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). The term is most often applied to liquids but also applies ...

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 greater than or equal to every element of . Dually, a lower bound or minorant of is defined to be an element ...

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

hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...

nitrobenzene Nitrobenzene is an organic compound 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 on a large scale from benzene as a precursor t ...

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 is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine In organic chemistry, an aromatic amine is an organic compound consisting of an aroma ...

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

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc 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 periodi ...

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 In thermodynamics, the limit of local stability with respect to small fluctuations is clearly defined by the condition that the second derivative of Gibbs free energy is zero. : 0 The locus of these points (the inflection point within a G-x or G- ...

and

binodal In thermodynamics, the binodal, also known as the coexistence curve or binodal curve, denotes the condition at which two distinct phases may coexist. Equivalently, it is the boundary between the set of conditions in which it is thermodynamicall ...

(or coexistence) curves. The UCST is in general dependent on pressure. The

phase separation Phase separation is the creation of two distinct phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. Colloids are formed by phase separation, though n ...

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 (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

solutions also have a

lower critical solution temperature The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible in all proportions. The word ''lower'' indicates that the LCST is a lower bound to a t ...

(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 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 polymerization is given by ...

and

polydispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an incons ...

. The seminal statistical mechanical model for the UCST of polymers is the

Flory–Huggins solution theory Flory–Huggins solution theory is a lattice model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation ...

. By adding soluble impurities the upper critical solution temperature increases and lower critical solution temperature decreases.

References

Critical phenomena {{chemistry-stub

Polymer-solvent mixtures

See also

References