UNIQUAC

UNIQUAC regression of activity coefficients (chloroform/methanol mixture)

In statistical thermodynamics, UNIQUAC (a portmanteau of universal quasichemical) is an activity coefficient model used in description of phase equilibria.[1] [2] The model is a so-called lattice model and has been derived from a first order approximation of interacting molecule surfaces. The model is, however, not fully thermodynamically consistent due to its two-liquid mixture approach.[2] In this approach the local concentration around one central molecule is assumed to be independent from the local composition around another type of molecule.

The UNIQUAC model can be considered a second generation activity coefficient because its expression for the excess Gibbs energy consists of an entropy term in addition to an enthalpy term. Earlier activity coefficient models such as the Wilson equation and the non-random two-liquid model (NRTL model) only consist of enthalpy terms.

Today the UNIQUAC model is frequently applied in the description of phase equilibria (i.e. liquid–solid, liquid–liquid or liquid–vapor equilibrium). The UNIQUAC model also serves as the basis of the development of the group contribution method UNIFAC,[3] where molecules are subdivided into functional groups. In fact, UNIQUAC is equal to UNIFAC for mixtures of molecules, which are not subdivided; e.g. the binary systems water-methanol, methanol-acryonitrile and formaldehyde-DMF.

A more thermodynamically consistent form of UNIQUAC is given by the more recent COSMOSPACE and the equivalent GEQUAC model.[4]

  1. ^ Abrams, Denis S.; Prausnitz, John M. (1975). "Statistical thermodynamics of liquid mixtures: A new expression for the excess Gibbs energy of partly or completely miscible systems". AIChE Journal. 21 (1): 116–128. doi:10.1002/aic.690210115. ISSN 0001-1541.
  2. ^ a b Maurer, G.; Prausnitz, J.M. (1978). "On the derivation and extension of the uniquac equation". Fluid Phase Equilibria. 2 (2): 91–99. doi:10.1016/0378-3812(78)85002-X. ISSN 0378-3812.
  3. ^ Fredenslund, Aage; Jones, Russell L.; Prausnitz, John M. (1975). "Group-contribution estimation of activity coefficients in nonideal liquid mixtures". AIChE Journal. 21 (6): 1086–1099. doi:10.1002/aic.690210607. ISSN 0001-1541.
  4. ^ Egner, K.; Gaube, J.; Pfennig, A. (1997). "GEQUAC, an excess Gibbs energy model for simultaneous description of associating and non-associating liquid mixtures". Berichte der Bunsengesellschaft für Physikalische Chemie. 101 (2): 209–218. doi:10.1002/bbpc.19971010208. ISSN 0005-9021.