Consolute solution temperature

The cloud point is close to, but not necessarily equal to the lower consolute solution temperature for polydisperse nonionic surfactants (97). These are equal if the surfactant is monodisperse. Since the lower consolute solution temperature is like a critical point for liquid—liquid mixtures, the dilute and coacervate phases have the same composition, and the volume fraction of solution which the coacervate comprises is a maximum at this temperature (98). If a coacervate phase containing a high concentration of surfactant is desired, the solution should be at a temperature well above the cloud point. 

The cloud point phenomena as a lower consolute solution temperature is becoming better understood in terms of critical solution theory and the fundamental forces involved for pure nonionic surfactant systems. However, the phenomena may still occur if some ionic surfactant is added to the nonionic surfactant system. A challenge to theoreticians will be to model these mixed ionic/nonionic systems. This will require inclusion of electrostatic considerations in the modeling. 

Early researchers sought to choose appropriate surfactants for mobility control from the hundreds or thousands that might be used, but very little of the technology base that they needed had yet been created. Since then, work on micellar/polymer flooding has established several phase properties that must be met by almost any EOR surfactant, regardless of the application. This list of properties includes a Krafft temperature that is below the reservoir temperature, even if the connate brine contains a high concentration of divalent ions (i.e., hardness tolerance), and a lower consolute solution temperature (cloud point) that is above the reservoir temperature. 

FIGURE 8.4 Chain length dependence of the upper critical consolute solution temperature for (1) polystyrene in cyclohexane and (2) polyisobutylene in di-isobutyl ketone (data from Schultz, A.R. and Flory, P.J., J. Am. Chem. Soc., 74, 4760, 1952), and the lower critical solution temperature for (3) polyoctene-1 in w-pentane (data from Kinsinger, J.B. and Ballard, L.R, Polym. Lett., 2, 879, 1964). 

In a similar manner to finding a lower consolute solution temperature, can a lower consolute solution volume fraction be found ... 

LEE Lee, H.-O., Ban, Y.-B., and Kim, J.-D., Upper and lower consolute solution temperatures of the pseudo-binaiy mixtures of polydispersed polymethyl methacrylate and n-butylehloride, Korean J. Chem. Eng, 8,147, 1988. 

The third type of system gives a closed solubility curve and therefore possesses both an upper and lower critical solution temperature. The first case of this type to be established was that of nicotine and water the solubility curve is illustrated in Fig. I, 8, 3. The lower and upper consolute temperatures are 60 8° and 208° respectively below the former and above the latter the two liquids are completely miscible. 

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. 

It is widely known that poly(N-isopropylacrylamide), poly(IPAAm), in water has a lower critical solution temperature (LCST) at 32 °C. LCST was originally observed in PEG solutions a long time ago. Rowlinson et al. [40] (1957) explained the lower consolute temperature for PEG in water in terms of negative entropies. The first paper on the LCST of poly(IPAAm) at about 31 °C was presented by Heskins and Guillet in 1968 [41]. They reported that aqueous solution of poly(IPAAm) showed phase separation above this temperature, and ascribed it primarily to an entropy effect on the basis of thermodynamical considerations. 

CRITICAL CONCENTRATION. When two immiscible liquids are heated in contact with cacti other their mutual solubility is usually increased until, al Ihe critical solution temperature, they become consolule. The composition of the two solutions immediately before they become consolute is termed the critical concentration. See also Critical Solution Temperature. 

As the temperature is raised, the solubility of phenol in water increases, whereas that of water in phenol also increases. Ultimately at a certain temperature, the two conjugate solutions change into one homogeneous solution. This temperature is known as critical solution temperature or consolute temperature. The value of consolute temperature for this system is 68.3°, and the composition is 33% phenol and 67% water. Above 68.3°, the two liquids are completely miscible in all proportions. The variation of mutual solubility of water and phenol with temperature is shown in fig. (16). The solubility of phenol in water increases with rise of temperature along die curve AB, while the solubility of water in phenol increases along CB. The two curves do not intersect each other, but meet at a certain point B, known as C.S.T. 

The curve AB shows the decreasing solubility of triethylamine in water while curvers CB shows the decreasing solubility of water in triethylamine. The two curves meet at B, which is the lower critical solution temperature or lower consolute temperature of the system. Any point within the area ABC will give two liquid layers, while any point outisde the area ABC will give a homogeneous solution. 

As the temperature increases, the compositions of the two layers come closer together because the solubility of phenol in water or vice versa increases with the increase in temperature. At the temperature of 66.8°C, for example, the compositions of the water-rich phase and the phenol-rich phase are identical. There is one homogeneous solution whose composition is represented by c (e.g., 0.37 weight fraction of phenol). The temperature tc is known as the critical solution temperature or upper consolute temperature, above which two liquids in all proportions are completely miscible. 

Another solubility phenomenon that may depend partly on H bonding is consolute temperature or critical solution temperature formation in mixtures that have a composition region in which they are im -miscible. The region narrows as the temperature is changed. Above an upper consolute temperature, or below a lower consolute temperature, the components are miscible over the entire composition range. Figure 2-11 shows such loops for 2,4- and 2,5-dimethylpyridine in water, as reported by Andon and Cox (45a). Other pyridine-water systems are... 

For the hydrocarbon--CO2 systems studied here, at pressures above the critical pressure (7.383 MPa) and above the critical temperature (304.21 K) of C02 the isobaric x,T coexistence plots of liquid and vapor phases form simple closed loops. The minimum occurs at the lower consolute point or the Lower Critical Solution Temperature (LCST). Since pressure is usually uniform in the vicinity of a heat transfer surface, such diagrams serve to display the equilibrium states possible in a heat transfer experiment. 

ATow.i Octanol-water partition coefficient of species i Kc Concentration-based partition coefficient of species i Kx Mole fraction-based partition coefficient of species i r Lower consolute or lower critical solution temperature (K) Upper consolute or upper critical solution temperature (K) X Set of liquid-phase mole fractions X, X2,. n Osmotic pressure (kPa)... 

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. ... 

This is the upper consolute temperature for a Margules mixture. Note that the Margules equation does not have a lower critical solution temperature (i.e., there is no rolution of Eq. 11.2-11 for which Eq. 11.2-10b is satisfied). Thus the two partially miscible liquid phases of a Margules mixture cannot be made to combine by lowering the temperature. 

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