Upper critical solution point

The system FLO - C02> H20 - ELS and H20 - (C HjOoO can be described by assuming cross-associationf The particular temperature dependence of the solubility for diethyl ether was reproduced by the calculation without making it the object of a fitting process. This suggests that the method might be able to describe systems with both an upper and lower critical solution point. 

From absolute zero (0°K) to 25°C, most hydrophilic solute remains separated in water to an upper critical solution or upper consolute temperature (Tc) (Glasstone and Lewis, 1963) whereupon they merge. In the opposite direction (from high to low temperature), solute and solvent or two solute phases in a common solvent may remain separated to a lower Tc, where they again merge. Many cellulose derivatives have a lower Tc in the vicinity of 45°C. The lower and upper Tc are called cloud points because of the incipient cloudiness observed there. This incipient cloudiness in a formerly translucent dispersion is evidence that the solute has emerged from a secondary minimum on its way to a gel (Walstra et al., 1991). 

Kubota, K. Abbey, K. M. Chu, B., "Static and Dynamical Properties of a Polymer Solution with Upper and Lower Critical Solution Points. NBS 705 Polystyrene in Methyl Acetate," Macromolecules, 16, 137 (1983). 

Figure 3.1. Upper and lower critical solution points.

From the above table it is clear that the lower critical solution temperature is raised, and the upper critical solution temperature is lowered, by increase of pressure. Under j>res,sure of 830 kgm. per sq. cm. the two critical solution points coincide. Under pressures higher than this, complete miscibility exists at all temperatures. A similar behaviour is found in the case of water and methylethylketone. 

Fig. 14.2 Behavior of (average) chemical potential (xb) in a mixture of two liquid components depending upon temperature (top) and the corresponding phase diagram at constant pressure with an upper critical solution point (bottom).

Fig. 14.3 Applying the lever rule using the example of a phase diagram with an upper critical solution point.

Some systems have both an upper and a lower critical solution point (Fig. 14.5). These kinds of systems are mostly found at higher pressures. It is therefore plausible to assume that aU systems having a lower critical solution point will also exhibit an upper critical solution point if the temperature and pressure are high... 

The Txx diagram shown in Figure 8.20 is typical of most binary liquid-liquid systems the two-phase curve passes through a maximum in temperature. The maximum is called a consolute point (also known as a critical mixing point or a critical solution point), and since T is a maximum, the mixture is said to have an upper critical solution temperature (UCST). A particular example is phenol and water, shown in Figure 9.13. At T > T, molecular motions are sufficient to counteract the intermolecular forces that cause separation. 

The upper critical solution point for furfural-water lies at 122.7 C., 51 wt. per cent furfural. Calculate the van Laar and Margules constants from this datum, and compare with the values of activity coefficient at x = 0 and 1.0 obtained from vapor-liquid data, Chemical Engineers Handbook. Explain the results in terms of the applicability of the van Laar and Margules equations to this system. 

Figure 10.12 shows our theoretical description of what we see in the experiments of methyl cellulose in water. The upper half of the miscibility loop is beyond the range of the experimental observation, so that we see the binodal of the LCST only. The sol-gel line intersects the binodal from below. However, there is no lower critical solution point. Instead a new inverted tricritical point (TCP) exists. 

In this and the next section we consider liquid-liquid phase separation in liquid mixtures terminating in either an upper or a lower critical solution point. Since the pressure does not affect concentration fluctuations we neglect in first approximation the contribution of the pressure to the independent scaling fields, hi and... 

In some liquid mixtures one may encounter a closed solubility loop between an upper critical solution point with temperature Ju and concentration Xu and a lower critical solution point with temperature Jl and concentration Xl. One can obtain a quantitative representation of such closed solubility loops if the temperature variable lAri is replaced by " At ji = T j-T) T-T IT jTi. This procedure has been applied successfully in the revised-scaling approximation i.e., without a contribution proportional to IATulI ), but with the addition of a correction-to-scaling contribution proportional to 1A7ul as discussed in Section 10.3.5 ... 

KUB Kubota, K., Abbey, K.M., and Chu, B., Static and dynamical properties of a polymer solution with upper and lower critical solution points. NBS 705 polystyrene in methyl acetate, MacrowotecMtes, 16, 138, 1983. 

The highest point in the tie-line region at temperature Tc is called an upper critical solution point or an upper consolute point. It has a number of properties similar to those of the gas-liquid critical point in Figure 1.5. If a mixture has the same overall composition as that of the consolute point, it will be a two-phase system at a temperature below the consolute temperature. If its temperature is gradually raised, the meniscus between the phases becomes diffuse and disappears, in the same way that the meniscus... 

The theory of isotopic mixtures developed in Ch. XIX predicts an upper critical solution point for liquid He —He< mixtures at about 1 K. The estimate given by Prigogine, Bingen and Bellemans [1954] was 0.7 K. This prediction has now be verified by the experi-... 

It should be noted that the modern view is that all partially miscible liquids should have both a lower and upper critical solution temperature so that all such systems really belong to one class. A closed solubility curve is not obtain in all cases because the physical conditions under normal pressure prevent this. Thus with liquids possessing a lower C.S.T., the critical temperature (the critical point for the liquid vapour system for each component, the maximum temperature at which liquefaction is possible) may be reached before the consolute temperature. Similarly for liquids with an upper C.S.T., one or both of the liquids may freeze before the lower C.S.T. is attained. 

Influence of added substances upon the critical solution temperature. For a given pressure the C.S.T. is a perfectly defined point. It is, however, affected to a very marked extent by the addition of quite a small quantity of a foreign substance (impurity), which dissolves either in one or both of the partially miscible liquids. The determination of the consolute temperature may therefore be used for testing the purity of liquids. The upper consolute temperature is generally employed for this purpose. 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

Detailed measurements of the solubility between the lower and upper critical end points have been made only for the solutions in ethylene of naphthalene,14 hexachlorethane,30 and />-iodochloro-benzene.21 Atack and Schneider2 have used dilute solutions of the last-named substance to study the formation of clusters near the gas-liquid critical point of ethane. 

Figure 8.17 Vapor fugacity for component 2 in a liquid mixture. At temperature T, large positive deviations from Raoult s law occur. At a lower temperature, the vapor fugacity curve goes through a point of inflection (point c), which becomes a critical point known as the upper critical end point (UCEP). The temperature Tc at which this happens is known as the upper critical solution temperature (UCST). At temperatures less than Tc, the mixture separates into two phases with compositions given by points a and b. Component 1 would show similar behavior, with a point of inflection in the f against X2 curve at Tc, and a discontinuity at 7V...

Point c is a critical point known as the upper critical end point (UCEP).y The temperature, Tc, where this occurs is known as the upper critical solution temperature (UCST) and the composition as the critical solution mole fraction, JC2,C- The phenomenon that occurs at the UCEP is in many ways similar to that which happens at the (liquid + vapor) critical point of a pure substance. For example, at a temperature just above Tc. critical opalescence occurs, and at point c, the coefficient of expansion, compressibility, and heat capacity become infinite. 

The critical point (Ij of the two-phase region encountered at reduced temperatures is called an upper critical solution temperature (UCST), and that of the two-phase region found at elevated temperatures is called, perversely, a lower critical solution temperature (LCST). Figure 2 is drawn assuming that the polymer in solution is monodisperse. However, if the polymer in solution is polydisperse, generally similar, but more vaguely defined, regions of phase separation occur. These are known as "cloud-point" curves. The term "cloud point" results from the visual observation of phase separation - a cloudiness in the mixture. 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

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