Handbook of Solvents - George Wypych - ChemTech - Ventech!

1424 Aydin K. Sunol and Sermin G. Sunol
1. The filled circles are pure component critical points
2. The triangles are critical end points, lower (L) and upper (U)
Type I mixtures have continuous gas-liquid critical line and exhibit complete miscibility
of the liquids at all temperatures. Mixtures of substances with comparable critical properties
or substances belonging to a homologous series form Type I unless the size difference
between components is large. The critical locus could be convex upward with a maximum
or concave down with a minimum. Examples of Type I mixtures are: Water + 1-propanol,
methane + n-butane, benzene + toluene, and carbon dioxide + n-butane.
Type II have systems have liquid-liquid immiscibility at lower temperatures while locus
of liquid-liquid critical point (UCST) is distinct from gas liquid critical line. Examples
include: water + phenol, water + tetralin, water + decalin, carbon dioxide + n-octane, and
carbon dioxide + n-decane.
When the mutual immiscibility of two components becomes large, the locus of liquid-liquid
critical solution moves to higher temperatures and it eventually interacts with
gas-liquid critical curve disrupting the gas-liquid locus. This particular class is type III and
some examples include: water + n-hexane, water + benzene, carbon dioxide + n-tridecane,
and carbon dioxide + water.
Type IV systems have three critical curves, two of which are VLL. If the hydrocarbon
mixtures differ significantly in their critical properties, they conform to type IV or V. The
primary difference between Type IV and V is that type IV exhibits UCST and LCST while
type V has LCST only. One important class of systems that exhibit type IV behavior is solvent
polymer mixtures such as cyclohexane + polystyrene. Other examples of type IV include
carbon dioxide + nitrobenzene and methane + n-hexane while ethane with ethanol or
1-propanol or 1-butanol exhibit type V behavior.
Type VI systems are composed of complex molecules with hydrogen bonding or other
strong intermolecular forces and result in behavior where LCST and UCST are at temperatures
well removed from gas-liquid critical temperature of the more volatile component.
Types IV and I are of particular interest in representing behavior exploited in supercritical
extraction. There are many more possible classes and subclasses, especially where
azeotropic behavior variations are involved, as discussed in the works of Rowlinson 10 and
King. 11 More on the phase behavior applicable to dense gases can be found in reviews 12,13
and specialized texts 14 in this area.
Supercritical extraction often
involves separation of relatively
non-volatile components,
often in the solid phase, through
selective solubility in the supercritical
gasses. Thus, the critical
temperatures of the pure components
are likely to be significantly
different and the critical temperature
of the solvent is likely to be
lower than the triple point temperature
of the solute. The implication
is that there is no common
temperature range where both
Figure 21.1.6 Phase behavior of dissimilar systems.