The upper critical solution temperature (UCST) or upper consolute temperature is the

critical temperature Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine *Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing in ...

above which the components of a mixture are

miscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). Such substances are said to be miscible (etymologically ...

in all proportions. The word ''upper'' indicates that the UCST is an

upper bound In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is every element of . Dually, a lower bound or minorant of is defined to be an element of that is less ...

to a temperature range of partial miscibility, or miscibility for certain compositions only. For example,

hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...

nitrobenzene Nitrobenzene is an aromatic nitro compound and the simplest of the nitrobenzenes, with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced ...

mixtures have a UCST of , so that these two substances are miscible in all proportions above but not at lower temperatures. Examples at higher temperatures are the

aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...

-water system at (at pressures high enough for liquid water to exist at that temperature), and the

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

system at (a temperature where both metals are liquid). A solid state example is the palladium-hydrogen system which has a solid solution phase (H₂ in Pd) in equilibrium with a hydride phase (PdH_n) below the UCST at 300 °C. Above this temperature there is a single solid solution phase. In the phase diagram of the mixture components, the UCST is the shared maximum of the concave down spinodal and binodal (or coexistence) curves. The UCST is in general dependent on pressure. The

phase separation Phase separation is the creation of two distinct Phase (matter), phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. This type of phase separation is kn ...

at the UCST is in general driven by unfavorable energetics; in particular, interactions between components favor a partially demixed state.

Polymer-solvent mixtures

Some

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

solutions also have a lower critical solution temperature (LCST) or lower bound to a temperature range of partial miscibility. As shown in the diagram, for polymer solutions the LCST is higher than the UCST, so that there is a temperature interval of complete miscibility, with partial miscibility at both higher and lower temperatures. The UCST and LCST of polymer mixtures generally depend on polymer

degree of polymerization The degree of polymerization, or DP, is the number of structural unit, monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymeriza ...

and polydispersity. The seminal statistical mechanical model for the UCST of polymers is the Flory–Huggins solution theory. By adding soluble impurities the upper critical solution temperature increases and lower critical solution temperature decreases.

References

{{Reflist, 30em Critical phenomena

Polymer-solvent mixtures

See also

References