Lower and upper critical solution

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. 

A few systems with both lower and upper critical solution temperatures are tabulated below., ... 

PAG Pagonis, K. and Bokias, G., Simultaneous lower and upper critical solution temperature-type co-nonsolvency behaviour exhibited in water-dioxane mixtures by linear copolymers and hydrogels containing Y-isopropylaciylamide and N,N-dimethylacrylamide, Polym. Int., 55, 1254, 2006. 

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

The available experimental data show a wide diversity of lower and upper critical solution temperature variation with increasing pressure. If the critical curves Li = L2 do not have the hypercritical solution point, they should intersect a crystallization surface at high pressures and end in the nonvariant critical point Li = L2-S (Valyashko, 1990a), as it was found for acetonitrile (C2H3N) - H2O and other binary mixtures (Schneider, 1964, 1970). 

Plumper, F. A., Schmalz, A., Ballauf, M. and Muller, A. H. E. (2007) Tuning the thermoresponsiveness of weak polyelectrolytes by pH and hght Lower and upper critical-solution temperature of poly(N,N-dimethylaminoethyl methacrylate),/ Am Chem Soc, 127,14538-14539, doi 10.1021/ja074720i. 

Figure 4.19 Schematic phase diagrams showing lower and upper critical solution temperatures, LCST and UCST.

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