Hypercritical point

Figure 5. A PS/l-component solvent mixture with two hypercritical points, (a Left) 21.5% PS8300 in cis/trans//l/l-l,4-dimethylcyclohexane(DMCH) exhibiting only Tjiyp in this experimental range, (b Right) 7% PS575,000 in trans-l,4-DMCH showing both Thypl and Phyp - Modified from ref 7 and used with permission.

The present data extend over a wide range and in no sense carefully sample the immediate vicinity of the hypercritical points. Further, over the range of these experiments. 

Rg. 6.9 (a) Hypercritical point where a miscibility loop shrinks to a point (n = 37). (b) Double critical... 

However, the miscibility loop shrinks with a decrease in the molecular weight, and eventually vanishes at a certain critical molecular weight (n = 37 for the Figure 6.9(a)). This vanishing loop is called the hypercritical point (HCP). 

Very large isotope effects like those shown in Fig. 5.12 seem to be limited to the hypercritical regions of phase diagrams, i.e. not too far from thermodynamic divergences of the type (dP/dT)c = 00 or (dT/dP)c = 00 (i.e. pressure-double critical points (p-DCP) or temperature-double critical points (T-DCP), respectively). 

Figure 1. Demixing diagrams for PS in 0-solvents and poor solvents (schematic). The variable X might be pressure, Mw D/H ratio in solvent or solute, etc. See text for a further discussion, (a, top left) PS in a 0-solvent (monodisperse approximation). For X=Mw - the X=0 intercepts of the upper and lower heavy lines drawn through the minima or maxima in the demixing curves define 0Land0u, respectively, (b, top right) PS in a poor solvent (monodisperse approximation). The heavy dot at thecenterlocates the hypercritical (homogeneous double critical) point. (c, bottom right) The effect of polydispersity. BIN=binoda] curve, CP=cloud point curve, SP=spinodal, SHDW=shadow curve. See text Modified from ref. 6 and used with permission.

Figure 4. Critical Demixing isopleths for PS/methylcyclohexane/n-heptane solutions. Parts b and c show the diagrams in the vicinity of the hypercritical (homogeneous double critical) points. Modified from ref 4 and used with permission.

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). 

Figure 1.18 p-T projection of limited immiscibility regions ( closed-loop or b type) with the hypercritical solution points in the systems methylketone (C4H8O) - H2O (1), 2-butanol (C4H10O) - H2O (2), 2-butoxyethanol (C6H14O2) - H2O (3) and 2-methylpyridine (C6H7N) - H2O (4) (Schneider, G.M. (1973) In Water - A Comprehensive Treatise , edt. F. Franks, Plenum Press, v.2, ch.6, pp. 381 04.). 

The experimental and theoretical studies of fluid phase equilibria in this type of binary systems demonstrate that the critical curve NKb may have several extremes in temperature and pressure, and not only one pressure maximum as in the version, shown in Figure 1.20. Some of these extremes are due to continuous phase transition fi om Li = L2 to L = G other ones are due to the hypercritical solution points. 

Influence of salts on ternary immiscibility regions. Figures 1.39a,b show how the closed-loop immiseibility region (type b) with the hypercritical solution point, observed... 

Since shrinkage and cracking are produced by capillary forces, Kistler [94] reasoned that those problems could be avoided by removing the liquid from the pores above the critical temperature (T ) and critical pressure (P ) of the liquid. As indicated in the phase diagram in Fig. 32, there is no longer any distinction between the liquid and vapor phases the densities become equal, there is no liquid-vapor interface and no capillary pressure. In the process of supercritical (or hypercritical) drying, a sol or wet gel is placed into an autoclave and heated along a path such as the one indicated in Fig. 32. The pressure and temperature are increased in such a way that the phase boundary is not crossed once the critical point is passed, the solvent is vented at a... 

---