Lower 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. 

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). 

Myrat, C. D. Rowlinson, J. S., "The Separation and Fractionation of Polystyrene at a Lower Critical Solution Point," Polymer, 6, 645 (1965). 

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. 

Some systems exhibit a lower critical solution point (Fig. 14.4). At higher temperatures (and depending upon the composition), two phases can be present. At lower temperatures, the two substances are totally miscible. An example of this... 

Fig. 14.4 Phase diagram of a system with a lower 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... 

MYR Myrat, C.D. and Rowlinson, J.S., The separation and fractionation of polystyrene at a lower critical solution point, Polymer, 6, 645, 1965. 

Polymers in coexisting phases show different molar-mass distributions which are also different from that of the initial homogeneous system (Figure 10.15). This effect is called fractionation effect and can be used for the production of tailor-made polymers [2, 39, 40], The phase with a lower polymer concentration contains the major part of the polymers with a lower molar mass. The cloud-point curve always corresponds to the molar-mass distribution of the initial polymer, but the first droplets of the formed coexisting new phase never do so (with the exception of the critical solution point) and, hence, they are not located on the cloud-point curve but on the shadow curve. 

Rowlinson, J.S. and Freeman, PJ. (1961) Lower critical solution points on hydrocarbon mixtures. Pure Appl. Chem., 2,329-334. 

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. 

Goldstein, R. E. 1984. On the theory of lower critical solution points in hydrogen-bonded mixtures, 80 5340-5341. 

Classes IV and V are observed only for certain values of the Lennard-Jones parameters. The same sequence Is found for quad-rupolar/nonpolar mixtures. The potential models considered here fail to account for the class VI systems, i.e., those with low temperature lower critical solution points. Such behavior is believed to arise from strong unlike pair forces, and presumably requires different potential models than those used here. 

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. 

Fig. 10. The mole fraction of carbon dioxide in saturated solutions in air at — 110°C (above the lower critical end point). The full line is the experimental curve of Webster and the dashed curves are 1, an ideal gas mixture 2, an ideal gas mixture with Poynting s correction and 3, the solubility calculated from Eq. 8 and the principle of corresponding states.

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...

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. 

An interesting family of polymeric ligands show inverse temperature dependence of solubihty in water, i.e. they can be precipitated from aqueous solutions by increasing the temperature above the so-called cloud point. Typically these ligands contain poly(oxyalkylene) chains, but the phenomenon can be similarly observed with poly(N-isopropyl acrylamide) derivatives (e.g. 132) and methylated cyclodextrins, too. At or above their cloud points these compounds fall off the solution, due to the break-up and loss of the hydration shell which prevents aggregation and precipitation of their molecules. Conversely, upon cooling below this temperature (also called the lower critical solution temperature, LCST) these substances dissolve again. 

Lower Critical Solution Temperatures LCSTs were determined from plots of optical density at 600 nm versus temperature for 0.03% solutions of each polymer in PBS and were defined as the temperature at which Asoo = 0.1. Temperatures were raised at less than 0.3 C per minute and were measured with a thermometer that had been calibrated against an NBS primary standard thermometer. LCSTs for Figure 6 were determined from the cloud points of 0.01% solutions. 

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. 

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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