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Order-Disorder Phenomena and Phase Separation

Published

Author(s)

Benjamin P. Burton

Abstract

Order-disorder and phase separation in condensed solutions of two or more components are similar processes that are typically driven by energetics of opposite sign. The components may be: different atomic species in alloys or solid solutions; atoms and vacancies in a lattice gas [1]); adsorbed atoms and vacant adsorption sites on a crystal surface; different molecules in a polymer blend[2] or liquid crystal; magnetic moments in a ferro-, ferri-, or antiferro-magnet; electric dipoles in a ferroelectric crystal. At sufficiently high temperatures, all solutions disorder, unless they melt or vaporize first, because the disordered state has greater configurational entropy, and therefore lower Gibbs energy at high temperatures: G = H - TS (1) where: G = Gibbs energy; H = enthalpy; T = absolute temperature; S = entropy. It is sufficient to deal only with configurational contributions to thermodynamic functions such as Gibbs energy (Gc), energy (Ec), enthalpy (Hc = Ec + PV), and entropy (Sc). Thus, the usual situation is that Ec, drives ordering at low-T, while the -TSc term favors disorder at high-T. A generic counter trend occurs, however, in many polymer-solvent systems and polymer blends[3] which typically exhibit miscibility gaps with lower critical temperatures (LCT) for phase separation, and sometimes with upper critical temperatures (UCT) as well. As discussed by Sanchez and Panaylotou[3], the occurrence of LCT's can be traced to the greater compressibility of concentrated polymer-solvent solutions relative to the approximately incompressible dilute polymer- or solvent-rich solutions; i.e. to generic composition dependence in the PV term.
Citation
Encyclopedia of Materials: Science & Technology
Volume
7

Keywords

Order-disorder, phase separation

Citation

Burton, B. (2001), Order-Disorder Phenomena and Phase Separation, Encyclopedia of Materials: Science & Technology (Accessed June 9, 2023)
Created December 31, 2000, Updated February 19, 2017