass into the other with change of experimental conditions, such as temperature and, less frequently, of pressure. 

The theory of the process can best be illustrated by considering the operation, frequently carried out in the laboratory, of extracting an orgaiuc compound from its aqueous solution with an immiscible solvent. We are concerned here with the distribution law or partition law which, states that if to a system of two liquid layers, made up of two immiscible or slightly miscible components, is added a quantity of a third substance soluble in both layers, then the substance distributes itself between the two layers so that the ratio of the concentration in one solvent to the concentration in the second solvent remains constant at constant temperature. It is assumed that the molecular state of the substance is the same in both solvents. If and Cg are the concentrations in the layers A and B, then, at constant temperature ... 

Ben2onitri1e [100-47-0] C H CN, is a colorless Hquid with a characteristic almondlike odor. Its physical properties are Hsted in Table 10. It is miscible with acetone, ben2ene, chloroform, ethyl acetate, ethylene chloride, and other common organic solvents but is immiscible with water at ambient temperatures and soluble to ca 1 wt% at 100°C. It distills at atmospheric pressure without decomposition, but slowly discolors in the presence of light. 

Because of the relative instabiUty of FeO, the reduction to metallic Fe occurs at a much lower temperature and appreciable CO2 is present in the product gas. The high temperature required for the reaction of MnO and C results in the formation of essentially pure CO the partial pressures of CO2 and Mn are <0.1 kPa (1 X 10 atm). The product of this reaction is manganese carbide (7 3) [12076-37-8J, Mn C, containing 8.56% carbon. Assuming immiscibility of the metal and carbide, Mn should be obtainable by the reaction of MnO and Mn C at 1607°C. However, at this temperature and activity of Mn, the partial pressure of Mn vapor is approximately 10 kPa (0.1 atm) which would lead to large manganese losses. 

A beryUium concentrate is produced from the leach solution by the counter-current solvent extraction process (10). Kerosene [8008-20-6] containing di(2-ethylhexyl)phosphate [298-07-7] is the water-immiscible beryUium extractant. The slow extraction of beryUium at room temperature is accelerated by warming. The raffinate from the solvent extraction contains most of the aluminum and aU of the magnesium contained in the leach solution. 

Steam distillation is used to lower the distillation temperatures of high boiling organic compounds that are essentially immiscible with water. If an organic compound is immiscible with water, both Hquids exert fliU vapor pressure upon vaporization from the immiscible two-component Hquid. At a system pressure of P, the partial pressures would be ... 

Alloys exhibit physical properties, the values of which are typically the weighted average of those of its constituents. In particular, the blend exhibits a single glass-transition temperature, often closely obeying semitheoretically derived equations. Blends of two compatibiLized immiscible polymers exhibit physical properties which depend on the physical arrangement of the constituents and thus maybe much closer to those of one of the parent resins. They will also typically exhibit the two glass-transition temperatures of their constituent resins. 

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