Liquid solutions upper critical solution temperature

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. 

We consider a binary liquid mixture of components 1 and 3 to be consistent with our previous notation, we reserve the subscript 2 for the gaseous component. Components 1 and 3 are completely miscible at room temperature the (upper) critical solution temperature Tc is far below room temperature, as indicated by the lower curve in Fig. 27. Suppose now that we dissolve a small amount of component 2 in the binary mixture what happens to the critical solution temperature This question was considered by Prigogine (P14), who assumed that for any binary pair which can be formed from the three components 1, 2 and 3, the excess Gibbs energy (symmetric convention) is given by... 

For example, 0 describes the temperature dependence of composition near the upper critical solution temperature for binary (liquid + liquid) equilibrium, of the susceptibility in some magnetic phase transitions, and of the order parameter in (order + disorder) phase transitions. 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (solid 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, Ca , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are C02 -hexane and C02 benzene. More complicated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (liquid—liquid) immiscibility lines, and even three-phase (liquid—liquid—gas) immiscibility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include C02—hexadecane and C02 H20 Class IV, C02 nitrobenzene Class V, ethane— -propanol and Class VI, H20— -butanol. 

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... 

For salts with univalent ions, Eq. (4) predicts critical points near room temperature for systems with e 5 [72]. Liquid-liquid immiscibilities in several electrolyte solutions are known to satisfy this criterion [5, 71, 72]. Note that these gaps do not necessarily possess an upper critical solution temperature (UCST). Theory can rationalize a lower critical solution temperature (LCST) as well, if the product esT decreases with increasing temperature. 

Figure 14.10 The five types of (fluid + fluid) phase diagrams according to the Scott and van Konynenburg classification. The circles represent the critical points of pure components, while the triangles represent an upper critical solution temperature (u) or a lower critical solution temperature (1). The solid lines represent the (vapor + liquid) equilibrium lines for the pure substances. The dashed lines represent different types of critical loci. (l) [Ar + CH4], (2) [C02 + N20], (3) [C3H8 + H2S],...

Fig. 8.6. Liquid-liquid equilibria of alcohol-ionic liquid mixtures [105], The left side shows the LLE curves of l-butyl-3-methyl-imidazolium-PF6 mixtures with alcohols (ethanol, blue 1-propanol, red and 1-butanol, green symbols). The experimental curves (solid symbols) show a shape different from the calculated LLE curves, but the upper critical-solution temperatures (UCST) are surprisingly well met. On the right side, the trends of the UCST with a modification of the 1-alkyl-group of the anion (butyl = 4, octyl = 8) is shown. Again, the COSMO-RS predictions (open symbols, same color code as on the left) are in surprisingly good agreement with the experimental data.

Not all systems behave as described in the preceding paragraphs. Some the upper critical solution temperature is never attained, because a vapor/F critical temperature is reached first. In other cases the liquid solubilities in" with a decrease in temperature. In this event a lower critical solution tempo exists, unless solid phases appear first. There are also systems which exhibit upper and lower critical solution temperatures. 

Figure 13.15 is drawn for a single constant pressure equilibrium phase compositions, and hence the locations of the lines, change with pressure, but the general nature of the diagram is the same over a range of pressures. For the majority of systems the species become more soluble in one another as the temperature increases, as indicated by lines CG and DH of Fig. 13.15. If this diagram is drawn for successively higher pressures, the corresponding three-phase equilibrium temperatures increase, and lines CG and DH extend further and further until they meet at the liquid/liquid critical point Af, as shown by Fig. 13.16. The temperature at which this occurs is known as the upper critical solution temperature, and at this temperature the two liquid phases become identical and merge into a single phase.

Meier et al. [13] assumed that above 353 K the upper critical solution temperature for the a-tocopherol/carbon dioxide system could be reached, above which the liquid and supercritical gas phase are completely soluble in each other. Measurements of phase equilibria carried out by Hoffmann-La Roche AG and our investigations contradict the assumption mentioned. Two coexisting phases are still present at 423 K and 27.5 MPa (see Figure 4). 

An example of this kind of behaviour is found in the formic acid 4-triethylamine system, ) while certain intermetallic compounds such as KZui2, KPbg show similar behaviour except that the liquid phase has an upper critical solution temperature. J... 

Consider diffusion in a binary liquid mixture exhibiting an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) (see Fig. 3.1). Let us take a mixture at the critical composition x at point A just above the UCST. Any concentration fluctuation at A will tend to be smeared out due to the effects of diffusion in this homogeneous mixture. On the other hand, any fluctuation of a system at point B, infinitesimally below the UCST, will lead to separation in two phases. Similarly, the mixture at point D, just below the LCST is stable whereas the mixture at point C, just above the LCST is unstable and will separate into two phases. 

The view that the general form of the concentration-temperature curve is that of a closed ring is supported by the fact that pairs of liquids which, under atmospheric pressure, give curves exhibiting only a lower or an upper critical solution temperature, give closed-ring solubility curves when 200 the pressure is increased 190"... 

When the third bst ce dissnlms m only one of the two liquids the mutual solubility of the latter is diminished, and the temperature at which the system becomes homogeneous is raised, in the case of liquids having an upper critical solution temperature, and lowered in the case of liquids having a lower critical solution temperature. The elevation (or the lowering) of temperature depends not only on the nature and amount of the added substance, but also on the composition of the liquid mixture. When the two liquids are present in the proportions of the critical composition, it is found that, for concentrations of the addendum (non-electrolyte) less than about 0 i molar, the elevation (or depression) of the critical solution temperature is nearly proportional to the amount added. The elevation (or depression) of the critical solution temperature for small equi-molecular quantities of different substances is, however, not constant, but depends on the nature of the substance added. In the following tables are given the values for the elevation of the critical solution temperature of phenol and water by naphthalene (soluble only in phenol) and by potassium chloride (soluble only in water). E represents the molecular elevation of the critical solution temperature —... 

Generally, liquid-liquid phase equilibrium (or phase separation) occurs only over certain temperature ranges, bounded above by the upper consolute or upper critical solution temperature, and bounded below by the lower consolute or lower critical solution temperature. These critical solution temperatures are indicated on the liquid-liquid phase diagrams given here. All partially miscible mixtures should exhibit either one or both consolute temperatures however, the lower consolute temperature may be obscured by the freezing of the mixture, and the upper consolute temperature will not be observed if it is above the bubble point temperature of the mixture, as vaporization will have instead occurred. ... 

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