Liquid lines, single-phase

When a quantity of pure solid is totally dissolved in a liquid, a single phase is obtained, which consists of the two components. In this system, only one degree of freedom (which is the solute concentration) is possible, and that condition persists as the solute concentration varies from zero to saturation. This behavior is represented by the A-B segment of Fig. 5. When the data are plotted so as to illustrate the dependence of the solution composition on the system composition, one obtains a straight line (the A-B segment) with a slope of unity. Since the saturation limit is defined only with respect to a solid phase, if no undissolved solid is present, the system is undefined. 

It is pertinent to mention that Eq. (0.41) describes practically the whole region of the liquid state (single-phase and along the saturation line) from (o = 2.0 to the line of crystallization. In the zone of a> = 1.9-2.0 the equation merges with Eq. (0.35). 

The other place where the constitution is not fully defined is where there is a horizontal line on the phase diagram. The lead-tin diagram has one line like this - it runs across the diagram at 183°C and connects (Sn) of 2.5 wt% lead, L of 38.1% lead and (Pb) of 81% lead. Just above 183°C an alloy of tin -i- 38.1% lead is single-phase liquid (Fig. 3.5). Just below 183°C it is two-phase, (Sn) -i- (Pb). At 183°C we have a three-phase mixture of L -I- (Sn) -I- (Pb) but we can t of course say from the phase diagram what the relative weights of the three phases are. 

Carson and Katz5 studied another part of the methane-propane-water system. These authors investigated its behavior when an aqueous liquid, a hydrocarbon liquid, a gas, and some solid were present. It was found that the system was univariant so that the solid consisted of a single phase only. This phase is a hydrate which proved to contain methane and propane in various ratios. They then concluded that these hydrates behaved as solid solutions. It is clear that Carson and Katz measured a part of the four-phase line HllL1L2G. 

In Figure 8.20, a single-phase liquid mixture is present in the area above the (solid + liquid) equilibria lines. Since pressure is fixed, two degrees of freedom are present in this region so that temperature and mole fraction can be varied independently. 

A salt hydrate consists of two components, the salt (e.g. CaCL) and water (e.g. 6H2O). The single phase of the salt hydrate is first heated up from point 1 (solid) to point 2. At point 3 the liquidus line is crossed and the material would be completely liquid. Upon heating or cooling, between point 2 and 3,2 phases are formed, the liquid and a small amount of a phase with less water (point 4). If these phases differ in density, this can lead to macroscopic separation of the phases and therefore concentration differences of the chemicals forming the PCM material (points 5 and Figure 104 right). 

Plotted in Fig. 10 are phase separation lines, which were obtained with the gradient oven for epoxies cured in the presence of the above solvents [88]. At the left side of these lines, no phase separation occurs and the materials stay transparent. The right side of these phase separation lines gives the temperature and composition ranges where phase separation occurs. The onset of the phase separation, which gives one single point on the phase separation line, can easily be detected for each concentration, as the samples become opaque as a consequence of the formation of liquid domains in the pm-range. 

The other single phase region is the liquid L. In addition to the two phase a+0 region, there are two other two phase regions L + a and L + 0. Just as in the isomorphous diagram the solidus and liquidus lines are connected by tie-lines of constant temperature. In a like manner, the a + 0 region is also considered to possess tie-lines joining the two solid-solution or solvus curves. 

The phase behaviour of many polymer-solvent systems is similar to type IV and type HI phase behaviour in the classification of van Konynenburg and Scott [5]. In the first case, the most important feature is the presence of an Upper Critical Solution Temperature (UCST) and a Lower Critical Solution Temperature (LCST). The UCST is the temperature at which two liquid phases become identical (critical) if the temperature is isobarically increased. The LCST is the temperature at which two liquid phases critically merge if the system temperature is isobarically reduced. At temperatures between the UCST and the LCST a single-phase region is found, while at temperatures lower than the UCST and higher than the LCST a liquid-liquid equilibrium occurs. Both the UCST and the LCST loci end in a critical endpoint, the point of intersection of the critical curve and the liquid liquid vapour (hhg) equilibrium line. In the two intersection points the two liquid phases become critical in the presence of a... 

Avery common occurrence is that, in the liquid phase, the components are completely miscible, whereas in the sohd phase, the components are only partially miscible, usually in small ranges around the pure components. This is illustrated in Fig. 10. Except for the single-phase sohd solution regions in the vicinity of the pure solid components, this diagram is similar to Fig. 10 of Chapter 8. It shows a eutectic, which freezes to a mixture of fine crystals of the two solid solutions. These three coexisting phases are represented by a horizontal line on the phase diagram. 

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